Which cytokine is essential for T-cell proliferation and is also necessary for the production of CD25-positive regulatory T cells?

Correct Answer: A

Rationale: IL-2 is critical for T-cell proliferation and the development of CD25+ regulatory T cells, which express the IL-2 receptor alpha chain (CD25).

Correct Answer: B

Rationale: IgA deficiency is a common primary immunodeficiency associated with recurrent mucosal infections and linked to autoimmune conditions like celiac disease.

Correct Answer: B

Rationale: Mature naive B cells co-express IgM and IgD on their surface, preparing them to respond to antigens.

Correct Answer: C

Rationale: C1q deficiency is strongly associated with a lupus-like syndrome due to impaired clearance of immune complexes, leading to autoimmunity. C3 deficiency causes severe bacterial infections, C5-9 deficiencies lead to neisserial infections, and C1 inhibitor deficiency causes angioedema, not lupus-like features.

Correct Answer: B

Rationale: SCID is the most severe congenital immunodeficiency, with absent T and B cell function, leading to early, life-threatening infections.