Steroid synthesis: lipoid congenital adrenal hyperplasia: 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 · Steroid synthesis: lipoid congenital adrenal hyperplasia

Sample item

A phenotypically female infant (46,XY karyotype) has vomiting, hyponatraemia, hyperkalaemia and marked skin pigmentation at 2 weeks. ACTH and renin are very high, and cortisol, aldosterone, 17-hydroxyprogesterone and androgens are all undetectable. The adrenals are enlarged and lipid-laden. What is the defect?

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

The answer and the reasoning

C. Steroidogenic acute regulatory protein, which carries cholesterol into mitochondria

The StAR protein moves cholesterol from the outer to the inner mitochondrial membrane, where side-chain cleavage forms pregnenolone, the first step of all steroid hormones. Its loss in lipoid CAH leaves every adrenal and gonadal steroid deficient, so XY infants are undervirilised, with salt wasting, and cholesterol esters pile up in the adrenals. The other enzymes block downstream steps so that some steroids persist.

Option by option

  • A. 17-alpha-hydroxylase

    17-alpha-hydroxylase deficiency causes hypertension and hypokalaemia from excess mineralocorticoid, and cortisol deficiency without salt wasting.

  • B. 21-hydroxylase

    21-hydroxylase deficiency gives high 17-hydroxyprogesterone and virilisation of females.

  • C. Steroidogenic acute regulatory protein, which carries cholesterol into mitochondria (correct)

    Absent StAR blocks all steroidogenesis, explaining undetectable 17-hydroxyprogesterone and female genitalia in a 46,XY infant.

  • D. 11-beta-hydroxylase

    11-beta-hydroxylase deficiency gives raised 11-deoxycorticosterone and hypertension with virilisation.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, Steroidogenic acute regulatory protein, which carries cholesterol into mitochondria. The StAR protein moves cholesterol from the outer to the inner mitochondrial membrane, where side-chain cleavage forms pregnenolone, the first step of all steroid hormones.

Why is "17-alpha-hydroxylase" not the answer?

17-alpha-hydroxylase deficiency causes hypertension and hypokalaemia from excess mineralocorticoid, and cortisol deficiency without salt wasting.

Why is "21-hydroxylase" not the answer?

21-hydroxylase deficiency gives high 17-hydroxyprogesterone and virilisation of females.

Why is "11-beta-hydroxylase" not the answer?

11-beta-hydroxylase deficiency gives raised 11-deoxycorticosterone and hypertension with virilisation.

What concept does this question test?

It tests Steroid synthesis: lipoid congenital adrenal hyperplasia in Biochemistry. Its loss in lipoid CAH leaves every adrenal and gonadal steroid deficient, so XY infants are undervirilised, with salt wasting, and cholesterol esters pile up in the adrenals. The other enzymes block downstream steps so that some steroids persist.

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 Steroid synthesis: lipoid congenital adrenal hyperplasia?

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.