A 6-year-old presents with the appearance of being intoxicated while walking. Opthlamoscopic findings reveal broken capillaries in the sclera of the eye. Lab work reveals IgA deficiency. Which of the following is the most likely?

Correct Answer: A

Rationale: Ataxia telangiectasia presents with ataxia, telangiectasia (broken capillaries), and often IgA deficiency.

Correct Answer: D

Rationale: The secretory piece (or secretory component) is unique to secretory IgA, such as that found in breast milk, and protects it from degradation in mucosal environments. Serum IgM lacks this feature.

Correct Answer: D

Rationale: X-linked agammaglobulinemia is caused by mutations in the Bruton's tyrosine kinase (BTK) gene, impairing B-cell development.

Correct Answer: B

Rationale: Rhesus D hemolytic disease is a Type II hypersensitivity reaction, involving antibody-mediated destruction of red blood cells.

Correct Answer: B

Rationale: Type II hypersensitivity involves antibodies binding to cell surface antigens, leading to complement-mediated or phagocytic destruction.