In an asthmatic attack, which of the following is expected?

Correct Answer: C

Rationale: Asthma is an obstructive airway disease characterized by reversible bronchoconstriction, inflammation, and mucus production. During an asthmatic attack, narrowed airways increase resistance, particularly during expiration, when dynamic compression exacerbates airflow limitation, producing wheezing most prominent on expiration, not inspiration. The work of breathing increases significantly as patients struggle against this resistance and reduced airflow, requiring greater effort from respiratory muscles like the diaphragm and intercostals to maintain ventilation. Bronchodilators (e.g., albuterol) are the mainstay of treatment, relaxing bronchial smooth muscle to relieve constriction, so they are not contraindicated. Forced expiratory volume in 1 second (FEV1) decreases during an attack due to obstruction, not increases, as airflow is impeded. The increased work of breathing is a consistent expectation, reflecting the physiological burden of overcoming narrowed airways and trapped air, distinguishing it from the incorrect options that misalign with asthma's acute presentation.

Correct Answer: B

Rationale: Mixed expired PCO2 (PECO2) reflects CO2 in exhaled air, diluted by dead space ventilation. Given dead space (VD) is one-third of tidal volume (VT), VD/VT = 1/3. The Bohr equation relates physiological dead space to CO2: VD/VT = (PaCO2 - PECO2) / PaCO2, where PaCO2 (arterial PCO2) is 45 mmHg. Substituting: 1/3 = (45 - PECO2) / 45. Solving: 45 / 3 = 45 - PECO2, so 15 = 45 - PECO2, and PECO2 = 45 - 15 = 30 mmHg. This assumes physiological dead space equals anatomic here, as no alveolar dead space is specified. Intuitively, if one-third of each breath doesn't participate in gas exchange (PCO2 ~0 mmHg in inspired air), the expired CO2 is diluted from arterial levels (45 mmHg) to two-thirds strength (30 mmHg), matching the calculation. Options like 45 mmHg imply no dead space effect, while 20 mmHg overestimates dilution. Thus, 30 mmHg aligns with the given ratio and respiratory physiology principles.

Correct Answer: D

Rationale: Pulmonary fibrosis, a restrictive disease, stiffens lungs with interstitial scarring. The FEV1/FVC ratio is normal or high (≥80%) because both FEV1 and FVC decrease proportionally, unlike obstructive diseases. Increased pulmonary vascular resistance occurs as fibrosis compresses capillaries, raising resistance. Peak expiratory flow (PEF), when corrected for reduced lung volume, can remain normal or above, as airflow isn't obstructed, just limited by volume. However, residual volume (RV) decreases in pulmonary fibrosis (e.g., from 1.5 L to <1 L) due to stiff lungs limiting all volumes, contrasting with obstructive diseases (e.g., COPD) where RV increases from air trapping. Increased RV doesn't fit fibrosis's restrictive pattern, where reduced elasticity shrinks residual air, not expands it, making this the inconsistent value among the set, reflecting the disease's impact on lung mechanics.

Correct Answer: D

Rationale: In a healthy upright individual, regional ventilation varies due to gravity and pleural pressure gradients. At rest, intrapleural pressure (IPP) is more negative at the apex (~-10 cm H2O) than the base (~-2.5 cm H2O) due to lung weight, making apices less compliant (stiffer) and bases more compliant (easier to expand). During quiet breathing (VT = 400 ml, RR = 12/min), the diaphragm's downward pull preferentially ventilates the lower zones, where compliance is higher and initial volume lower, allowing greater volume change (ΔV). Studies (e.g., West) show lower lobes receive ~4 times more ventilation per unit volume than apices. Thus, ventilation is greatest in the lower zones, followed by middle, then upper (Lower > Middle > Upper). Equal ventilation ignores gravity's effect, and Upper > Middle > Lower reverses the gradient. For this boy, VA = (400 - ~120 ml VD) × 12 = ~3.36 L/min, distributed predominantly to the base, making this the best description.

Correct Answer: C

Rationale: Residual volume (RV) is the air left in the lungs after maximal expiration (~1-1.5 L), preventing collapse and measurable only indirectly (e.g., helium dilution). It's not just after tidal expiration that's FRC (~2.5-3 L), including RV plus ERV, making that false. In COPD, RV increases (e.g., to 2-3 L) due to air trapping from obstructed airways and lost elasticity, not decreases. In pulmonary fibrosis, a restrictive disease, RV decreases (e.g., to <1 L) as stiff lungs limit all volumes, including residual air, making this correct. RV doesn't stay constant lifelong aging and disease alter it but in health, it's relatively stable, though this isn't the strongest fit. Fibrosis's reduction reflects restricted lung expansion, contrasting with obstructive hyperinflation, making it the accurate statement amid options misaligned with RV's physiological behavior.