A newborn girl develops vomiting, dehydration and hyponatraemic hyperkalaemic shock in the second week of life. Examination shows clitoromegaly with partial labial fusion. Plasma renin activity is markedly elevated. Which single biochemical measurement provides the definitive diagnosis?

• A Serum dehydroepiandrosterone sulphate
• B Urinary free cortisol excretion over 24 hours
• C Morning plasma ACTH level
• D Serum 17-hydroxyprogesterone concentration ✓

Explanation

This is classic salt-wasting congenital adrenal hyperplasia from 21-hydroxylase deficiency, the commonest form, in which impaired cortisol synthesis drives ACTH excess and shunting of precursors into androgen production. Markedly elevated 17-hydroxyprogesterone is the diagnostic hallmark and directly reflects the enzymatic block. Elevated ACTH merely indicates adrenal insufficiency generically and cannot distinguish the enzyme defect, while DHEA-S rises in several forms of virilising adrenal pathology and lacks specificity for the 21-hydroxylase block.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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