Question 1 · Hard
A 6-year-old boy, whose parents are first cousins, is referred to a hospital in Abu Dhabi with an unsteady gait that began when he started walking and is slowly getting worse. Since the age of 4 he has had fine red vessels on the whites of both eyes, and he has had repeated sinusitis and two pneumonias. Results: IgG 6.2 g/L (reference 5.0-15.0), IgA below 0.07 g/L (0.3-2.0), IgM 1.4 g/L (0.4-1.8); lymphocytes 1.0 × 10^9/L (1.5-6.5) with low CD3+ T cells; platelets 290 × 10^9/L with a normal MPV; serum alpha-fetoprotein 165 µg/L (reference below 10). Liver tests and an abdominal ultrasound are normal. The most likely diagnosis is:
- A22q11.2 deletion syndrome
- BAtaxia-telangiectasia
- CSelective IgA deficiency
- DWiskott-Aldrich syndrome
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Answer: B · Ataxia-telangiectasia
Progressive cerebellar ataxia from early childhood, telangiectasia of the conjunctivae and sinopulmonary infections are the classic triad of ataxia-telangiectasia, an autosomal recessive defect of the ATM DNA-repair gene, which fits the consanguinity. The laboratory pattern supports it: low IgA, T-cell lymphopenia and a raised AFP with a normal liver, a finding present in most patients after the age of 2 that makes AFP a useful screening test. Selective IgA deficiency does not cause ataxia, lymphopenia or a high AFP. Wiskott-Aldrich syndrome gives few, small platelets with eczema, and 22q11.2 deletion presents in infancy with heart defects and hypocalcemia, not ataxia. Because the patient's cells are very sensitive to ionizing radiation, X-rays and CT scans should be kept to a minimum.
- A: 22q11.2 deletion presents in infancy with heart defects and low calcium, not ataxia.
- C: IgA deficiency alone does not explain the ataxia, lymphopenia or raised AFP.
- D: Wiskott-Aldrich gives few, small platelets; here the count and MPV are normal.