Antigen presenting cells (APCs) are required for T-cell recognition of specific antigen and activation. APCs accomplish this task by presenting antigen in the context of which of the following molecules?

Correct Answer: C

Rationale: APCs present antigens on MHC molecules to T cells.

Correct Answer: D

Rationale: Bone marrow or cord blood transplantation is curative for WAS, correcting the underlying genetic defect affecting immunity, platelets, and eczema.

Correct Answer: C

Rationale: Cytomegalovirus can cause prolonged bone marrow suppression, leading to chronic neutropenia, unlike the others, which typically cause transient effects.

Correct Answer: A

Rationale: Chédiak-Higashi syndrome (silvery hair, infections, ataxia) is characterized by neutropenia due to defective neutrophil granule function.

Correct Answer: D

Rationale: Cartilage-hair hypoplasia causes T-cell deficiency but not necessarily lymphopenia; the others suppress lymphocyte counts via infection or stress.