All the following regarding the maxillary air sinuses are correct EXCEPT:

Correct Answer: B

Rationale: Maxillary sinuses drain into the middle meatus (A), are innervated by V2 branches (C), and risk fistulae from molar extraction (D) due to thin floor proximity. They lie inferior and lateral to the nasal cavity, anterior to the pterygopalatine fossa (B), not posterior B is false, as the fossa is behind the sinus, making it the exception.

Correct Answer: D

Rationale: Spermiogenesis transforms spermatids into spermatozoa: acrosome forms (A) from Golgi, aiding egg penetration; cytoplasm is shed (B) via residual bodies; nucleus condenses (C) for streamlined DNA delivery. Morula formation (D), a blastomere stage in early embryogenesis, is unrelated to spermiogenesis, which occurs in seminiferous tubules. D is false morula is an embryonic, not spermatogenic, process, making it the exception among spermiogenesis steps.

Correct Answer: C

Rationale: Most CO₂ diffusing into blood in systemic capillaries converts to bicarbonate ions in red blood cells (RBCs), making choice C correct. Only about 5-10% remains dissolved as CO₂ (choice A) due to low solubility. Around 20-25% forms carbamino compounds (choice B) by binding to hemoglobin's amino groups, but this is less dominant. The majority (65-70%) enters RBCs, where carbonic anhydrase catalyzes its reaction with H₂O to form carbonic acid (H₂CO₃), which dissociates into H⁺ and HCO₃⁻ (bicarbonate). ' CO₂ doesn't combine directly with hemoglobin significantly carbamino formation is specific. Choice E occurs minimally in plasma (slow without enzyme) compared to RBCs. Bicarbonate then exits RBCs via the chloride shift, maintaining pH balance. This process's efficiency in RBCs, driven by enzymatic speed and volume, ensures C is the primary fate of CO₂ in systemic capillaries.

Correct Answer: B

Rationale: To abolish hypoxemia (PaO₂ < 60 mmHg, here ≈ 50 mmHg), inspired O₂ (FiO₂) must raise PAO₂ to ≈100 mmHg. From the alveolar gas equation: PAO₂ = FiO₂ × (760 - 47) - (80 / 0.8). Set PAO₂ = 100: 100 = FiO₂ × 713 - 100; 200 = FiO₂ × 713; FiO₂ ≈ 0.28. Normal FiO₂ = 0.21 (21%), so increase = 0.28 - 0.21 = 0.07 (7%, choice B). Choice A (5%) yields PAO₂ ≈ 85 mmHg, insufficient; C (10%) overshoots to 121 mmHg; D (15%) is excessive (157 mmHg). A 7% rise (to 28% O₂) restores normoxemia without overcompensation, matching physiological needs under hypoventilation, making B correct.

Correct Answer: C

Rationale: In the upright lung, the ventilation/perfusion (V̇/Q̇) ratio is highest at the apex (choice C), due to gravity's effect. Ventilation is greater at the base (choice A is false), as dependent alveoli expand more. Perfusion is also greater at the base (choice B is false), with blood flow higher in lower zones (e.g., 4:1 base-to-apex ratio). V̇/Q̇ at the apex (≈3) exceeds the ideal 0.8, being ‘abnormally high' relative to perfect matching, while the base is ≈0.6. PO₂ is highest at the apex (choice D is false), as high V̇/Q̇ over-ventilates relative to perfusion; bases have lower PO₂ due to lower V̇/Q̇. pH follows PO₂ trends, not bases (choice E false). C's high V̇/Q̇ reflects physiological mismatch, driving regional gas exchange differences.