Which person would be expected to have the largest PAO2-PaO2 gradient? (A stands for alveolar and a-stands for arterial)

Correct Answer: B

Rationale: The PAO2-PaO2 gradient (alveolar-arterial O2 difference) is normally ~5-10 mmHg due to efficient diffusion. In pulmonary fibrosis, thickened alveolar walls impair O2 transfer, dropping PaO2 (e.g., to 60 mmHg) while PAO2 (~100 mmHg, per alveolar gas equation) stays closer to normal, widening the gradient (e.g., 40 mmHg). During exercise, a normal person's ventilation and perfusion match, keeping the gradient small despite higher O2 use. Anemia lowers O2-carrying capacity, not diffusion, so PaO2 ≈ PAO2, maintaining a normal gradient. At 5000 m, low atmospheric PO2 reduces both PAO2 and PaO2 (e.g., 50 vs. 45 mmHg), keeping the gradient small. Fibrosis's diffusion barrier creates the largest gradient, as O2 struggles to cross, a hallmark of restrictive disease affecting gas exchange, unlike other scenarios.

Correct Answer: D

Rationale: In an upright healthy child, gravity gradients affect ventilation. At rest (VT = 400 ml, RR = 12/min), intrapleural pressure is more negative at the apex (~-10 cm H2O) than base (~-2.5 cm H2O), making apices less compliant and bases more so. Diaphragm movement ventilates lower zones most, where compliance and volume change peak. Studies (e.g., West) show lower lobes get ~4 times more ventilation per unit volume than apices. Thus, ventilation is Lower > Middle > Upper. Equal distribution ignores gravity; Upper > Middle > Lower reverses it. VA = (400 - ~120 ml VD) × 12 ≈ 3.36 L/min, mostly basal, making this the best description of regional ventilation in a healthy boy.

Correct Answer: B

Rationale: Melting of polar ice reducing albedo is a positive feedback mechanism. Ice's high albedo (~0.6-0.9) reflects sunlight; melting exposes darker ocean/land (~0.1), absorbing more heat, accelerating warming and further melting amplifying climate change (e.g., Arctic amplification, ~2-3× global rate). Increased cloud cover (negative feedback) reflects sunlight (~50 W/m²), cooling Earth false. Vegetation growth absorbs CO2, mitigating warming (negative) false. Cooling oceans enhance CO2 solubility, reducing greenhouse effect (negative) false. Ice-albedo feedback's self-reinforcing loop, per climate models (e.g., IPCC), distinguishes it as positive, intensifying temperature rise, a critical dynamic in polar regions.

Correct Answer: D

Rationale: The pterygopalatine fossa hosts the maxillary artery (A, via pterygomaxillary fissure), maxillary nerve (B, via foramen rotundum), and pterygopalatine ganglion. The ganglion receives preganglionic parasympathetic fibers from the facial nerve (VII) via the greater petrosal nerve (C), and postganglionic sympathetic fibers from the deep petrosal nerve (carotid plexus), not the lesser petrosal (D), which supplies the parotid via IX. D is incorrect sympathetic input is via deep petrosal, making it the exception.

Correct Answer: B

Rationale: Simple cuboidal epithelium, a single layer of cube-shaped cells, is specialized for secretion and absorption. It lines thyroid gland follicles (B), where it secretes thyroid hormones, and other structures like kidney tubules. The intestine (A) has simple columnar epithelium for nutrient absorption, with taller cells and microvilli. The epidermis (C) is stratified squamous, designed for protection, not a single layer. The trachea (D) features pseudostratified columnar epithelium with cilia and goblet cells for mucus clearance. B is correct thyroid follicles match the description, unlike the others, which serve different functions requiring distinct epithelial types.