Clinical Scenario:
A 7-day-old exclusively breastfed male neonate is brought with poor feeding, lethargy, and recurrent vomiting. He was well until the fifth day of life. Examination reveals hypotonia and depressed neonatal reflexes. Laboratory investigations show:
Blood glucose: 92 mg/dL
Serum ammonia: 850 µmol/L
Arterial blood gas: pH 7.42
Serum lactate: Normal
Urine ketones: Negative
What is the most likely diagnosis?
A. Fatty acid oxidation defect
B. Organic acidemia
C. Urea cycle defect
D. Galactosemia
E. Maple syrup urine disease
Correct answer & Explanation:
Correct Answer
C. Urea cycle defect
Explanation
The hallmark findings are:
Neonatal encephalopathy
Severe hyperammonemia
Normal glucose
Normal lactate
Absence of significant metabolic acidosis
This combination is classic for a urea cycle defect.
