Metabolism USMLE Step 1 Practice Question
A 6-month-old male presents with recurrent vomiting, irritability, and poor weight gain over 2 months. Vital signs show HR 128/min, RR 26/min, temp 37.2°C, and BP 85/55 mmHg. Physical examination reveals brittle hair and mild hepatomegaly; no rash noted. Laboratory studies demonstrate ammonia 156 μmol/L (normal <50), and plasma amino acid analysis shows elevated argininosuccinate. Serum transaminases are normal. Which enzyme deficiency best explains these findings?
Answer choices
- APhenylalanine hydroxylase
- BArgininosuccinate lyaseCorrect answer
- CMethionine synthase
- DOrnithine transcarbamylase
- EArginase
- FCarbamoyl phosphate synthetase I
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