Metabolism USMLE Step 1 Practice Question

A 2-day-old newborn presents with severe metabolic acidosis (pH 7.18), vomiting, hypotonia, and altered mental status. Vital signs show tachycardia (HR 145/min), tachypnea (RR 32/min), and hypothermia (36.2°C). Laboratory analysis reveals elevated serum lactate (8.5 mmol/L) and alanine (850 μmol/L), with normal blood glucose. Head ultrasound shows no structural abnormalities. The infant has received no medications. These findings are most consistent with a deficiency of which enzyme?