Reproduction: testosterone synthesis in Leydig cells: 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 · Reproduction: testosterone synthesis in Leydig cells

Sample item

In a Leydig cell, LH binding raises cyclic AMP and promotes the first, rate-limiting step of steroid synthesis. What is that step?

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

The answer and the reasoning

D. Transfer of cholesterol into the mitochondria by StAR protein

Steroidogenesis begins when StAR protein carries cholesterol to the inner mitochondrial membrane, where P450scc forms pregnenolone. Aromatase and 5-alpha reductase act later, on testosterone.

Option by option

  • A. Conversion of testosterone to oestradiol by aromatase

    Aromatisation of testosterone is a later reaction and is not the step controlled by LH.

  • B. Conversion of testosterone to dihydrotestosterone by 5-alpha reductase

    5-alpha reduction occurs in target tissues and after testosterone has been made.

  • C. Binding of testosterone to androgen-binding protein

    Androgen-binding protein is made by Sertoli cells and holds androgen in the tubule lumen.

  • D. Transfer of cholesterol into the mitochondria by StAR protein (correct)

    Correct. Cholesterol delivery to the inner mitochondrial membrane is rate-limiting.

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.

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Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is D, Transfer of cholesterol into the mitochondria by StAR protein. Steroidogenesis begins when StAR protein carries cholesterol to the inner mitochondrial membrane, where P450scc forms pregnenolone.

Why is "Conversion of testosterone to oestradiol by aromatase" not the answer?

Aromatisation of testosterone is a later reaction and is not the step controlled by LH.

Why is "Conversion of testosterone to dihydrotestosterone by 5-alpha reductase" not the answer?

5-alpha reduction occurs in target tissues and after testosterone has been made.

Why is "Binding of testosterone to androgen-binding protein" not the answer?

Androgen-binding protein is made by Sertoli cells and holds androgen in the tubule lumen.

What concept does this question test?

It tests Reproduction: testosterone synthesis in Leydig cells in Physiology. Aromatase and 5-alpha reductase act later, on testosterone.

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 Reproduction: testosterone synthesis in Leydig cells?

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.