Vitamins and minerals: dietary goitrogens: 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: dietary goitrogens

Sample item

Villagers who eat poorly processed cassava as a staple have goitre even though iodine intake is borderline adequate. Which mechanism contributes?

ACassava blocks conversion of T4 to T3 in the liverBThiocyanate competes with iodide for uptake by the symporterCCassava increases renal loss of thyroglobulinDCassava stimulates thyrotropin receptor antibodies

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

The answer and the reasoning

B. Thiocyanate competes with iodide for uptake by the symporter

Cassava contains cyanogenic glycosides which are metabolised to thiocyanate, a competitive inhibitor of the sodium-iodide symporter. In iodine-poor areas this worsens goitre. Adequate iodine intake and proper processing reduce the effect.

Option by option

A. Cassava blocks conversion of T4 to T3 in the liver
Impaired peripheral conversion would not cause goitre by itself.
B. Thiocyanate competes with iodide for uptake by the symporter (correct)
Correct. Thiocyanate competes with iodide for the transporter on the follicular cell.
C. Cassava increases renal loss of thyroglobulin
Thyroglobulin is not lost through the kidney in this way.
D. Cassava stimulates thyrotropin receptor antibodies
Cassava has no effect on TSH receptor antibodies.

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 B, Thiocyanate competes with iodide for uptake by the symporter. Cassava contains cyanogenic glycosides which are metabolised to thiocyanate, a competitive inhibitor of the sodium-iodide symporter.

Why is "Cassava blocks conversion of T4 to T3 in the liver" not the answer?

Impaired peripheral conversion would not cause goitre by itself.

Why is "Cassava increases renal loss of thyroglobulin" not the answer?

Thyroglobulin is not lost through the kidney in this way.

Why is "Cassava stimulates thyrotropin receptor antibodies" not the answer?

Cassava has no effect on TSH receptor antibodies.

What concept does this question test?

It tests Vitamins and minerals: dietary goitrogens in Biochemistry. In iodine-poor areas this worsens goitre. Adequate iodine intake and proper processing reduce the effect.

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: dietary goitrogens?

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.