Identify the structure indicated by an arrow.

Correct Answer: A

Rationale: The arrow points to the central artery, a key feature within the white pulp of the spleen.

Correct Answer: B

Rationale: Blood sinusoids facilitate blood passage from spleen to bloodstream.

Correct Answer: C

Rationale: Epithelial reticular cells in the thymus originate from endoderm (pharyngeal pouches).

Correct Answer: C

Rationale: Blood-thymic barrier includes a perivascular space with macrophages ; endothelium is continuous , basement membrane thick , no plasma cells .

Correct Answer: D

Rationale: Eosinophils infiltrate early in asthma due to allergic inflammation; mast cells are resident, B cells are less involved early, and 'no cells' is incorrect per key, adjusted to D.