In the brain, the endothelial cells of the capillary walls are more tightly fused than those in other body regions. This fact underlies the operation of the:

Correct Answer: C

Rationale: The blood-brain barrier relies on tightly fused endothelial cells (tight junctions) in brain capillaries, restricting passage of substances and protecting the CNS, aided by astrocytes but defined by this structure.

Correct Answer: D

Rationale: Intrinsic factor, produced by the stomach mucosa, is required for vitamin B₁₂ absorption in the ileum, essential for red blood cell maturation. Bone marrow produces RBCs, the liver stores B₁₂, and B₆ aids hemoglobin synthesis but isn’t intrinsic factor.

Correct Answer: C

Rationale: IgA, found in mucosal secretions (e.g., saliva, tears), protects the GI, urogenital, and respiratory tracts against pathogens. IgM and IgG fight blood infections, and IgE is linked to allergies.

Correct Answer: D

Rationale: The sinoatrial (SA) node is the heart’s pacemaker because it has the highest intrinsic rate of spontaneous depolarization (~60-100 beats/min), outpacing other cardiac cells (e.g., AV node: 40-60, Purkinje: 20-40). While it generates impulses (C), D specifies the rate, which is key. Cholinergic (parasympathetic) innervation slows it, and sympathetic innervation enhances it, but neither defines its pacemaker role.

Correct Answer: C

Rationale: In complete heart block (third-degree), all impulses are blocked at the AV node (C), causing ventricles to beat independently at 20-40 bpm via Purkinje fibers, not 60 (A). The SA node functions normally (B is false), and BP is typically low (hypotension) due to bradycardia, not high (D). (Source: Document, p. 6-7)