Clinical scenario: Abdominal pain | Limb weakness | Reddish brown urine
An 8-year-old boy is evaluated for recurrent episodes of severe abdominal pain, vomiting, irritability, and progressive weakness of the lower limbs. During attacks, he develops hypertension and tachycardia. There is no fever or meningism. Urine collected during an episode becomes dark reddish-brown after standing in light. Serum ammonia, lactate, and blood glucose are normal.
What is the most likely diagnosis?
A. Acute intermittent porphyria
B. Hereditary fructose intolerance
C. Methylmalonic acidemia
D. Wilson disease
E. X-linked adrenoleukodystrophy
Correct answer & Explanation:
Correct Answer: A. Acute intermittent porphyria
Explanation
Acute intermittent porphyria (AIP) is caused by porphobilinogen deaminase deficiency, leading to accumulation of δ-aminolevulinic acid (ALA) and porphobilinogen (PBG).
Although uncommon in children, it is an important metabolic disorder tested in postgraduate examinations because of its characteristic presentation:
- Recurrent severe abdominal pain with no surgical cause
- Autonomic dysfunction (hypertension, tachycardia)
- Peripheral motor neuropathy
- Neuropsychiatric manifestations
- Urine that darkens on standing due to oxidation of porphyrin precursors
- No hyperammonemia or metabolic acidosis
Diagnosis is confirmed by markedly increased urinary porphobilinogen during an acute attack.
Why the other options are incorrect
- B. Hereditary fructose intolerance – Presents after fructose ingestion with hypoglycemia, vomiting, jaundice, and hepatomegaly.
- C. Methylmalonic acidemia – Typically causes metabolic acidosis, ketosis, hyperammonemia, and recurrent metabolic decompensation.
- D. Wilson disease – Causes chronic liver disease, movement disorders, and Kayser–Fleischer rings rather than acute abdominal crises.
- E. X-linked adrenoleukodystrophy – Presents with adrenal insufficiency and progressive cerebral demyelination, not episodic abdominal pain.
High-Yield Pearl
A child or adolescent with recurrent unexplained abdominal pain, autonomic instability, motor neuropathy, and urine that darkens on standing should prompt consideration of acute intermittent porphyria. The first-line diagnostic test during an attack is urinary porphobilinogen.
