A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 5 year-old-boy with a history of severe allergies and recurrent sinusitis presents with foul-smelling, fatty diarrhea. He is at the 50th percentile for height and weight. The boy's mother reports that he has had several such episodes of diarrhea over the years. He does not have any known history of fungal infections or severe viral infections. Which of the following is the most likely underlying cause of this boy's presentation?
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A
Thymic aplasiaIncorrect. Thymic (DiGeorge) aplasia features T-cell deficiency, recurrent viral/fungal infections, hypocalcemia, and cardiac anomalies - none of which are described.
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B
Hyper IgE syndromeIncorrect. Hyper-IgE (Job) syndrome shows recurrent staphylococcal cold abscesses, eczema, retained primary teeth, and characteristic facies - not the picture here.
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C
Severe combined immune deficiencyIncorrect. SCID presents in early infancy with failure to thrive and severe infections of all types (viral, bacterial, fungal); this child is growing normally with no severe viral/fungal disease.
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D
IgA deficiencyCorrect. Selective IgA deficiency presents with recurrent sinopulmonary infections, GI infections (Giardia causing steatorrhea), and atopic disease, with normal growth - the most common primary immunodeficiency.
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E
Chronic granulomatous diseaseIncorrect. CGD presents with recurrent catalase-positive bacterial and fungal infections (S. aureus, Aspergillus) and granuloma formation due to defective NADPH oxidase; sinopulmonary infection with GI Giardia and atopy fits IgA deficiency better.
↑ Tap an answer to reveal the reasoning
Answer: D. This 5-year-old with recurrent sinopulmonary infections, severe atopic disease (allergies), and chronic steatorrhea (foul-smelling fatty diarrhea suggesting GI involvement, e.g., Giardia infection) - with normal growth, no fungal infections, and no severe viral infections - has the classic phenotype of selective IgA deficiency.
IgA deficiency is the most common primary immunodeficiency (1 in ~300-600 in Western populations). Most patients are asymptomatic, but symptomatic patients present with: recurrent sinopulmonary infections (decreased mucosal IgA), GI infections (especially Giardia lamblia, which causes the steatorrhea here), allergic/atopic disease, autoimmune conditions (celiac disease, SLE), and anaphylaxis with IgA-containing blood products (these patients can form anti-IgA antibodies). Diagnosis is decreased serum IgA with normal IgG and IgM. Treatment is largely supportive (treating individual infections); pooled immunoglobulin is contraindicated because it contains IgA.
Distractors: thymic aplasia (DiGeorge) presents in infancy with severe T-cell deficiency, recurrent viral and fungal infections, hypocalcemia (tetany), and cardiac defects - not seen here. Hyper-IgE (Job) syndrome shows recurrent S. aureus cold abscesses, eczema, retained primary teeth, and very high IgE. SCID presents in early infancy with failure to thrive and life-threatening infections of all types (viral, bacterial, fungal); this child is at the 50th percentile and lacks severe viral/fungal infections.
**Why each option:**
**A.** Wrong. Thymic (DiGeorge) aplasia features T-cell deficiency, recurrent viral/fungal infections, hypocalcemia, and cardiac anomalies - none of which are described.
**B.** Wrong. Hyper-IgE (Job) syndrome shows recurrent staphylococcal cold abscesses, eczema, retained primary teeth, and characteristic facies - not the picture here.
**C.** Wrong. SCID presents in early infancy with failure to thrive and severe infections of all types (viral, bacterial, fungal); this child is growing normally with no severe viral/fungal disease.
**D.** Correct. Selective IgA deficiency presents with recurrent sinopulmonary infections, GI infections (Giardia causing steatorrhea), and atopic disease, with normal growth - the most common primary immunodeficiency.