A 13-month-old male presented with vomiting, fever, and irritability. On examination his temperature was 40°C, lethargic, with atrophied tonsils, clear tympanic membrane, clear chest and neck stiffness. The parents gave past history of pneumonia at the age of 6 and 9 months for which he was admitted to hospital and treated with i.v. antibiotics for 10 days. Of the following, the MOST likely diagnosis is

Correct Answer: A

Rationale: Recurrent bacterial infections and absent tonsils suggest agammaglobulinemia, likely X-linked, due to severe B-cell deficiency.

Correct Answer: D

Rationale: CD46 (membrane cofactor protein) inhibits C3 convertase by promoting its degradation, regulating the complement system to prevent excessive activation.

Correct Answer: C

Rationale: T cells recognize antigens only when presented on MHC molecules by antigen-presenting cells, not as soluble antigens directly.

Correct Answer: C

Rationale: NK cells target cells lacking MHC class I, a mechanism viruses use to evade CD8 T cells.

Correct Answer: D

Rationale: Each Fab has one antigen-binding site, but with 3 CDRs per chain (light and heavy), there are 6 CDRs total contributing to binding.