Clinical scenario: poor feeding| vomiting |rapid breathing | hyperammonemia
A 12-day-old previously healthy male infant is brought to the emergency department with poor feeding, recurrent vomiting, excessive sleepiness, and rapid breathing for the past 24 hours. He has been treated with intravenous antibiotics for presumed neonatal sepsis without improvement. On examination, he is lethargic, hypotonic, and afebrile. There is no hepatomegaly or dysmorphic feature.
Investigations reveal:
- pH 7.55
- PaCO₂ 22 mmHg
- Blood glucose 92 mg/dL
- Serum lactate 1.8 mmol/L
- Plasma ammonia 690 μmol/L
Which of the following is the most likely diagnosis?
A. Maple syrup urine disease
B. Methylmalonic acidemia
C. Ornithine transcarbamylase deficiency
D. Propionic acidemia
E. Pyruvate dehydrogenase deficiency
Correct answer & Explanation
Correct Answer: C. Ornithine transcarbamylase deficiency
Explanation
This infant has the classic pattern of a urea cycle disorder:
- Male neonate
- Initially well, then deteriorates after protein feeding
- Severe hyperammonemia
- Respiratory alkalosis (hyperventilation induced by ammonia)
- Normal glucose
- Normal lactate
- No significant metabolic acidosis
Among the options, OTC deficiency is the most likely diagnosis because it is the most common urea cycle disorder and classically presents in this way.
Why not the others?
- Maple syrup urine disease → ketosis, encephalopathy, maple syrup odor, not isolated severe hyperammonemia.
- Methylmalonic acidemia → hyperammonemia plus high anion gap metabolic acidosis, elevated lactate may occur.
- Propionic acidemia → hyperammonemia with metabolic acidosis and ketosis, not respiratory alkalosis.
- Pyruvate dehydrogenase deficiency → persistent lactic acidosis without marked hyperammonemia.
