NEET 2026 re-exam Biology answer key with solutions: questions 91–180

The human vertebral column is made of 26 serially arranged vertebrae (after fusion of the sacral and coccygeal vertebrae into single bones in the adult), running from the neck to the pelvis.

• (1) 7 is the number of cervical vertebrae (neck region) only, not the whole column.
• (2) 12 is the number of thoracic vertebrae, the chest region alone; students confuse this single-segment count with the whole 26-vertebra column.
• (4) 206 is the total number of bones in the adult human skeleton, not the vertebral count -- a common mix-up.

A lichen is a symbiotic association between a fungal partner (mycobiont) and a photosynthetic alga or cyanobacterium (phycobiont); the alga makes food and the fungus provides shelter and absorbs water/minerals.

• (2) Sponges (Porifera) are multicellular animals, not a fungus-alga association.
• (3) Mycorrhiza is a symbiotic fungus-root association with higher plants, not with algae.
• (4) Chrysophytes are a group of algae themselves (diatoms, golden algae), not a symbiosis.

Schleiden (a botanist) and Schwann (a zoologist) together proposed the cell theory, later extended by Virchow's principle that cells arise only from pre-existing cells.

• (2) Robert Brown is known for discovering the cell nucleus, not for formulating cell theory; students confuse this famous cell-biology find with the theory's origin.
• (3) Singer and Nicolson proposed the fluid mosaic model of the membrane, unrelated to cell theory.
• (4) Leeuwenhoek first observed and described free-living cells (like bacteria), but did not formulate the theory.

Prokaryotic (70S) ribosomes are built of a 50S and a 30S subunit (a, true); many bacteria carry extra-chromosomal circular plasmid DNA (b, true); mesosomes are infoldings of the plasma membrane seen in prokaryotes (c, true). Peroxisomes are membrane-bound organelles found only in eukaryotic cells, so (d) is false. Correct set: (a), (b), (c).

• (1) (a) is a true prokaryotic feature -- 70S ribosomes split into 50S and 30S subunits; this option wrongly drops it from the correct combination.
• (2) (b) plasmids do occur in many prokaryotes, a true statement; this option wrongly excludes it, keeping only two of the three correct statements.
• (3) Wrongly includes (d) peroxisomes, which prokaryotes lack (no membrane-bound organelles).

The brain (part of the CNS) is wrapped in three meninges -- outer dura mater, middle arachnoid, and inner pia mater -- so options 1-3 are all CNS coverings. The pericardium is the membrane covering the heart, part of the circulatory system, not the CNS.

• (1) Arachnoid is the middle cranial meninx, a genuine CNS covering.
• (2) Dura mater is the outer cranial meninx, a genuine CNS covering.
• (3) Pia mater is the inner meninx in direct contact with brain tissue.

The mitochondrion has two membranes; the inner membrane folds inward as cristae and encloses the central fluid space called the matrix, which contains the enzymes of the Krebs cycle, mitochondrial DNA and ribosomes.

• (2) Cytosol is the fluid of the whole cell outside organelles, not enclosed by the mitochondrial inner membrane.
• (3) Mucus is a secretion of epithelial cells, unrelated to mitochondria.
• (4) Aqueous humor is the fluid of the eye chamber, unrelated to mitochondria.

Cristae are infoldings of the mitochondrial inner membrane (A-II). Cisternae are the disc-shaped stacked sacs of the Golgi apparatus (B-IV). Thylakoids are flat membranous sacs inside the chloroplast stroma (C-I). Phospholipids are the basic structural molecules of the cell membrane (D-III).

• (1) Swaps cristae with cell membrane and cisternae with mitochondria -- wrong pairing.
• (3) Puts thylakoids with cell membrane and phospholipid with stroma sacs -- wrong pairing.
• (4) Puts cristae with Golgi sacs and cisternae with cell membrane -- wrong pairing.

Chromoplasts are the coloured plastids (other than green) that store fat-soluble pigments such as carotene and xanthophylls, giving flowers/fruits their yellow, orange or red colour.

• (1) Chloroplasts store chlorophyll for photosynthesis, not primarily xanthophyll.
• (3) Aleuroplasts are colourless leucoplasts that store protein, not pigments; students mix up plastid names that all sound like 'stores something'.
• (4) Amyloplasts are colourless leucoplasts that store starch, easily confused with the pigment-storing chromoplast since both are specialised plastids.

The pituitary is anatomically divided into adenohypophysis and neurohypophysis (a, true). Its pars distalis secretes six trophic hormones including FSH (b, true) and prolactin, while pars intermedia secretes MSH (c, true). So (d), which claims no prolactin is secreted, is false -- prolactin IS one of the adenohypophyseal hormones. Correct set: (a), (b), (c).

• (1) Leaves out (c), which is also correct (MSH is secreted by pars intermedia).
• (3) Wrongly includes (d), which is false since prolactin is secreted.
• (4) This option leaves out (a), the true fact that the pituitary is anatomically split into adenohypophysis and neurohypophysis, keeping only two of the three correct statements.

RuBisCO's oxygenase activity adds O2 to RuBP, splitting it into one 3-carbon molecule of 3-phosphoglycerate and one 2-carbon molecule of phosphoglycolate; the latter is metabolised in the photorespiratory pathway.

• (1) Phosphoenolpyruvate is the CO2 acceptor in C4 plants (PEP carboxylase reaction), unrelated to RuBisCO's oxygenase step.
• (3) Oxaloacetate is the first stable product of C4 photosynthesis, not of photorespiration.
• (4) Malate is a C4-pathway intermediate transported to bundle sheath cells, not a photorespiration product.

Prions are infectious proteinaceous particles with no nucleic acid; the most notable prion disease is bovine spongiform encephalopathy (mad cow disease) in cattle and its human analogue, Creutzfeldt-Jakob disease.

• (2) Viroids are naked, low-molecular-weight infectious RNA (no protein coat) that cause plant diseases, not mad cow disease.
• (3) Aspergillus is a fungus causing infections like aspergillosis, not mad cow disease.
• (4) Mycoplasma are wall-less bacteria causing diseases like pneumonia, not mad cow disease.

The reaction-centre pigment of both photosystems is a special form of chlorophyll a. In PS I it absorbs maximally at 700 nm, hence named P700; in PS II the corresponding chlorophyll a absorbs at 680 nm (P680).

• (1) Chlorophyll b is an accessory pigment that funnels light energy to chlorophyll a; it is not the reaction-centre pigment.
• (3) Xanthophylls are accessory carotenoid pigments, not the reaction-centre pigment.
• (4) Carotenoids in general are accessory pigments, not the reaction-centre pigment.

Frogs are amphibians with dual respiration -- in water, the moist, vascularised skin acts as the respiratory surface for cutaneous respiration (dissolved O2 diffuses in through the skin); on land, they switch to buccal and pulmonary (lung) respiration.

• (2) Buccal cavity respiration operates on land through air movement in the mouth, not the primary mode in water.
• (3) Lungs are used mainly for respiration on land, not the primary aquatic mode.
• (4) Frogs do not have a trachea-based tracheal respiratory system like insects.

Proceeding proximal to distal down the hindlimb: the femur (thigh bone) articulates at the knee where the patella (kneecap) sits, then the tibia-fibula (leg bones), and finally the tarsals (ankle bones). This gives Femur-Patella-Tibia-Tarsal.

• (1) Places patella after tibia, which is out of proximal-to-distal order (patella sits at the knee, before the tibia).
• (2) Starts with patella before femur, reversing the correct proximal-to-distal order.
• (4) Places tarsal (ankle) before patella and tibia, badly out of sequence.

Phyllotaxy (from Greek phyllon=leaf, taxis=arrangement) describes the pattern in which leaves are arranged on a stem or branch -- alternate, opposite, or whorled.

• (2) Flower arrangement on the floral axis is called inflorescence, not phyllotaxy.
• (3) Fruit arrangement on an axis has no dedicated term like phyllotaxy; students wrongly stretch the leaf-arrangement word to other plant parts.
• (4) Sepal arrangement in the bud (aestivation) is a separate floral term, not phyllotaxy.

Starch is a storage polysaccharide used for energy storage (A-II). Antibodies are proteins that fight infection (B-I). Concanavalin A is a well-known lectin protein from jack bean (C-IV). GLUT-4 is a membrane transport protein that enables glucose transport into cells (D-III).

• (1) Swaps starch/antibody roles and mismatches Concanavalin A with glucose transport.
• (3) This option swaps lectin Concanavalin A with the glucose-transporter Glut-4, mixing up a plant defence protein with a membrane transport protein.
• (4) Keeps starch/antibody wrong and mismatches lectin/transport pair.

Statement I correctly states the textbook definition of radial symmetry. Statement II is false: in Echinodermata only the sessile/free-living adults are radially symmetrical, while their free-swimming larvae are bilaterally symmetrical -- a classic case of larval symmetry differing from the adult.

• (1) Wrong because Statement II is false (larvae are bilateral, not radial).
• (2) Statement I is the standard definition of radial symmetry and is true, not false; only Statement II (about echinoderm larvae) is wrong, so calling both incorrect overcorrects.
• (4) Wrong because Statement I is actually correct, not incorrect.

Because their functions are coordinated through vesicle trafficking, the endoplasmic reticulum, Golgi complex, lysosomes and vacuole are grouped together as the endomembrane system. Mitochondria, chloroplasts and peroxisomes are excluded because their functions are not coordinated with this network.

• (2) Wrongly includes chloroplast and peroxisomes, which are NOT part of the endomembrane system.
• (3) Lists only excluded organelles (mitochondria, chloroplast, peroxisomes), none of which belong to the endomembrane system.
• (4) Wrongly includes chloroplast and peroxisomes instead of ER and lysosomes.

Glycolysis breaks down one molecule of glucose into two molecules of pyruvic acid. For 206 molecules of glucose, the yield is 206 x 2 = 412 molecules of pyruvic acid.

• (1) 206 would be the yield only if 1 glucose gave 1 pyruvate, ignoring the 1:2 stoichiometry.
• (2) 309 does not correspond to any simple multiple of 206 tied to the correct 1:2 ratio.
• (3) 103 is half of 206, the reverse of the actual 1:2 relationship.

2,4-Dichlorophenoxyacetic acid (2,4-D) is a synthetic auxin widely used as a herbicide -- it kills broad-leaved dicot weeds in monocot crop fields (e.g., lawns, cereal crops) without harming the monocots.

• (2) Kinetin is a cytokinin used to promote cell division and delay senescence, not a herbicide.
• (3) Abscisic acid is the stress/dormancy hormone, used to induce dormancy, not as a herbicide.
• (4) Gibberellin promotes growth (bolting, seed germination); it is not used as a herbicide.

Statement I is correct: unlike bryophytes/pteridophytes, gymnosperm gametophytes have no independent free-living existence -- they remain retained within the sporangia on the sporophyte. Statement II is also correct: gymnosperm literally means 'naked seed' -- the ovules/seeds are not enclosed in an ovary/fruit wall.

• (2) Wrong because both statements are individually verifiable as true.
• (3) Statement II is also true -- gymnosperm literally means 'naked seed', so the seeds are genuinely uncovered; this option wrongly rejects that correct fact.
• (4) Statement I is actually true -- gymnosperm gametophytes stay retained within the sporangia; this option mistakenly reverses which statement holds.

The four basic bacterial shapes are: spherical = coccus/cocci (A-II), rod-shaped = bacillus/bacilli (B-IV), comma-shaped = vibrio (C-I), and spiral = spirillum (D-III, matching itself with 'Spirilla' as the plural form/shape name).

• (1) This option pairs spherical bacteria with vibrio (the comma shape) and rods with spirilla (the spiral shape), swapping two of the four shape names.
• (2) Mismatches spherical with spirilla and comma with vibrio pairing order.
• (3) This option pairs rod-shaped bacteria with vibrio and comma-shaped bacteria with bacilli, crossing the two shape-name pairs.

Solanaceae flowers are bisexual, actinomorphic (radially symmetric) (a, true); calyx has five gamosepalous (united) sepals, persistent (b, true); androecium has five epipetalous stamens attached to the corolla tube (c, true). The gynoecium is bicarpellary, syncarpous with a SUPERIOR ovary, not inferior, so (d) is false.

• (2) Wrongly isolates (d), which is actually false for Solanaceae (ovary is superior).
• (3) Drops the also-correct statement (c) about epipetalous stamens.
• (4) Wrongly includes false statement (d) and drops true statement (a).

Historically: Priestley (1770s) first showed plants restore something in air that is used up by burning/breathing (role of air); Ingenhousz then showed it is specifically the green parts, in sunlight, that release oxygen; Sachs demonstrated glucose (as starch) is produced during photosynthesis; Engelmann later used a prism and Cladophora with aerotactic bacteria to map the action/absorption spectrum of chlorophyll a and b to specific wavelengths. So the correct historical order is: role of air, release of oxygen, production of glucose, absorption spectra.

• (1) Places the absorption-spectra work (a late, refined finding) first, before even the basic 'role of air' discovery -- reverses history.
• (3) Puts absorption spectra before the role-of-air experiment, which came first historically.
• (4) Puts glucose production before the role-of-air and oxygen-release experiments that actually preceded it.

The SAN is the natural pacemaker of the heart -- it generates the maximum number of action potentials among the nodal tissue, about 70-75 per minute, setting the normal resting heart rate (average ~72 beats/min).

• (1) 28-30/min is far below normal SAN firing rate -- would represent severe bradycardia.
• (3) 100-110/min is above the normal resting rate, closer to mild tachycardia.
• (4) 120-140/min is well above normal resting heart rate, seen only during exertion/tachycardia.

One turn of the Calvin cycle fixes one CO2; producing a single molecule of glucose (6 carbons) requires the fixation of six CO2 molecules, i.e., six turns of the cycle. So three molecules of glucose need 3 x 6 = 18 turns.

• (1) 6 turns of the Calvin cycle fix only enough CO2 for one glucose molecule; students stop at the per-glucose count instead of scaling it up to three.
• (2) 3 turns is far too few; not even one glucose can be made in 3 turns (needs 6).
• (3) 1 turn fixes only a single CO2, nowhere near enough carbon for even one glucose.

During coagulation, the enzyme thrombin converts soluble plasma fibrinogen into insoluble fibrin threads that form the clot network trapping blood cells -- the conversion is fibrinogen to fibrin, never the reverse. So statement 4, which reverses this direction, is the incorrect one.

• (1) True -- coagulation is indeed the physiological response to vessel injury.
• (2) This statement is factually correct -- the clot's mesh is built of fibrin threads formed from fibrinogen; it does not fit the 'incorrect statement' the question asks for.
• (3) True -- this correctly states the actual direction of conversion (fibrinogen to fibrin).

For mango (Mangifera indica): Family = Anacardiaceae (A-III), Genus = Mangifera (B-V), Class = Dicotyledonae (C-II), Phylum/division = Angiospermae (D-IV), Order = Sapindales (E-I). Note that strictly, 'Angiospermae' is classified botanically as a division rather than a phylum, but the NTA-keyed option 4 is the one matching the standard NCERT taxonomic-category table for mango.

• (1) Mismatches every category except D; wrong pairing throughout.
• (2) This option assigns Anacardiaceae to genus and Mangifera to family, reversing the true family-genus pair for mango's classification.
• (3) This option assigns genus to Anacardiaceae and family to Sapindales (which is really the order), mixing up several taxonomic ranks for mango.

The standard taxonomic hierarchy, from the most specific (lowest rank, smallest group of organisms) to the most general (highest rank), is: Species -> Genus -> Family -> Order -> Class -> Phylum -> Kingdom. That is exactly (g),(a),(e),(c),(b),(d),(f).

• (2) Starts with genus, skipping species, and scrambles the remaining order.
• (3) This sequence places order and phylum ahead of family and genus, breaking the true rank order species-genus-family-order-class-phylum-kingdom.
• (4) Runs the hierarchy essentially backwards, starting from kingdom.

Marginal placentation (ovules on the ridge along the ventral suture, single carpel) = pea (A-IV). Axile placentation (ovules on a central axis, multilocular ovary) = tomato (B-II). Parietal placentation (ovules on the inner ovary wall) = Argemone/mustard family (C-I). Free central placentation (ovules on a central axis without septa) = primrose/Dianthus (D-III).

• (1) This option pairs marginal placentation with tomato (an axile example) and axile with pea (a marginal example), swapping the two.
• (2) This option pairs axile placentation with primrose (a free-central example) and free-central with pea, crossing those two types.
• (3) This option pairs free-central placentation with tomato (an axile example) and axile with primrose, reversing the two.

Pteridophytes are divided into four classes -- Psilopsida, Lycopsida, Sphenopsida (represented by Equisetum, the horsetail) and Pteropsida -- so Sphenopsida is a pteridophyte class.

• (1) Bryophytes are the mosses/liverworts group, classified separately (not into Sphenopsida etc).
• (2) Angiosperms are flowering seed plants, not divided into Sphenopsida.
• (3) Gymnosperms are naked-seed plants, a separate group from pteridophyte classes.

Photorespiration DOES occur in C3 plants (it is essentially a C3-plant phenomenon caused by RuBisCO's oxygenase activity), so (a) is false. Photorespiration takes up O₂ and gives off CO₂ (the reverse of statement b), so (b) is false. Phosphoglycolate is indeed the initial 2-carbon product (c, true). Unlike photosynthesis, photorespiration involves no synthesis of ATP or NADPH (d, true).

• (1) Wrongly includes false statement (a); photorespiration does occur in C3 plants.
• (3) Wrongly includes false statement (b), which reverses the actual gas exchange.
• (4) Both (a) and (b) included here are false statements about photorespiration.

Smooth ER (SER) lacks ribosomes on its surface (hence 'smooth') and is the major site for synthesis of lipids, including steroidal hormones; rough ER, by contrast, has ribosomes and is the main protein-synthesis site.

• (1) Ribosome studding is the hallmark of ROUGH ER, not smooth ER.
• (3) Active protein synthesis is a rough-ER function, not smooth ER's.
• (4) Carbohydrate synthesis mainly occurs in chloroplasts (in plants), not smooth ER.

In the Islets of Langerhans, the alpha-cells secrete glucagon and the beta-cells secrete insulin -- alpha-cells do NOT secrete insulin, so statement (2) is the incorrect one. Glucagon indeed stimulates glycogenolysis in the liver, raising blood glucose (hyperglycemic hormone).

• (1) This statement is factually correct -- alpha-cells do secrete glucagon; it is not the incorrect statement the question asks to identify.
• (3) This is a correct fact -- glucagon does trigger glycogenolysis in the liver; it does not fit the 'incorrect statement' being sought.
• (4) This statement is accurate -- insulin is secreted by beta-cells, not alpha-cells; picking it confuses a true fact for the required incorrect one.

A genus comprises a group of closely related species that share more characters in common with each other than with species of other genera -- e.g., Panthera includes lion, tiger, leopard.

• (1) An individual organism is described at the species/organism level, not a genus.
• (2) A population is a group of interbreeding individuals of one species, a different (lower) concept than genus.
• (4) A group of related families is the definition of an ORDER, not a genus.

Prokaryotes include bacteria, blue-green algae (cyanobacteria), and Mycoplasma (wall-less bacteria) -- all lack a membrane-bound nucleus. Fungi, however, are eukaryotic organisms with a true nucleus and membrane-bound organelles.

• (1) Bacteria are the textbook example of a prokaryote, lacking a membrane-bound nucleus; picking this confuses a prokaryote example with the 'not a prokaryote' answer being sought.
• (2) Blue-green algae (cyanobacteria) are prokaryotic despite the name 'algae'.
• (3) Mycoplasma is a wall-less prokaryote, the smallest known living cell.

Gibberellins cause 'bolting' -- rapid internode elongation just before flowering -- in rosette-habit plants such as cabbage and beet.

• (1) Abscisic acid is the growth-inhibiting/stress hormone, promoting dormancy, not elongation.
• (3) Indole butyric acid is an auxin mainly used to promote rooting, not bolting.
• (4) Ethephon releases ethylene, associated with fruit ripening/senescence, not internode elongation.

Interphase is subdivided into G1 (Gap 1, cell growth), S (Synthesis, DNA replication) and G2 (Gap 2, further growth/preparation), after which the cell enters M phase (mitosis) for division. So the correct order is G1-S-G2-M.

• (1) Places G2 before S, but DNA synthesis (S) must occur between G1 and G2.
• (2) Places M in the middle of interphase, but mitosis only happens after G2, at the end of the cycle.
• (4) This option lists the interphase-to-mitosis order in reverse, starting from S and ending at G1 instead of the true G1-S-G2-M sequence.

Fusion of protoplasms of two gametes is plasmogamy (A-II); subsequent fusion of the two nuclei is karyogamy (B-III); meiosis in the resulting zygote generates haploid spores (C-I).

• (2) This option assigns karyogamy to spore formation and meiosis to nuclear fusion, swapping the true B and C pairings.
• (3) This option calls protoplasm fusion karyogamy and nuclear fusion plasmogamy, reversing the two fusion terms.
• (4) Mismatches all three -- reverses the logical sequence plasmogamy->karyogamy->meiosis.

Statement I is correct -- the class name Reptilia derives from Latin repere/reptum (to creep or crawl), reflecting their locomotion. Statement II is false: while most reptiles have a three-chambered heart, crocodiles are the exception with a four-chambered heart, so 'all' reptiles cannot be said to have three chambers.

• (1) Wrong because Statement II is false (crocodiles are an exception).
• (2) Statement I is a verified true fact -- Reptilia's name does derive from the Latin for creeping or crawling; calling both statements false wrongly rejects it.
• (4) Statement I is actually correct, not false; this option wrongly reverses which of the two statements holds true.

Statement I is correct: diakinesis, the final sub-stage of prophase I, is marked by full chromosome condensation and disappearance of the nucleolus/nuclear envelope. Statement II is false: meiosis I is a REDUCTIONAL division involving pairing of homologues, crossing over and separation of homologous chromosomes -- fundamentally different from mitosis. It is meiosis II that resembles mitosis (equational division, sister chromatids separate).

• (1) Statement II is false -- meiosis I is a reductional division unlike mitosis; it is meiosis II, not meiosis I, that resembles mitosis, so calling both statements true is wrong.
• (2) Wrong because Statement I is a correct, textbook fact about diakinesis.
• (4) Wrong because Statement I is actually correct, not incorrect.

Chordates are fundamentally defined by a notochord, a dorsal, hollow nerve cord, paired pharyngeal gill slits, and a post-anal tail. So 'absence of gills' is actually the opposite of the true defining feature (pharyngeal gill slits ARE present at some stage), making it the characteristic that does NOT apply to chordates.

• (1) A notochord is a genuine defining chordate feature, present at some life stage in every chordate; picking it confuses a true trait with the 'not a characteristic' answer sought.
• (2) A dorsal (not ventral) nerve cord is a genuine chordate feature.
• (4) A post-anal tail at some stage of life is a genuine chordate feature.

Arithmetic growth follows Lt = L0 + rt, where L0 is initial length, r is the constant growth rate, and t is time. Here Lt = 20 + 30 x 7 = 20 + 210 = 230 cm.

• (1) 50 cm would result from only one day's growth added to L0 (20+30), ignoring the other 6 days.
• (2) 170 cm undercounts the total added growth (would come from a smaller/incorrect multiplier).
• (4) 460 cm overshoots -- it looks like doubling the correct 230 cm answer, not the linear formula's result.

By percentage weight of the human body, oxygen contributes about 65.0%, carbon about 18.5%, hydrogen about 9.5%, and nitrogen about 3.3%. So in descending order: Oxygen > Carbon > Hydrogen > Nitrogen, i.e., (a),(b),(c),(d).

• (2) Places hydrogen (9.5%) ahead of oxygen (65%) and carbon (18.5%), badly out of order.
• (3) Starts with carbon, ranking it above oxygen, the actual highest-percentage element.
• (4) This ordering places carbon (about 18.5% by weight) ahead of oxygen (about 65%), reversing the two highest-contributing elements.

In frogs, ten pairs of cranial nerves arise directly from the brain (compared to twelve pairs in humans/mammals).

• (1) 6 pairs undercounts the frog's cranial nerves; students may recall only a partial list of named nerves instead of the full ten pairs.
• (2) 9 pairs is close to but one short of the true count of ten cranial-nerve pairs in frogs, a common off-by-one slip.
• (4) 12 pairs is the number in humans/higher vertebrates, not frogs.

Streptokinase, produced by the bacterium Streptococcus and modified by genetic engineering, is used clinically as a 'clot buster' to dissolve blood clots in patients who have had a myocardial infarction (heart attack).

• (2) Penicillin is an antibiotic that kills bacteria; it has no clot-dissolving action.
• (3) Cyclosporin A is an immunosuppressant used to prevent organ-transplant rejection, not a thrombolytic.
• (4) Statins lower blood cholesterol to slow plaque formation; they do not dissolve an existing clot.

Bt toxin exists in the bacterium as an inactive protoxin crystal. Once an insect larva ingests it, the alkaline pH of the insect gut solubilises the crystal, converting the protoxin into its active, insecticidal form, which then binds to the gut epithelium and kills the insect.

• (2) An acidic gut environment would not solubilise the crystal; it is the alkaline gut of the target insect that activates the toxin.
• (3) Proteases act on the toxin only after it has been solubilised; they are not what triggers the initial activation.
• (4) Nucleases act on nucleic acids and play no role in activating a protein toxin.

Down's syndrome is trisomy of chromosome 21 (an extra copy, 47 chromosomes total), not the loss of an X-chromosome. Turner's syndrome is caused by the loss of one X-chromosome (45, X0 / monosomy), not an additional chromosome copy. Both statements have swapped the two disorders' causes, so both are incorrect.

• (1) Both statements have the disorders' causes swapped: Down's syndrome comes from an extra chromosome 21, and Turner's syndrome from a missing X, the reverse of what is claimed here.
• (3) Statement I is also incorrect, not just Statement II -- Down's syndrome is caused by an extra chromosome 21, not a missing X-chromosome.
• (4) Statement II is also incorrect, not just Statement I -- Turner's syndrome results from losing an X-chromosome, not gaining an extra chromosome.

Syphilis, gonorrhoea and genital warts are classic sexually transmitted infections (STIs), caused respectively by Treponema pallidum, Neisseria gonorrhoeae and Human papilloma virus. Tuberculosis is caused by Mycobacterium tuberculosis and spreads through air (droplet infection), not sexual contact.

• (1) Syphilis, caused by the bacterium Treponema pallidum, is a genuine sexually transmitted disease; picking it confuses an STI with the 'not sexually transmitted' answer sought.
• (3) Gonorrhoea, caused by Neisseria gonorrhoeae, is a well-known sexually transmitted disease; it does not fit the 'not sexually transmitted' answer the question asks for.
• (4) Genital warts, caused by human papilloma virus, spread through sexual contact; picking it confuses a genuine STI with the disease that is NOT sexually transmitted.

Each human sperm has a long tail (flagellum) whose whip-like lashing propels the sperm forward through the female reproductive tract, so sperm motility is due to flagellar movement.

• (2) Ciliary movement (coordinated beating of many short cilia) moves fluid/ovum along the oviduct, not the sperm itself.
• (3) Amoeboid movement, using pseudopodia, is seen in cells like leucocytes, not in sperm.
• (4) Sperm have no muscle tissue; there is no muscular movement involved.

NCERT states natural selection can lead to stabilisation (more individuals acquire the mean character value), directional change (individuals shift towards one extreme) or disruption (individuals move towards both extremes). Genetic drift is a separate mechanism of evolution driven by chance sampling of alleles, not an outcome of natural selection itself, so (b) is excluded.

• (1) Directional change and disruption are also valid outcomes of natural selection, so 'only (a)' is too narrow.
• (3) Genetic drift (b) is a distinct, chance-based evolutionary mechanism, not a consequence of natural selection.
• (4) Disruption (d) is also a recognised outcome of natural selection and should be included.

Intra Cytoplasmic Sperm Injection (ICSI) is the assisted-reproduction procedure in which a sperm is directly injected into the cytoplasm of the ovum to form an embryo in the laboratory.

• (1) GIFT places a donor's ovum in the fallopian tube of a woman who cannot produce ova but can support fertilisation and pregnancy; it does not involve direct sperm injection.
• (2) ZIFT is the transfer of a zygote or early embryo (up to 8 blastomeres) into the fallopian tube, done after normal fertilisation, not by injecting sperm.
• (4) Embryo transfer places an already-formed embryo into the uterus/fallopian tube; it does not describe the fertilisation step itself.

Sperm leave the testis through a chain of ducts: rete testis, then vasa efferentia, then epididymis, then vas deferens. The infundibulum is the funnel-shaped, fimbriated opening of the fallopian tube that collects the ovum after ovulation, so it belongs to the female reproductive system, not the male.

• (1) Rete testis is a network of tubules inside the testis that is part of the male duct system.
• (2) Epididymis is the coiled duct that stores and matures sperm, a male accessory duct.
• (3) Vasa efferentia are the ducts connecting the rete testis to the epididymis, part of the male system.

NCERT's Amazon rain forest figures are: more than 40,000 plant species, about 3,000 fish species, about 1,300 bird species, 427 mammal species, 378 reptile species and more than 1,25,000 invertebrate species. Ranking these five groups in descending order gives invertebrates > plants > fishes > birds > mammals, i.e. (d) > (a) > (c) > (b) > (e).

• (1) Places fishes ahead of plants and invertebrates, which is incorrect; invertebrates (>1,25,000) and plants (>40,000) far outnumber fishes (~3,000).
• (3) Places mammals first, but mammals (427) are the least numerous of the five groups listed.
• (4) Places birds first and invertebrates too low; invertebrates vastly outnumber birds.

Statement I is correct: rapid secretion of LH (the LH surge) at mid-cycle induces rupture of the Graafian follicle, i.e. ovulation. Statement II is incorrect because the remnant of the Graafian follicle forms the corpus luteum, whose main hormone is progesterone, not estrogen (estrogen is secreted mainly by the growing follicle before ovulation).

• (1) Statement II wrongly names estrogen instead of progesterone as the corpus luteum's secretion.
• (2) Statement I correctly describes the LH surge causing ovulation, so it cannot be called incorrect.
• (4) It is Statement I that holds true (the LH surge causes ovulation), not Statement II; this option wrongly swaps which statement is the correct one.

Organisms that eat the producers (plants) directly occupy the second trophic level and are called primary consumers; these are the herbivores.

• (1) Parasites live on/in a host and feed on it; they are not defined by trophic position as primary consumers.
• (2) Predators can occupy any consumer level depending on what they eat; the term itself does not mean primary consumer.
• (4) Carnivores eat other animals (secondary/tertiary consumers), not producers, so they are not primary consumers.

Under Hardy-Weinberg equilibrium, genotype frequencies follow p^2 + 2pq + q^2 = 1, where p is the frequency of allele A. Given p = 0.1, the frequency of the AA genotype = p^2 = (0.1)^2 = 0.01.

• (2) 0.02 would come from an arithmetic slip (e.g. doubling p rather than squaring it).
• (3) 0.10 is simply the allele frequency p itself, not the AA genotype frequency p^2.
• (4) 0.99 does not equal p^2; it is close to a miscalculation of q^2 or of 'not-A' proportions.

Excess growth hormone in adults causes acromegaly (A-III). Luteinizing hormone induces ovulation of the mature Graafian follicle (B-IV). Vasopressin (ADH) promotes reabsorption of water and electrolytes in the kidney (C-I). Oxytocin stimulates uterine contraction during child birth (D-II). This gives A-III, B-IV, C-I, D-II.

• (1) Swaps C and D: vasopressin (not oxytocin) governs water/electrolyte reabsorption, and oxytocin (not vasopressin) causes uterine contraction.
• (3) Assigns acromegaly to oxytocin (D-III) and uterine contraction to growth hormone (A-II), both wrong.
• (4) Assigns ovulation to excess growth hormone (A-IV) and acromegaly to LH (B-III), both wrong.

The opening between the right atrium and right ventricle is guarded by a valve formed of three muscular flaps (cusps), called the tricuspid valve.

• (1) The bicuspid (mitral) valve sits on the left side, between left atrium and left ventricle, so it cannot be the answer for the right side.
• (3) Semilunar valves guard the openings of the ventricles into the pulmonary artery and aorta, not the atrio-ventricular opening.
• (4) The sino-atrial node is the heart's pacemaker tissue in the right atrial wall, not a valve.

Lower invertebrates such as sponges, coelenterates and flatworms exchange O2 and CO2 by simple diffusion over their entire body surface because they lack specialised respiratory organs.

• (2) A moist cuticle is used by earthworms for gas exchange, not sponges.
• (3) Tracheal tubes are the gas-exchange system of insects; sponges lack these.
• (4) Gills are used by aquatic arthropods and molluscs for gas exchange, not by sponges.

A typical angiosperm anther is bilobed, and each lobe has two theca (making it dithecous overall). A longitudinal groove runs lengthwise separating the two theca of a lobe.

• (2) 6 theca per lobe does not correspond to any standard anther description in NCERT.
• (3) 8 theca per lobe is far in excess of the dithecous condition described for a typical anther.
• (4) 12 theca per lobe is not supported by the NCERT description of a bilobed, dithecous anther.

A signal from the CNS travels via a motor neuron to the neuromuscular junction (motor end plate), where the neurotransmitter acetylcholine is released. This generates an action potential in the sarcolemma, initiating muscle contraction.

• (2) Acetyl coenzyme A is a metabolic intermediate (e.g. entering the Krebs cycle), not the neuromuscular transmitter.
• (3) Cyclic GMP is a second messenger in some signalling pathways (e.g. vision), not the neuromuscular junction transmitter.
• (4) Cyclic AMP is a second messenger for many hormones, not the neurotransmitter released at the neuromuscular junction.

The regulatory gene i of the lac operon is constitutively (always) expressed, continuously producing the repressor protein. When lactose (via allolactose) is present, it binds the repressor and inactivates it, so the repressor cannot bind the operator, allowing transcription of the structural genes.

• (2) Lactose inactivates the repressor (prevents it from binding the operator); it does not activate the repressor to bind.
• (3) Gene i has its own separate promoter from the shared promoter of the structural genes z, y and a; they are not all under one common promoter.
• (4) Lactose (via allolactose), not galactose, acts as the inducer of the lac operon.

The two synergid cells at the micropylar end of the embryo sac have a special cellular thickening called the filiform apparatus, which guides the entry of the pollen tube into one of the synergids so the male gametes can be discharged.

• (1) Antipodals are three cells at the chalazal end of the embryo sac with no known role in guiding the pollen tube.
• (3) The central cell contains the two polar nuclei and is where triple fusion occurs, but it does not guide pollen tube entry.
• (4) The polar nucleus/nuclei fuse with a male gamete during triple fusion; they play no role in guiding the pollen tube.

Non-albuminous seeds have no residual endosperm because it is completely used up by the developing embryo; pea is the standard NCERT example. Wheat, maize and barley retain endosperm and are albuminous seeds.

• (1) Wheat is a classic example of an albuminous (endosperm-retaining) seed.
• (2) Maize retains endosperm as the major food-storage tissue and is albuminous.
• (3) Barley retains endosperm as its food reserve at maturity, making it an albuminous seed like wheat and maize, not the non-albuminous example being asked for.

The angiosperm endosperm is triploid (3n), formed by triple fusion of one male gamete (n) with the two polar nuclei (n + n) of the central cell. Given 2n = 36, n = 18, so the endosperm's chromosome number = 3n = 3 x 18 = 54.

• (1) 18 is just n (the haploid number), not the triploid endosperm number.
• (2) 36 is the diploid (2n) number of the parent sporophyte/zygote, not the endosperm, which is triploid.
• (4) 72 would be 4n, which does not correspond to the triploid nature of angiosperm endosperm.

NCERT states the descending limb of Henle's loop is permeable to water but almost impermeable to electrolytes (a-true), and the counter-current arrangement of the loop concentrates the urine (b-true). NCERT also describes active/passive NaCl transport in the ascending limb (d-true). Statement (c), read as a blanket claim that Na+ AND water are reabsorbed together in the loop, does not match NCERT: water leaves only via the descending limb, while the ascending limb is impermeable to water and reabsorbs only Na+/electrolytes, so the two are not co-reabsorbed in the same segment. NCERT therefore supports (a), (b) and (d); we publish the NTA key, option 4, as given.

• (1) Omits (d), which is a correct statement about ascending-limb electrolyte transport.
• (2) Includes (c), which wrongly implies simultaneous Na+ and water reabsorption in the same segment of the loop, and omits (a), a directly quoted NCERT fact.
• (3) Includes (c) for the same reason as above, and omits (d), a valid NCERT-supported statement.

The human vertebral column, from the skull downward, is differentiated into cervical (7), thoracic (12), lumbar (5), sacral (1, fused) and coccygeal (1, fused) regions, in that order, so the correct head-to-toe sequence is cervical, thoracic, lumbar, sacrum.

• (1) Places sacrum before lumbar vertebrae, reversing their actual head-to-toe order.
• (2) Lists the regions in exactly the reverse (toe-to-head) order.
• (3) Places lumbar vertebrae before thoracic vertebrae, which is incorrect; thoracic precedes lumbar.

Heckel's embryological 'biogenetic law' (that embryonic stages recapitulate ancestral adult forms) was proposed as support for evolution but was later disapproved after careful study by Karl Ernst von Baer, so it is not valid evidence for evolution. Fossil records, and divergent evolution producing homologous structures, are genuine, currently accepted lines of evidence.

• (1) Convergent evolution of analogous structures (wings of birds/insects) does illustrate adaptive evolution by natural selection, unlike Heckel's disproved recapitulation idea.
• (2) Fossil records are a standard, currently accepted line of paleontological evidence for evolution.
• (4) Divergent evolution producing homologous structures (e.g. forelimbs) is a core, currently accepted evidence for common ancestry.

NCERT states Homo sapiens arose in Africa, not Australia, and moved across continents, so Statement I is incorrect. Modern Homo sapiens arose during the ice age between 75,000 and 10,000 years ago, so Statement II is correct.

• (1) Statement I wrongly places the origin of Homo sapiens in Australia instead of Africa.
• (2) Statement II is factually correct per NCERT's stated time range, so it cannot be called incorrect.
• (3) It is Statement I that is wrong -- Homo sapiens arose in Africa, not Australia; this option mistakenly flags the correct time-range statement (Statement II) instead.

Population growth rate r = (b - d) = 0.002 - 0.002 = 0. Since dN/dt = rN = 0, the population size does not change over time regardless of the number of generations, so after 10 generations the population remains 10 million cells.

• (1) 1 million would imply the population declined to a tenth, which contradicts r = 0 (no net change).
• (2) 5 million would imply the population halved, again inconsistent with a zero net growth rate.
• (4) 100 million would imply tenfold growth, but with birth rate equal to death rate there is no net growth.

PCR has three steps -- denaturation (strand separation by heat), annealing (primers bind/anneal to the single-stranded template) and extension (Taq polymerase extends the primers). Primer binding to the DNA strands occurs specifically in the annealing step.

• (1) Denaturation is the heating step that separates the double-stranded DNA into single strands; primers are not bound yet at this stage.
• (2) Extension is when the polymerase synthesises new DNA from the already-bound primers, not when primers first bind.
• (4) Ligation is a separate step (used e.g. when joining DNA fragments/vectors), not part of the three-step PCR cycle.

Assertion A is correct -- the logistic model, which incorporates a carrying capacity, is considered more realistic than the unrestricted exponential model. Reason R is also correct and is exactly why: since resources for growth are finite and become limiting, growth cannot continue exponentially forever, which is precisely the assumption built into the logistic model.

• (2) R directly explains why the logistic model is more realistic (finite resources cap growth), so it is the correct explanation, not an unrelated true fact.
• (3) Reason R (finite resources) is a true, NCERT-stated premise, not an incorrect one.
• (4) Assertion A is also correct, not false; the logistic model genuinely is considered more realistic.

Australian marsupials underwent adaptive radiation in isolation to fill ecological niches similar to those filled by placental mammals elsewhere. Because this independent, parallel radiation in geographically isolated areas produced superficially similar (but not closely related) forms, it is termed convergent evolution.

• (1) Divergent evolution describes one ancestral form giving rise to different structures/species adapting to different needs (e.g. Darwin's finches), not two separate lineages converging on similar forms.
• (3) Founder effect is genetic drift arising when a small group starts a new, isolated population; it does not describe the parallel adaptive radiation of marsupials and placental mammals.
• (4) Genetic drift is a chance-driven change in allele frequency in small populations, unrelated to this large-scale radiation comparison.

Lymphocytes are made and mature in bone marrow and the thymus, which is why these two count as primary lymphoid organs. Tonsils and spleen are secondary lymphoid organs, where mature lymphocytes interact with antigen and proliferate into effector cells, so the correct pair is (b) and (c).

• (1) Bone marrow (a) is a primary, not secondary, lymphoid organ.
• (3) Thymus is where T-lymphocytes mature, making it a primary lymphoid organ; students confuse it with secondary organs since both terms relate to lymphocyte development.
• (4) Bone marrow (a) and thymus (d) are where lymphocytes form and mature, i.e. primary, not secondary, lymphoid organs; neither belongs in the secondary category.

During pregnancy the placenta also works as a hormone-producing organ, releasing hCG, human placental lactogen (hPL), estrogens and progestogens. Luteinizing hormone (LH), by contrast, is secreted by the anterior pituitary, not the placenta.

• (1) hCG (human chorionic gonadotropin) is a classic placental hormone, used as the basis of pregnancy tests.
• (2) Estrogen is secreted by the placenta during pregnancy, among other sources.
• (3) Progesterone is secreted by the placenta (as progestogens) to maintain pregnancy.

In eukaryotes, RNA polymerase II transcribes the precursor of mRNA, the heterogeneous nuclear RNA (hnRNA), which is subsequently processed into mature mRNA.

• (1) RNA polymerase I transcribes rRNA (28S, 18S and 5.8S), not precursor mRNA.
• (3) RNA polymerase III transcribes tRNA, 5S rRNA and other small nuclear RNAs, not precursor mRNA.
• (4) DNA polymerase synthesises DNA during replication; it does not transcribe RNA at all.

Plasmids are circular, extrachromosomal DNA molecules found in bacterial cytoplasm that can replicate independently of the bacterial chromosome (autonomously replicating). Both statements correctly describe plasmids, so both are correct.

• (2) Both statements are accurate descriptions of plasmids, so they cannot both be incorrect.
• (3) Statement II is also correct (plasmids are indeed extrachromosomal), not incorrect.
• (4) Statement I is also correct (plasmids do replicate autonomously), not incorrect.

An 'O' blood group child (genotype ii) needs an 'i' allele from each parent. A father with genotype IAIB has no 'i' allele at all to pass on, so this parental combination can never produce an 'O' (ii) child, unlike the other three combinations, each of which has an 'i' allele available from both parents.

• (1) IAi (father) x IBi (mother) can each contribute an i allele, giving an ii ('O') child -- this combination is possible.
• (2) IAi x IAi can both contribute an i allele, giving an ii child -- possible.
• (3) IBi x IBi can both contribute an i allele, giving an ii child -- possible.

Human and bat forelimbs share the same basic bone arrangement (humerus, radius, ulna, carpals, metacarpals, phalanges) despite performing different functions, which is exactly what makes them homologous -- structures with a common origin/basic plan that diverged for different needs. So A is true, R is true, and R is the correct explanation of A.

• (2) R is not merely a coincidental true fact -- shared anatomical structure is precisely the criterion that defines homology, so it does explain A.
• (3) Reason R is also true, not false; the similar bone arrangement is well documented.
• (4) Assertion A is also true; forelimb homology between humans and bats is a standard NCERT example of divergent evolution.

Colostrum, the milk secreted during the initial days of lactation, is rich in IgA antibodies, which provide passive immunity to the newborn.

• (1) IgG is the antibody that crosses the placenta to give the fetus passive immunity before birth, not the antibody characteristic of colostrum.
• (2) IgM is typically the first antibody produced in a primary immune response, not the one highlighted as abundant in colostrum.
• (4) IgD functions mainly as a B-cell surface receptor and is not associated with colostrum.

NCERT gives this exact case as an example of interspecific competition: the Abingdon tortoise in the Galapagos Islands went extinct within a decade of goats being introduced, and this was apparently because the goats were more efficient browsers/competitors for the same food resource. So A is true, R is true, and R is the correct explanation of A.

• (2) R is not a separate, unrelated true fact -- NCERT explicitly attributes the tortoise's extinction to the goats' greater browsing efficiency, so R does explain A.
• (3) Reason R is also correct, as directly stated in NCERT, not incorrect.
• (4) Assertion A is also correct -- the extinction of the Abingdon tortoise following goat introduction is the NCERT example itself.

After the Graafian follicle ruptures at ovulation, the released secondary oocyte (ovum) is surrounded by a membrane called the zona pellucida.

• (1) Endometrium is the innermost lining layer of the uterine wall, not a covering of the ovum.
• (2) Zona radiata is not the term used by NCERT for the ovum's covering at ovulation; the text specifically names the zona pellucida.
• (4) Chorion is the outermost extraembryonic membrane surrounding the developing embryo, formed later, not the ovum's covering at ovulation.

NCERT defines: both species harmed = competition (A-III); one harmed, other benefited = predation (B-I); both species benefited = mutualism (C-II); one benefited, other unaffected = commensalism (D-IV). This gives A-III, B-I, C-II, D-IV.

• (1) Pairs B with commensalism and D with predation, both mismatched relative to the NCERT definitions.
• (2) Pairs A with predation and B with mutualism, which reverses the harm/benefit pattern each interaction actually involves.
• (3) Pairs A with mutualism (both benefited) when A actually describes both species being harmed (competition).

Mendel crossed pure-breeding tall and dwarf pea plants and found all F1 progeny were tall, none dwarf (A is correct). This happens precisely because tall is the dominant allele and dwarf the recessive one for this contrasting trait, so the dominant trait alone is expressed in F1 (R correctly explains A).

• (2) R is exactly the mechanism (dominance) that produces the F1 result described in A, so it is the correct explanation, not an unrelated fact.
• (3) Reason R correctly states the dominant/recessive relationship for stem height as established by Mendel's experiments.
• (4) Assertion A correctly reports Mendel's observed F1 result (all tall).

Since DNA is enclosed within cell membranes and associated with other macromolecules, cells must be broken open to release and isolate the DNA (R). This is done using enzymes such as lysozyme for bacterial cells and cellulase for plant cells (A), so R correctly explains why A's enzymatic treatments are needed.

• (2) R is not a coincidental fact -- it is precisely why enzymes like lysozyme and cellulase are used, so it does explain A.
• (3) Reason R is a true, NCERT-stated premise about needing to disrupt cells to isolate genetic material.
• (4) Assertion A correctly matches lysozyme to bacteria and cellulase to plant cells as per NCERT.

Morphine, derived from the opium poppy, is a very effective sedative and painkiller and is widely used for patients who have undergone surgery.

• (1) Interferons are cytokines that act as part of the innate immune barrier against viral infection, not surgical painkillers.
• (2) Antibiotics restrict bacterial growth/infection; they have no sedative or analgesic action.
• (4) Anti-retroviral drugs are used against retroviral infections such as HIV, not as sedatives or painkillers.

Energy flows only one way, from producers to consumers/decomposers (a-true). The pyramid of energy is always upright and can never be inverted, since energy is lost as heat at every transfer (b-true). However, energy transfer between trophic levels follows the 10 per cent law, not a 1% law, so (c) is false.

• (1) Includes (c), which misstates the 10 per cent law as a '1% law'.
• (3) Includes (c), the incorrect '1% law' claim, though (a) is correctly included.
• (4) Omits (a), which is a correct NCERT statement, and includes the incorrect (c).

Plasmodium reproduces asexually in liver cells and then in RBCs; the sexual stages (gametocytes) form in RBCs and are taken up by the female Anopheles mosquito during a blood meal, and fertilisation (fusion of gametes) then takes place in the mosquito's gut.

• (1) Plasmodium reproduces asexually, not sexually, in liver cells.
• (2) Plasmodium reproduces asexually in RBCs, causing their rupture; sexual fusion happens later in the mosquito.
• (3) Gametocytes develop in human RBCs, not in the mosquito gut; it is fertilization that occurs in the mosquito gut after the gametocytes are taken up.

Transformation is the process of transferring DNA (a vector/plasmid) into a host bacterium (A-II). The cloning site is where restriction enzymes cut the vector to insert foreign DNA (B-I). Selection of transformed cells (recombinants) is done using antibiotic resistance markers (C-IV). Ori (origin of replication) is the sequence where the host's machinery begins replicating the plasmid (D-III). This gives A-II, B-I, C-IV, D-III.

• (2) Swaps A and B: transformation is about DNA transfer into the host, not about restriction-enzyme recognition, which belongs to the cloning site.
• (3) Assigns replication to transformation (A-III) and antibiotic selection to cloning site (B-IV), both mismatched.
• (4) Assigns antibiotic function to transformation (A-IV) and restriction enzyme to Ori (B-I is actually correct here but C and D are swapped incorrectly relative to the rest of the mapping), which does not match the NCERT roles overall.