AIDS and cancer are high-frequency NEET topics where terminology traps cost marks. The common trap: confusing which test screens for HIV versus which confirms it — and mixing up what CD4+ T cell count actually tells a clinician.
AIDS — the biology. HIV (Human Immunodeficiency Virus) is a retrovirus that selectively destroys helper T lymphocytes (CD4+ T cells). Transmission occurs through sexual contact, contaminated blood/needles, and from infected mother to child (transplacental or through breast milk). HIV is NOT transmitted by touch, mosquito bites, or sharing food (NCERT Class 12 Biology Chapter 7, page 138).
The diagnostic sequence matters. ELISA (Enzyme-Linked Immunosorbent Assay) is the screening test — it is cheap, sensitive, and used first. A positive ELISA alone does not confirm HIV. Western blot is the confirmatory test. CD4+ T cell count is neither screening nor confirmatory — it monitors disease progression. When CD4+ count drops below 200 cells/µL, the condition is classified as AIDS.
Cancer — key facts. Cancer results from uncontrolled cell division caused by activation of oncogenes or inactivation of tumour suppressor genes. Benign tumours remain localised; malignant tumours (cancers) show metastasis — spread to distant sites via blood and lymph. Carcinogens include physical agents (UV, X-rays), chemical agents (tobacco smoke compounds), and biological agents (oncogenic viruses).
Prevention and detection. Cancer detection methods include biopsy, radiography, CT, MRI, and molecular markers. AIDS prevention centres on safe practices, screening of blood before transfusion, use of disposable needles, and public awareness — no vaccine is available for HIV as of the NCERT syllabus.
Watch out: NEET questions on AIDS frequently test the ELISA-vs-Western-blot-vs-CD4 distinction. Read the stem carefully for whether it asks about screening, confirmation, or monitoring — each maps to a different answer.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which of the following is the causative agent of AIDS?
Answer: B. HIV (Human Immunodeficiency Virus) is the retrovirus that causes AIDS. NCERT Class 12 Biology Chapter 7, page 141 explicitly identifies HIV as the causative organism.
Why A is wrong: A is wrong because HTLV (Human T-cell Lymphotropic Virus) causes adult T-cell leukaemia, not AIDS. The two retroviruses are distinct despite both targeting T cells.
Why C is wrong: C is wrong because Hepatitis B virus causes hepatitis B (liver inflammation), not AIDS. Both are blood-borne, which is why students sometimes confuse them.
Why D is wrong: D is wrong because Mycobacterium tuberculosis causes tuberculosis. TB is an opportunistic infection in AIDS patients, but the bacterium does not cause AIDS itself.
Which of the following is NOT a route of HIV transmission?
Answer: B. HIV is NOT transmitted through mosquito bites. NCERT Class 12 Biology Chapter 7, page 141 states HIV spreads through sexual contact, blood transfusion, contaminated needles, and mother-to-child routes — not through insect vectors.
Why A is wrong: A is wrong because sexual contact IS a documented route of HIV transmission — it is in fact the most common mode globally.
Why C is wrong: C is wrong because sharing contaminated needles IS a well-established route of HIV transmission, particularly among intravenous drug users.
Why D is wrong: D is wrong because transplacental transfer IS a recognised route — HIV can cross the placenta from infected mother to foetus during pregnancy.
Which type of cells does HIV primarily destroy in the human body?
Answer: C. HIV selectively infects and destroys helper T lymphocytes bearing the CD4 receptor. The progressive loss of CD4+ T cells cripples the immune system, leading to AIDS. NCERT Class 12 Biology Chapter 7, page 140.
Why A is wrong: A is wrong because B lymphocytes are not the primary target of HIV. B cells produce antibodies but lack the CD4 receptor that HIV uses for entry.
Why B is wrong: B is wrong because CD8+ cytotoxic T cells are not HIV's primary target. They actually attempt to kill HIV-infected cells as part of the immune response.
Why D is wrong: D is wrong because natural killer cells are innate immune cells and are not the primary target of HIV. HIV specifically uses the CD4 surface protein for attachment and entry.
A patient tests positive on ELISA for HIV. The physician orders a second test before confirming HIV status. Which test is most appropriate as the confirmatory step?
Answer: D. Western blot is the standard confirmatory test after a positive ELISA screen. ELISA is sensitive but can give false positives; Western blot provides specificity. NCERT Class 12 Biology Chapter 7 distinguishes screening from confirmation (trap: ELISA-vs-Western-blot-vs-CD4 confusion).
Why A is wrong: A is wrong because repeating the same screening test does not serve as confirmation — ELISA is a screening tool regardless of which lab runs it. Confirmation requires a different, more specific assay.
Why B is wrong: B is wrong because PCR for bacterial DNA is irrelevant — HIV is a virus, not a bacterium. While PCR for viral RNA/DNA can detect HIV, the standard confirmatory protocol after ELISA is Western blot.
Why C is wrong: C is wrong because CD4+ T cell count monitors disease progression (how advanced the immunodeficiency is), not whether HIV infection is present. A person can be HIV-positive with a normal CD4+ count early in infection (trap: CD4 count as diagnostic vs monitoring marker).
An HIV-positive patient has a CD4+ T cell count of 180 cells/µL. What does this value indicate about the patient's condition?
Answer: A. A CD4+ T cell count below 200 cells/µL is the threshold for classifying HIV infection as AIDS. At 180 cells/µL, the patient has crossed into the AIDS stage with severe immunodeficiency. NCERT Class 12 Biology Chapter 7, page 140.
Why B is wrong: B is wrong because CD4+ count does not validate or invalidate ELISA results — it is a monitoring marker for disease progression, not a diagnostic test. A low CD4+ count in an HIV-positive patient confirms worsening disease, not a testing error.
Why C is wrong: C is wrong because the asymptomatic phase of HIV typically has CD4+ counts well above 200 cells/µL. A count of 180 indicates severe depletion, not early-stage infection.
Why D is wrong: D is wrong because a CD4+ count of 180 indicates immune destruction, the opposite of natural immunity. Natural immunity would maintain or restore normal CD4+ counts (500–1500 cells/µL).
Which of the following correctly distinguishes a benign tumour from a malignant tumour?
Answer: C. Benign tumours remain confined to their site of origin and do not invade surrounding tissues. Malignant tumours (cancers) exhibit metastasis — they detach, travel through blood and lymph, and establish secondary growths at distant sites. NCERT Class 12 Biology Chapter 7.
Why A is wrong: A is wrong because it reverses the key distinction. Benign tumours do NOT metastasise — that is the defining feature of malignant tumours. This is a direct reversal trap common in NEET questions.
Why B is wrong: B is wrong because metastasis is exclusive to malignant tumours. If a tumour shows metastasis, it is by definition malignant, not benign.
Why D is wrong: D is wrong because the benign/malignant classification is based on behaviour (localised vs metastatic), not on the type of carcinogen. Both viruses and chemicals can cause malignant tumours.
A researcher discovers that a particular gene, when activated, causes uncontrolled cell proliferation. This gene is best classified as:
Answer: A. A gene whose activation leads to uncontrolled cell division is an oncogene. Oncogenes are mutated or overexpressed forms of normal proto-oncogenes. Tumour suppressor genes, by contrast, inhibit cell division — their inactivation (not activation) contributes to cancer. NCERT Class 12 Biology Chapter 7.
Why B is wrong: B is wrong because tumour suppressor genes INHIBIT cell division. Their inactivation (loss of function), not activation, leads to cancer. The question describes activation causing proliferation, which is the hallmark of an oncogene.
Why C is wrong: C is wrong because structural genes code for structural proteins (like collagen or keratin) and are not specifically associated with cell proliferation control.
Why D is wrong: D is wrong because a regulatory gene for apoptosis would control programmed cell death. While loss of apoptosis regulation can contribute to cancer, the question specifically describes activation causing proliferation — the oncogene definition.
A NEET question states: "An individual is diagnosed HIV-positive. Over the next several years, which sequence of events correctly describes disease progression?"
Answer: D. The correct progression is: initial detection (ELISA screening, confirmed by Western blot) → progressive destruction of CD4+ T cells → when CD4+ drops below 200 cells/µL, the person is classified as having AIDS → the severely immunocompromised individual becomes vulnerable to opportunistic infections. NCERT Class 12 Biology Chapter 7, page 140.
Why A is wrong: A is wrong because CD4+ count does not rise during HIV progression — it progressively falls. Recovery is not the natural course of untreated HIV infection. This option reverses the pathological trajectory.
Why B is wrong: B is wrong because the sequence is inverted. ELISA detects HIV antibodies regardless of CD4+ count, and Western blot does not become negative during disease progression — it confirms the presence of anti-HIV antibodies which persist.
Why C is wrong: C is wrong because a positive Western blot confirms HIV infection but does not mean immediate AIDS. AIDS is defined by CD4+ count falling below 200/µL, which takes years in the natural course. The sequence conflates diagnosis with disease stage (trap: confusing HIV-positive status with AIDS).
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Given
• ELISA result: positive• CD4+ T cell count: 450 cells/µL
Required
Determine the correct interpretation of these two results together.
Concept
ELISA is a screening test for HIV antibodies. CD4+ T cell count monitors immune status. AIDS is classified when CD4+ drops below 200 cells/µL. A positive ELISA requires Western blot confirmation before HIV-positive status is established.
Formula/Rule
• ELISA positive → proceed to Western blot for confirmation• CD4+ < 200 cells/µL → AIDS classification• CD4+ ≥ 200 cells/µL in an HIV-positive individual → HIV-positive but not yet AIDS
Substitution
CD4+ count = 450 cells/µL → 450 > 200 → does NOT meet AIDS threshold.
Calculation
This is a logical reasoning problem, not an arithmetic one. The two data points are evaluated against clinical thresholds:• Positive ELISA → HIV exposure likely (pending confirmation)• CD4+ = 450 → above the 200 threshold → not AIDS
Final answer
(B) The person is HIV-positive but has not progressed to AIDS.
(Strictly, the person requires Western blot confirmation of HIV status, but among the given options, B is the most accurate — the CD4+ count of 450 rules out AIDS.)
Common trap
Selecting option (A) by assuming a positive ELISA + any CD4+ reading = AIDS. The threshold is specifically <200 cells/µL. At 450, the immune system is impaired but has not collapsed to AIDS levels. Another trap: selecting (C) by assuming ELISA is unreliable — ELISA is highly sensitive; while false positives exist, a positive result is not "definitely" false.
Similar NEET-style question
"A patient shows positive Western blot and CD4+ count of 150 cells/µL. The patient is most likely suffering from: (A) Hepatitis B, (B) Tuberculosis, (C) AIDS, (D) Typhoid." Answer: (C) — Western blot confirms HIV, and CD4+ < 200 = AIDS.
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AIDS: HIV (retrovirus); attacks helper T (CD4) cells; transmission — sexual, blood, mother-to-child. Diagnosis: ELISA / Western blot. Cancer: uncontrolled cell division; benign vs malignant; oncogenes; metastasis. Hallmarks: contact inhibition lost.
-- NCERT Class 12 Biology, Ch. 7, p. 141These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
ELISA = screening (cheap, sensitive); Western blot = confirmation. CD4+ T cell count tracks disease progression.
Question on which test confirms HIV vs which monitors progression.
ELISA first (screening); Western blot confirms; CD4+ count = monitoring marker (<200/µL = AIDS).
More in Biology and Human Welfare: 3 exam traps and mistakes · 1 question pattern from its other lessons.
3 questions from NEET 2023, 2025. Answers verified against NTA official keys.
In which blood corpuscles, the HIV undergoes replication and produces progeny viruses?
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