An academic medical center's endocrinology and genetics board is reviewing complex cases p… — USMLE Step 3

An academic medical center's endocrinology and genetics board is reviewing complex cases presenting with differences in sex development and severe electrolyte abnormalities. Based on the integrated clinical and laboratory profiles provided, match each patient's presentation to the most accurate underlying primary enzymatic deficiency.

46,XX neonate with virilized external genitalia, blood pressure of 65/35 mm Hg, serum $Na^+$ 122 mEq/L, serum $K^+$ 6.8 mEq/L, and markedly elevated plasma renin activity. $21$ -hydroxylase ( $CYP21A2$ ) deficiency
46,XX neonate with virilized external genitalia, blood pressure of 105/65 mm Hg, serum $Na^+$ 148 mEq/L, serum $K^+$ 3.1 mEq/L, and suppressed plasma renin activity. $11\beta$ -hydroxylase ( $CYP11B1$ ) deficiency
46,XY adolescent presenting with primary amenorrhea, phenotypically female external genitalia, blood pressure of 145/95 mm Hg, serum $Na^+$ 146 mEq/L, serum $K^+$ 2.8 mEq/L, and unmeasurable estradiol and testosterone. $17\alpha$ -hydroxylase ( $CYP17A1$ ) deficiency
46,XY neonate with female external genitalia, blood pressure of 60/30 mm Hg, serum $Na^+$ 120 mEq/L, serum $K^+$ 7.1 mEq/L, and an elevated ratio of 17-hydroxypregnenolone to 17-hydroxyprogesterone. $3\beta$ -hydroxysteroid dehydrogenase ( $HSD3B2$ ) deficiency

Answer

Profile 1 matches 21-hydroxylase deficiency; Profile 2 matches 11-beta-hydroxylase deficiency; Profile 3 matches 17-alpha-hydroxylase deficiency; Profile 4 matches 3-beta-hydroxysteroid dehydrogenase deficiency.

Correct matching requires a systematic analysis of two main axes: the mineralocorticoid axis (reflected in BP, Na+, K+, and renin) and the androgen axis (reflected in karyotype vs. phenotype). 21-hydroxylase deficiency blocks aldosterone and cortisol, causing salt-wasting and shunting to androgens (virilized XX). 11-beta-hydroxylase deficiency blocks final cortisol and aldosterone steps but accumulates 11-deoxycorticosterone, causing hypertension, hypokalemia, and virilization. 17-alpha-hydroxylase deficiency blocks all sex steroids and cortisol, shifting everything to mineralocorticoids, causing hypertension and lack of secondary sexual characteristics in both sexes (XY appear female). 3-beta-HSD deficiency blocks early pathways, causing global deficiency of potent steroids, leading to salt-wasting and ambiguous/female genitalia in XY males.

Step-by-Step Solution

Analyze blood pressure and electrolytes (sodium, potassium, renin) to determine the patient's mineralocorticoid status.

Profiles 1 and 4 show mineralocorticoid deficiency (hypotension, hyponatremia, hyperkalemia, high renin). Profiles 2 and 3 show mineralocorticoid excess (hypertension, hypernatremia, hypokalemia, suppressed renin).

Establishing mineralocorticoid status narrows the enzymatic defect to either the aldosterone-producing pathways or upstream pathways that accumulate precursors with mineralocorticoid activity.

Evaluate the karyotype combined with the genital phenotype to determine the sex steroid (androgen) status.

Profiles 1 and 2 show androgen excess (virilized 46,XX). Profiles 3 and 4 show androgen deficiency (undermasculinized 46,XY).

Identifying whether there is a shunting toward or away from the sex steroid pathway pinpoints the exact horizontal or vertical block in the steroidogenesis cascade.

Synthesize mineralocorticoid and androgen status with specific precursor ratios to identify the definitive enzyme deficiency.

Low mineralocorticoid + High androgen = 21-hydroxylase. High mineralocorticoid + High androgen = 11-beta-hydroxylase. High mineralocorticoid + Low androgen = 17-alpha-hydroxylase. Low mineralocorticoid + Low potent androgen = 3-beta-hydroxysteroid dehydrogenase.

Each congenital adrenal hyperplasia variant produces a unique, mathematically deducible pattern of laboratory derangements.

Key Concept

Interpreting Adrenal Steroidogenesis Laboratory Profiles

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