The intrinsic factor necessary for the complete maturation of red cells is derived from:

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: 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)

Correct Answer: C

Rationale: An electrocardiogram (ECG, C) records the heart’s electrical activity via skin electrodes, showing rhythm and conduction. A (eye movement), B (brain activity), D (ultrasound imaging), and E (muscle activity) are unrelated to this function.

Correct Answer: A

Rationale: The aorta (A) carries oxygenated blood from the left ventricle to the body. The pulmonary artery (B) carries deoxygenated blood from the right ventricle, pulmonary vein (C) to the left atrium, and superior vena cava (D) to the right atrium.