Circulation Blood Groups Coagulation

8 MCQs9-step worked example
Source: NCERT Human PhysiologyPYQ coverage: NEET 2021, 2022, 2026Official key: NTA-verifiedLast updated: 25 Sep 2026

Circulation Blood Groups Coagulation, explained for NEET

A high-frequency trap here is swapping which blood group is the universal donor and which is the universal recipient. Group O has neither the A nor the B antigen on its RBCs, so no recipient's antibody finds anything to clump with — that makes O the universal donor. Group AB carries both antigens but has neither anti-A nor anti-B antibody in its plasma, so it can accept blood from any group without attacking the incoming RBCs — that makes AB the universal recipient (NCERT Class 11 Biology, Chapter 15, page 195).

The Rh system adds a second axis. About 80% of people carry the Rh antigen (Rh+ve); the rest are Rh-ve. An Rh-ve person exposed to Rh+ve blood forms anti-Rh antibodies. The exam-relevant case is an Rh-ve mother carrying an Rh+ve foetus: the placenta keeps the two bloods separated through the first pregnancy, so the mother is not sensitised until small amounts of foetal blood cross during delivery. It is therefore a subsequent pregnancy, not the first, that is at risk — her now-primed anti-Rh antibodies cross into the Rh+ve foetal blood and destroy foetal RBCs, causing erythroblastosis foetalis. Giving the mother anti-Rh antibodies right after the first delivery prevents this sensitisation (Chapter 15, page 196).

Coagulation follows a fixed direction that a rushed reading reverses. An injury triggers a cascade of plasma factors that forms the enzyme complex thrombokinase. Thrombokinase converts the inactive plasma factor prothrombin into active thrombin. Thrombin then converts soluble fibrinogen into fibrin threads, which trap blood cells into the clot. Calcium ions are required at multiple steps of this cascade (Chapter 15, page 196).

Watch out: thrombokinase acts on prothrombin, not on fibrinogen directly — a question asking "which enzyme converts fibrinogen to fibrin" wants thrombin, not thrombokinase.

Can you answer these Circulation Blood Groups Coagulation MCQs?

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

Group O individuals are called "universal donors" because their RBCs carry:

Show answer and why every option is right or wrong

Answer: D. O group RBCs carry neither the A nor the B antigen, so no recipient's anti-A or anti-B antibody finds an antigen to clump with, which is why O is the universal donor (NCERT Class 11 Biology, Chapter 15, page 195).

Why A is wrong: A is wrong because carrying both A and B antigens describes group AB's RBCs, the profile that makes a donor unsafe to A and B recipients, not the universal-donor profile.

Why B is wrong: B is wrong because anti-A and anti-B antibodies sit in group O's own plasma, not on its RBCs; donor safety is decided by the donor's RBC antigens meeting the recipient's antibodies, not the donor's own antibodies.

Why C is wrong: C is wrong because the Rh antigen belongs to a separate grouping system from ABO; it plays no part in why O is called the universal donor in the ABO system.

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MCQ 2Easy RecallPractice

The enzyme complex required to convert inactive prothrombin to active thrombin during blood clotting is called:

Show answer and why every option is right or wrong

Answer: A. Thrombokinase is the enzyme complex, itself formed through a cascade of plasma factors triggered at the injury site, that converts prothrombin into thrombin (NCERT Class 11 Biology, Chapter 15, page 196).

Why B is wrong: B is wrong because fibrinogen is the inactive plasma protein thrombin acts on later in the cascade, not the enzyme that activates prothrombin.

Why C is wrong: C is wrong because plasmin is not part of this NCERT account of coagulation; it belongs to clot breakdown, not the prothrombin-to-thrombin step described here.

Why D is wrong: D is wrong because heparin is described elsewhere in this chapter as a basophil secretion involved in inflammation, not the enzyme complex that activates prothrombin.

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MCQ 3Easy RecallPractice

Which ion does NCERT state plays a very important role in blood coagulation?

Show answer and why every option is right or wrong

Answer: B. NCERT states that calcium ions play a very important role in clotting (Chapter 15, page 196).

Why A is wrong: A is wrong because sodium is listed as a plasma mineral in the chapter's plasma-composition section, but it is not the ion singled out for a role in coagulation.

Why C is wrong: C is wrong because potassium is not named anywhere in this chapter's account of plasma composition or coagulation.

Why D is wrong: D is wrong because magnesium is listed among plasma minerals, but coagulation's specifically named ion is calcium, not magnesium.

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MCQ 4Direct ApplicationPractice

A person with blood group A needs a transfusion. Based on the ABO compatibility table, which donor group(s) can safely be used?

Show answer and why every option is right or wrong

Answer: C. Group A plasma carries the anti-B antibody, so it accepts RBCs carrying no B antigen: group A itself and group O, which carries neither antigen (Chapter 15, page 195, Table 15.1).

Why A is wrong: A is wrong because group B RBCs carry the B antigen, which group A's anti-B antibody would clump; B is a safe donor to B and AB, not to A.

Why B is wrong: B is wrong because AB RBCs carry the B antigen, which group A's anti-B antibody would attack; AB is not on A's safe-donor list.

Why D is wrong: D is wrong because ABO compatibility must be matched just as carefully as Rh; the table exists precisely because not every group can safely donate to every recipient.

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MCQ 5Direct ApplicationPractice

An Rh-ve woman is pregnant with her first Rh+ve child. Based on the placental barrier NCERT describes, what is the risk of erythroblastosis foetalis in this first pregnancy?

Show answer and why every option is right or wrong

Answer: D. The placenta keeps foetal and maternal blood separated through the first pregnancy, so the mother is not sensitised until foetal blood crosses during delivery — the first pregnancy itself carries low risk (Chapter 15, page 196).

Why A is wrong: A is wrong because NCERT describes the placenta as keeping the two circulations separated during pregnancy, with exposure occurring at delivery, not as a continuous crossing.

Why B is wrong: B is wrong because a person is not born sensitised to Rh; sensitisation requires prior exposure to Rh+ve blood, which has not yet happened before a first pregnancy.

Why C is wrong: C is wrong because Rh incompatibility is defined for an Rh-ve mother carrying an Rh+ve foetus, not the reverse — the risk in a first pregnancy is low, not the framing this option gives.

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MCQ 6Direct ApplicationPractice

Arrange the correct sequence of events in blood coagulation after an injury.

Show answer and why every option is right or wrong

Answer: A. An injury triggers a cascade of plasma factors that first forms thrombokinase; thrombokinase then converts prothrombin to thrombin, and thrombin finally converts fibrinogen to fibrin threads (Chapter 15, page 196).

Why B is wrong: B is wrong because it has thrombokinase forming after thrombin already exists, but thrombokinase is the enzyme complex needed to make thrombin in the first place — it must come before, not after.

Why C is wrong: C is wrong because it runs the fibrinogen-to-fibrin step before thrombin even exists, and has prothrombin activating thrombokinase rather than the reverse.

Why D is wrong: D is wrong because it starts from thrombin and has thrombin activate thrombokinase — the scrambled reversal this trap targets — when thrombokinase must act on prothrombin first to produce thrombin.

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MCQ 7Concept TrapPractice

A student says: "An Rh-ve mother's very first pregnancy with an Rh+ve foetus is destroyed by Rh incompatibility." What is incorrect about this claim?

Show answer and why every option is right or wrong

Answer: C. The placenta separates the two bloods during the first pregnancy; the mother is sensitised only when foetal blood crosses during delivery, so it is a subsequent pregnancy that is at risk, not the first (Chapter 15, page 196).

Why A is wrong: A is wrong because NCERT places the first exposure at delivery, so the first pregnancy itself is not what destroys the foetus — the claim as stated is not correct.

Why B is wrong: B is wrong because erythroblastosis foetalis is exactly the condition NCERT describes for Rh incompatibility; the claim's error is about timing, not about whether the condition exists.

Why D is wrong: D is wrong because the at-risk mother is Rh-ve (not Rh+ve) carrying an Rh+ve foetus; this option names the wrong parent as being at risk.

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MCQ 8CalculationPractice

A patient with blood group AB, Rh-ve needs an emergency transfusion, and only AB, Rh+ve blood is available. Based on ABO and Rh matching rules, is this transfusion safe?

Show answer and why every option is right or wrong

Answer: B. AB matches on the ABO system since the patient's plasma has no anti-A or anti-B antibody, but Rh is a separate matching requirement, and NCERT states Rh should also be matched before transfusions — an Rh-ve recipient can be sensitised by Rh+ve donor RBCs (Chapter 15, page 196).

Why A is wrong: A is wrong because 'universal recipient' describes only the ABO antigen–antibody logic (AB has no anti-A or anti-B); it does not extend to the separate Rh system, which must still be matched on its own.

Why C is wrong: C is wrong because AB plasma carries neither anti-A nor anti-B antibody, so ABO matching is not the problem here — the mismatch is on the Rh axis.

Why D is wrong: D is wrong because NCERT states Rh should also be matched before transfusions, not only in pregnancy; ignoring Rh in a transfusion risks the same kind of sensitisation.

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How do you solve a Circulation Blood Groups Coagulation question? A worked example

  1. 1

    Given

    An Rh-ve woman has already had one Rh+ve child, delivered normally, with no anti-Rh antibody injection given after that first delivery. She is now pregnant a second time with another Rh+ve foetus.

  2. 2

    Required

    Whether this second pregnancy is at risk of erythroblastosis foetalis, and why.

  3. 3

    Concept

    Rh incompatibility arises when an Rh-ve person's blood is exposed to the Rh antigen and forms anti-Rh antibodies. The placenta keeps maternal and foetal blood separated during pregnancy, so exposure normally happens only at delivery (Chapter 15, page 196).

  4. 4

    Formula

    Not applicable — this is a qualitative immunological mechanism, not a calculation; no formula from this topic is used.

  5. 5

    Substitution

    Apply the mechanism to this patient's history: during the first pregnancy the placenta kept the bloods separated, so no exposure occurred; at the first delivery, a small amount of the Rh+ve foetal blood leaked into the mother's circulation, and because no anti-Rh antibody injection was given afterward, her own immune system had time to build anti-Rh antibodies.

  6. 6

    Calculation

    Not a numeric calculation. Trace the exposure chain instead: first pregnancy (no exposure, placenta intact) → first delivery (foetal-to-maternal leak, mother sensitised) → second pregnancy (the mother's pre-formed anti-Rh antibodies cross the placenta and attack the Rh+ve foetal RBCs).

  7. 7

    Final answer

    Yes, this second pregnancy is at risk of erythroblastosis foetalis, because the mother was sensitised during the first delivery and her anti-Rh antibodies can now cross into the Rh+ve foetal blood and destroy foetal RBCs (Chapter 15, page 196).

  8. 8

    Common trap

    Students place the risk in the first pregnancy itself. NCERT's account places the sensitising exposure at the first delivery, so the danger appears from the second pregnancy onward — and only if no preventive anti-Rh antibody was given after the first delivery.

  9. 9

    Similar NEET-style question

    An Rh-ve mother is given anti-Rh antibodies immediately after the delivery of her first Rh+ve child. Is her next Rh+ve pregnancy still at the same risk of erythroblastosis foetalis? (Answer: No — the injected anti-Rh antibodies neutralise the Rh+ve foetal cells that leaked into the mother's blood during the first delivery before her own immune system can respond and build its own anti-Rh antibodies. Without that self-sensitisation, there is no pre-formed maternal antibody to cross the placenta in the next pregnancy, so the risk is much lower than in the unprotected case described in Step 7.)

What to remember before solving Circulation Blood Groups Coagulation questions

Key Fact

ABO Grouping

ABO grouping is based on the presence or absence of two surface antigens (chemicals that can induce immune response) on RBCs, namely A and B. The plasma of different individuals contains two natural antibodies (proteins produced in response to antigens). The distribution of antigens and antibodies in the four blood groups, A, B, AB and O, determines compatibility: group A has antigen A, antibody anti-B, and can donate to/receive from A and O; group B has antigen B, antibody anti-A, and works with B and O; group AB has both antigens A and B, no antibodies, and can accept from AB, A, B and O; group O has no antigens, antibodies anti-A and anti-B, and can donate to any group. During blood transfusion the donor's blood must be carefully matched with the recipient's to avoid clumping (destruction of RBC). Group O individuals are called universal donors; group AB individuals are called universal recipients.

-- NCERT Class 11 Biology, Chapter 15, p. 195
Key Fact

Rh Grouping

Another antigen, the Rh antigen, similar to one present in Rhesus monkeys (hence Rh), is observed on the surface of RBCs of the majority (nearly 80 per cent) of humans. Such individuals are called Rh positive (Rh+ve), and those in whom this antigen is absent are called Rh negative (Rh-ve). An Rh-ve person, if exposed to Rh+ve blood, will form specific antibodies against the Rh antigens; therefore, Rh group should also be matched before transfusions. A special case of Rh incompatibility has been observed between the Rh-ve blood of a pregnant mother and the Rh+ve blood of the foetus. Rh antigens of the foetus do not get exposed to the Rh-ve blood of the mother in the first pregnancy, as the two bloods are well separated by the placenta.

-- NCERT Class 11 Biology, Chapter 15, p. 196
Key Fact

Erythroblastosis Foetalis

During delivery of the first child, there is a possibility of exposure of the maternal blood to small amounts of Rh+ve blood from the foetus. In such cases, the mother starts preparing antibodies against the Rh antigen in her blood. In subsequent pregnancies, the Rh antibodies from the Rh-ve mother can leak into the blood of the Rh+ve foetus and destroy the foetal RBCs. This could be fatal to the foetus or could cause severe anaemia and jaundice to the baby; this condition is called erythroblastosis foetalis. It can be avoided by administering anti-Rh antibodies to the mother immediately after delivery of the first child.

-- NCERT Class 11 Biology, Chapter 15, p. 196
Key Fact

Coagulation of Blood

Blood exhibits coagulation or clotting in response to injury or trauma; this is a mechanism to prevent excessive loss of blood from the body. A dark reddish brown scum forms at the site of a cut over time: a clot or coagulam formed mainly of a network of threads called fibrins, in which dead and damaged formed elements of blood are trapped. Fibrins are formed by the conversion of inactive fibrinogens in the plasma by the enzyme thrombin. Thrombin, in turn, is formed from another inactive substance present in the plasma called prothrombin. An enzyme complex, thrombokinase, is required for this reaction, formed by a series of linked enzymic reactions (cascade process) involving factors present in the plasma in an inactive state. Injury stimulates platelets to release factors that activate coagulation; tissue factors at the injury site can also initiate coagulation. Calcium ions play a very important role in clotting.

-- NCERT Class 11 Biology, Chapter 15, p. 196

Where do students lose marks on Circulation Blood Groups Coagulation?

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

Universal donor vs universal recipient swap

Category: Similar Terms

Students swap which blood group is the universal donor (O) and which is the universal recipient (AB).

When it triggers

A question asks 'blood group O individuals are called ___' with 'universal recipients' as a distractor.

How to avoid

Group O has no A/B antigens on RBCs so it can be donated to any group (universal donor); group AB has no antibodies in plasma so it can accept from any group (universal recipient) (C11_BIO_18_FACT_11).

Trap

Coagulation cascade step order

Category: Similar Terms

Students reverse the order fibrinogen/prothrombin/thrombin/fibrin act in the clotting cascade, or forget thrombokinase is the enzyme complex, not thrombin itself.

When it triggers

A question asks 'thrombin converts ___ to ___' and offers 'prothrombin to fibrinogen' as a scrambled distractor.

How to avoid

Prothrombin is converted (via thrombokinase) to thrombin, and thrombin then converts inactive fibrinogen to fibrin threads that form the clot (C11_BIO_18_FACT_14).

Mistake

Students think the first Rh+ve pregnancy of an Rh-ve mother is at risk of erythroblastosis foetalis, missing that the placenta separates the bloods during the first pregnancy.

Root cause: concept gap

Correction

Rh antigens of the foetus do not get exposed to the mother's Rh-ve blood in the first pregnancy because the two bloods are separated by the placenta; exposure typically occurs during delivery, so risk arises in subsequent pregnancies unless anti-Rh antibodies are given after the first delivery (C11_BIO_18_FACT_12, C11_BIO_18_FACT_13).

Wrong option pattern

States the first pregnancy itself is destroyed by Rh incompatibility

Mistake

Students confuse thrombokinase (the enzyme complex activating the cascade) with thrombin (which converts fibrinogen to fibrin), or forget calcium's role.

Root cause: term confusion

Correction

Thrombokinase is the enzyme complex required to convert prothrombin to thrombin; thrombin then converts inactive fibrinogen to fibrin threads; calcium ions play a very important role in clotting (C11_BIO_18_FACT_14).

Wrong option pattern

Names thrombin as the step-activating enzyme complex instead of thrombokinase

More in Human Physiology: 29 exam traps and mistakes · 2 formulas · 1 question pattern from its other lessons.

Circulation Blood Groups Coagulation questions from past NEET papers

3 questions from NEET 2021, 2022, 2026. Answers verified against NTA official keys.

NEET 2022

Given below are two statements : Statement I : The coagulum is formed of network of threads called thrombins. Statement II : Spleen is the graveyard of erythrocytes. In the light of the above statements, choose the most appropriate answer from the options given below :

1Statement I is incorrect but Statement II is correct
2Both Statement I and Statement II are correct
3Both Statement I and Statement II are incorrect
4Statement I is correct but Statement II is incorrect
NTA Answer: Option 1(final)

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Sources

NCERT refs: Class 11 Biology Chapter 15, p.195

Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.

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