Vitamins and minerals: warfarin and the vitamin K cycle: practice question

A Biochemistry 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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Biochemistry · Vitamins and minerals: warfarin and the vitamin K cycle

Sample item

A man with atrial fibrillation on long-term warfarin has a prolonged prothrombin time. Warfarin impairs clotting factor activation by blocking which step in the vitamin K cycle?

ARecycling of vitamin K epoxide back to reduced vitamin KBHepatic uptake of vitamin K carried in chylomicron remnantsCColonic bacterial synthesis of menaquinone from dietary precursorsDIntestinal absorption of vitamin K together with micellar fat digestion

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

The answer and the reasoning

A. Recycling of vitamin K epoxide back to reduced vitamin K

Gamma-carboxylation of glutamate residues in factors II, VII, IX and X converts reduced vitamin K to its epoxide. Warfarin inhibits vitamin K epoxide reductase (VKORC1), so the reduced form is not regenerated and under-carboxylated, inactive factors are made. Vitamin K given in large amounts can partly overcome the block.

Option by option

A. Recycling of vitamin K epoxide back to reduced vitamin K (correct)
Correct. Inhibition of vitamin K epoxide reductase stops regeneration of the reduced cofactor.
B. Hepatic uptake of vitamin K carried in chylomicron remnants
Chylomicron transport of vitamin K is not altered by warfarin.
C. Colonic bacterial synthesis of menaquinone from dietary precursors
Gut bacterial synthesis is unaffected by warfarin; it is reduced by broad-spectrum antibiotics.
D. Intestinal absorption of vitamin K together with micellar fat digestion
Absorption is not affected by warfarin; this step is impaired in fat malabsorption or biliary obstruction.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, Recycling of vitamin K epoxide back to reduced vitamin K. Gamma-carboxylation of glutamate residues in factors II, VII, IX and X converts reduced vitamin K to its epoxide.

Why is "Hepatic uptake of vitamin K carried in chylomicron remnants" not the answer?

Chylomicron transport of vitamin K is not altered by warfarin.

Why is "Colonic bacterial synthesis of menaquinone from dietary precursors" not the answer?

Gut bacterial synthesis is unaffected by warfarin; it is reduced by broad-spectrum antibiotics.

Why is "Intestinal absorption of vitamin K together with micellar fat digesti…" not the answer?

Absorption is not affected by warfarin; this step is impaired in fat malabsorption or biliary obstruction.

What concept does this question test?

It tests Vitamins and minerals: warfarin and the vitamin K cycle in Biochemistry. Warfarin inhibits vitamin K epoxide reductase (VKORC1), so the reduced form is not regenerated and under-carboxylated, inactive factors are made. Vitamin K given in large amounts can partly overcome the block.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 Vitamins and minerals: warfarin and the vitamin K cycle?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.