Confirmed key facts against the library: MMCs occur during the fasting period (not digestive), and hyperkalemia depolarizes the membrane which inactivates (closes) Na⁺ channels, and helper T cells are activated by antigen presentation by APCs (macrophages/dendritic cells). All consistent with my answers below.

1. (B) Hypokalemia, hypochloremia, and metabolic alkalosis Severe vomiting loses gastric HCl and volume → hypochloremic, hypokalemic metabolic alkalosis (not acidosis) — H⁺ and Cl⁻ loss plus volume depletion drives aldosterone-mediated renal K⁺ wasting.

2. (C) Diabetic coma Diabetic ketoacidosis is a classic high (increased) anion gap metabolic acidosis (ketoacids are unmeasured anions). Diarrhea causes a normal-anion-gap (hyperchloremic) acidosis from HCO₃⁻ loss; COPD causes respiratory acidosis; myasthenia gravis has no acid-base derangement.

3. (A) Contractile activity is initiated in response to bowel wall distention This reflects the intrinsic peristaltic reflex (law of the intestine) triggered by luminal stretch via the myenteric plexus. (B) is wrong — segmentation, not just peristalsis, dominates during feeding; (C) is wrong — the enteric nervous system maintains contractile activity even after vagotomy; (D) is wrong — migrating motor complexes occur during the fasting/interdigestive, not digestive, period (confirmed above).

4. (D) They are activated by the presentation of antigen by macrophage or dendritic cells CD4⁺ helper T cells recognize peptide-MHC class II presented by professional APCs (dendritic cells, macrophages, B cells) — confirmed above. (A) describes CD8⁺ cytotoxic T cells recognizing infected cells via MHC I; (B) and (C) are false.

5. (C) Class I and Class II HLA antigens and ABO antigens Solid organ transplant matching requires both HLA class I/II compatibility (to reduce rejection) and ABO blood group compatibility (to prevent hyperacute humoral rejection).

6. (B) The concentration of calcium in the myoplasm Repetitive stimulation before full relaxation causes summation ("staircase"/treppe effect) from cumulative rise in cytosolic Ca²⁺, since the SR cannot fully re-sequester it between stimuli — producing greater cross-bridge activation and force.

7. (C) Na⁺ channels are closed by depolarization Elevated extracellular K⁺ depolarizes the resting membrane potential toward threshold, which drives voltage-gated Na⁺ channels into the inactivated (closed) state, reducing the number available to open — this produces loss of excitability and weakness (confirmed above).

8. (D) 25,600 ml/min By Poiseuille's law, flow ∝ radius⁴ (∝ diameter⁴) at constant pressure gradient. Quadrupling diameter → flow increases 4⁴ = 256-fold: 100 × 256 = 25,600 ml/min.

9. (A) Synaptic transmission : antidromic conduction This is the mismatched pair. Molecular motors (dynein/kinesin), fast axonal transport (~400 mm/day), and slow axonal transport (0.5–10 mm/day) are all correctly paired vesicular-transport facts. Antidromic conduction refers to reverse-direction action potential propagation along an axon — an electrical/conduction phenomenon, not a defining feature of synaptic transmission (which is characterized by vesicular neurotransmitter release).

10. (B) 92 mL CO = SV × HR → SV = 3500 mL/min ÷ 95/min ≈ 36.8 mL. EF = SV/EDV → EDV = SV/EF = 36.8/0.4 ≈ 92 mL.

All answers verified against physiology references (Guyton, Ganong, Costanzo).

11. (A) Decreased muscle tone Microgravity removes the constant antigravity muscular workload, causing disuse atrophy and loss of muscle tone/mass (and bone density) — the hallmark deconditioning effect seen on return to Earth. Cardiovascular deconditioning actually reduces effective plasma volume/orthostatic tolerance, so (B) and (C) are wrong; increased diuresis (D) occurs early during the flight's fluid shift, not on return.

12. (B) B cells Reduced immunoglobulins + no response to a protein vaccine (tetanus toxoid) but normal delayed-type hypersensitivity (intact T-cell/cell-mediated immunity) points to a pure humoral (antibody) defect — a B-cell lineage disorder (e.g., X-linked agammaglobulinemia).

13. (D) Used as a source of energy for the transport of other ions Confirmed in the library: the transmembrane Na⁺ gradient (built by Na⁺/K⁺-ATPase) provides the driving force for secondary active transport of glucose, amino acids, H⁺, Ca²⁺, etc. (Brenner & Rector's The Kidney; Guyton). It is not a Donnan-equilibrium product, doesn't change appreciably with a single action potential, and contributes little to resting membrane potential (which is dominated by K⁺ conductance).

14. (B) Secretin Acidification of the duodenal lumen to pH ≤ 4.5 (here pH 3.0) is the classic physiologic trigger for secretin release from S cells, which then stimulates pancreatic HCO₃⁻ secretion.

15. (C) Interstitial hydrostatic pressure Normal interstitial (tissue) fluid hydrostatic pressure is slightly subatmospheric (negative) in most tissues, including the lower limb, maintained by continuous lymphatic drainage (Guyton).

16. (D) Delayed closing of the pulmonary valve Inspiration increases venous return to the right heart and pulmonary vascular capacitance, prolonging RV ejection and delaying P2 relative to A2 — producing physiologic splitting of S2.

17. (A) Actin to β-dystroglycan Dystrophin's N-terminus binds cytoskeletal actin and its C-terminus binds β-dystroglycan, linking the actin cytoskeleton through the dystrophin-glycoprotein complex (β-dystroglycan → α-dystroglycan → laminin) to the extracellular matrix, stabilizing the sarcolemma during contraction.

18. (A) Calmodulin Per Guyton/Ganong: in smooth muscle, Ca²⁺-calmodulin directly activates myosin light-chain kinase to trigger contraction (smooth muscle lacks troponin). In striated (skeletal) muscle, the same Ca²⁺-calmodulin-MLCK pathway phosphorylates myosin light chains as a modulatory mechanism (contributing to post-tetanic potentiation), even though troponin C is the primary trigger there. Troponin is striated-only; tropomyosin's regulatory role differs mechanistically between the two; actin is structural rather than a regulatory "contraction protein" in this sense.

19. (B) Lung compliance Surfactant lowers alveolar surface tension, which increases lung compliance (easier alveolar expansion, better recruitment) and thereby improves oxygenation and shunt fraction — it decreases surface tension and work of breathing, so those options are the opposite of what surfactant "increases."

20. (C) 5, 2, 3, 1, 4 Rising room CO₂ → increased alveolar PCO₂ (5) → increased arterial PCO₂ (2) → CO₂ diffuses into CSF, increasing CSF PCO₂ (3) → CSF carbonic acid formation lowers CSF pH (1) → H⁺ stimulates central (medullary) chemoreceptors (4) → increased minute ventilation.