Haemostasis: antithrombin and heparin: practice question

A Physiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Physiology · Haemostasis: antithrombin and heparin

Sample item

A man with extensive deep vein thrombosis, whose father had recurrent venous thrombosis, shows almost no prolongation of aPTT despite escalating unfractionated heparin doses. Which deficiency explains this heparin resistance?

AAntithrombin, the cofactor through which heparin inactivates thrombin and factor XaBPlasminogen, the precursor of the fibrinolytic enzymeCProtein C, which degrades factors Va and VIIIaDFactor VIII, a cofactor of factor IXa

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. Antithrombin, the cofactor through which heparin inactivates thrombin and factor Xa

Heparin binds antithrombin and causes a conformational change that accelerates its inhibition of thrombin and factor Xa by about a thousand-fold. With inherited antithrombin deficiency, heparin has little substrate to act on, so the aPTT barely rises and thrombosis recurs. Treatment may need antithrombin concentrate or a direct thrombin inhibitor.

Option by option

A. Antithrombin, the cofactor through which heparin inactivates thrombin and factor Xa (correct)
Correct. Heparin depends on antithrombin for its anticoagulant effect.
B. Plasminogen, the precursor of the fibrinolytic enzyme
Plasminogen deficiency impairs clot lysis, but heparin action does not depend on it.
C. Protein C, which degrades factors Va and VIIIa
Protein C deficiency causes thrombophilia but does not blunt the heparin response.
D. Factor VIII, a cofactor of factor IXa
Factor VIII deficiency would prolong the aPTT and cause bleeding rather than thrombosis.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is A, Antithrombin, the cofactor through which heparin inactivates thrombin and factor Xa. Heparin binds antithrombin and causes a conformational change that accelerates its inhibition of thrombin and factor Xa by about a thousand-fold.

Why is "Plasminogen, the precursor of the fibrinolytic enzyme" not the answer?

Plasminogen deficiency impairs clot lysis, but heparin action does not depend on it.

Why is "Protein C, which degrades factors Va and VIIIa" not the answer?

Protein C deficiency causes thrombophilia but does not blunt the heparin response.

Why is "Factor VIII, a cofactor of factor IXa" not the answer?

Factor VIII deficiency would prolong the aPTT and cause bleeding rather than thrombosis.

What concept does this question test?

It tests Haemostasis: antithrombin and heparin in Physiology. With inherited antithrombin deficiency, heparin has little substrate to act on, so the aPTT barely rises and thrombosis recurs. Treatment may need antithrombin concentrate or a direct thrombin inhibitor.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Haemostasis: antithrombin and heparin?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.