Regarding pulmonary vascular resistance, which one of the following is true?

Correct Answer: C

Rationale: Pulmonary vascular resistance (PVR) reflects opposition to blood flow in the pulmonary circulation, influenced by lung volume and vessel mechanics. At high lung volumes (near TLC), extra-alveolar vessels stretch and narrow, and alveolar capillaries compress, increasing PVR. At low volumes (near RV), these vessels are less stretched and more patent, lowering PVR, though this isn't the queried truth. The true statement is that increased PVR can lead to right heart failure, as seen in conditions like pulmonary hypertension or fibrosis, where elevated resistance overworks the right ventricle, causing cor pulmonale. PVR isn't measured by routine pulmonary function tests (e.g., spirometry), which assess airflow and volumes, not vascular pressures cardiac catheterization is required instead. The link to right heart failure is critical, as chronic high PVR elevates pulmonary artery pressure, straining the heart's ability to pump against it, a key pathophysiological consequence distinguishing this option as true amid the others' inaccuracies.

Correct Answer: D

Rationale: Vital capacity (VC) is the maximum air exhaled after maximal inhalation, measured as inspiratory reserve volume (IRV, ~2-3 L), tidal volume (VT, ~0.5 L), and expiratory reserve volume (ERV, ~1-1.5 L), totaling ~4-5 L via spirometry. Sum of all lung volumes' is total lung capacity (TLC, ~6 L), including RV (~1-1.5 L), not VC. VT plus RV' (~2 L) omits IRV and ERV, far below VC. IRV plus ERV' (~3-4 L) excludes VT, underestimating VC. VC (IRV + VT + ERV) captures the full expirable volume, a key respiratory health metric, distinct from TLC or partial sums, reflecting the lung's functional capacity for deep breathing, widely used in clinical assessment.

Correct Answer: C

Rationale: Lung compliance (C = ΔV / ΔP) measures volume change per pressure unit, correctly defined. It's not maximal during quiet breathing (VT ~500 ml) compliance peaks at moderate volumes, declining near TLC due to stiffness, making that false but not the exception here. The statement more surface tension, more compliance' is incorrect high tension (e.g., no surfactant) reduces compliance by stiffening alveoli, per Laplace's law (P = 2T/r), as in RDS, while low tension increases it. Fibrosis decreases compliance via collagen stiffening true. Emphysema (not listed) raises compliance by elasticity loss. The surface tension error contradicts physiology compliance falls with rising tension, making it the exception, as surfactant's role is to enhance compliance by reducing tension, critical for understanding lung mechanics.

Correct Answer: D

Rationale: Pulmonary function tests (PFTs) differ by disease. Obstructive (e.g., COPD) reduces airflow, lowering FEV1 (<80% predicted) from airway narrowing, with increased residual volume (RV) and total lung capacity (TLC). Restrictive (e.g., fibrosis) limits expansion, also reducing FEV1 due to less volume, with decreased RV and TLC. Decreased FEV1 fits both obstructive from obstruction, restrictive from capacity loss. Decreased RV is restrictive-only (obstructive increases RV). Normal or above TLC fits obstructive (restrictive lowers it). Vascular resistance (not a PFT) rises in fibrosis, not decreases. Decreased FEV1's commonality reflects exhalation impairment across types, making it the shared value, critical for broad PFT interpretation.

Correct Answer: C

Rationale: The greenhouse effect occurs when greenhouse gases (e.g., CO2, CH4, H2O) absorb and re-emit infrared radiation from Earth's surface, trapping heat in the atmosphere. Ozone absorbs UV (~200-310 nm), protecting life, not driving greenhouse warming false. Cloud reflection (albedo ~0.3) cools by deflecting solar radiation, not trapping heat false. Conduction transfers heat to air, but radiation dominates atmospheric retention false. Absorption/re-emission of infrared (~4-100 μm) by gases increases atmospheric temperature (e.g., ~33°C warmer than without), per radiative forcing models (e.g., IPCC), making it the primary process, central to climate dynamics.