A 3-week-old neonate develops repeated vomiting and dehydration. Examination reveals ambiguous genitalia with fusion of the anterior portion of the labioscrotal folds. Ultrasonography reveals presence of uterus, ovaries, and fallopian tubes. Of the following, the MOST valuable test to establish the diagnosis is

Correct Answer: A

Rationale: Elevated 17-hydroxyprogesterone confirms congenital adrenal hyperplasia (e.g., 21-hydroxylase deficiency), common in 46,XX neonates with ambiguous genitalia and salt-wasting, as seen here. Other tests (B, C, D, E) are less diagnostic initially.

Correct Answer: C

Rationale: Glycogen storage disease IV (Andersen) causes hepatic failure due to amylopectin-like polysaccharide accumulation leading to cirrhosis. GSD I (A) and III (B) affect liver function but rarely cause failure, while MPS I (D) and II (E) primarily affect other systems.

Correct Answer: B

Rationale: Galactokinase deficiency leads to galactose accumulation forming galactitol, causing cataracts after the neonatal period. It lacks the severe systemic effects (A, D, E) or cognitive impact (C) of classic galactosemia.

Correct Answer: C

Rationale: Cystinuria impairs reabsorption of cystine, lysine, arginine, and ornithine (dibasic amino acids), but tryptophan, a neutral amino acid, is not affected.

Correct Answer: C

Rationale: MPS III (Sanfilippo) typically lacks hepatosplenomegaly, focusing on neurological deterioration, unlike other MPS types where organomegaly is common.