A 28-day-old girl presented with lethargy, poor feeding, and repeated vomiting for last 5 days; IV fluid and empirical antibiotics were started; later she develops repeated seizures not responding to IV calcium, glucose, B6, and anticonvulsant drugs. Septic screen is negative; serum ammonia is elevated with normal anion gap and normal pH. Family history reveals 2 siblings died with same scenario. Of the following, the MOST likely diagnosis is

Correct Answer: D

Rationale: Elevated ammonia with normal anion gap and pH, plus neonatal seizures and family history, points to urea cycle defects (e.g., OTC deficiency), unlike galactosemia (A), hyperglycinemia (B), organic acidemias (C), or PKU (D), which have different metabolic profiles.

Correct Answer: D

Rationale: MSUD, indicated by symptoms and urine odor, is best managed with a diet low in branched-chain amino acids to reduce toxic metabolites, more effective than hydration (A), dialysis (B), transplantation (C), or calories alone (D).

Correct Answer: D

Rationale: Tay-Sachs disease causes cherry-red spots, seizures, macrocephaly, and startle response from GM2 ganglioside accumulation, matching this case, unlike others (A, B, C, E).

Correct Answer: A

Rationale: Fabry disease is X-linked, unlike the autosomal recessive Krabbe (B), Gaucher (C), Tay-Sachs (D), and Niemann-Pick (E).

Correct Answer: B

Rationale: Liver biopsy showing glycogen accumulation and enzyme deficiency (glucose-6-phosphatase) definitively diagnoses GSD I, more specific than EMG (A), muscle biopsy (C), gene analysis (D), or fibroblasts (E).