Pathologic proteolysis is frequently the initiating cause of the defibrination syndrome:

Correct Answer: A

Rationale: Abruptio placentae (A) placental detachment triggers defibrination (DIC) via tissue factor release, consuming fibrinogen (e.g., <100 mg/dL), causing bleeding. Liver (B), surgery (C), cancers (D) contribute, but abruptio's acute proteolysis is frequent, key in nursing for FFP and obstetric emergency care.

Correct Answer: B

Rationale: Airway resistance is inversely proportional to the fourth power of the radius (Poiseuille's law), but total cross-sectional area also determines resistance across the respiratory tree. The trachea has a large diameter (~2 cm), but as a single tube, its cross-sectional area is limited (e.g., ~3-4 cm²). Bronchioles, though individually small (~1 mm), number in the thousands by the terminal stage, yet their collective area is still less than the alveoli. The alveoli, numbering ~300 million in adult lungs, have a tiny individual diameter (~0.2 mm) but an enormous total cross-sectional area (~70-100 m² during inspiration), vastly exceeding other structures. This massive area reduces airflow velocity and resistance to negligible levels at the alveolar level, where gas exchange occurs by diffusion, not flow. While resistance is highest in medium-sized bronchi due to turbulent flow, the alveoli's collective area minimizes overall resistance to air movement, making them the site of lowest resistance, contrasting with the trachea or bronchioles, which handle bulk airflow with higher resistance despite larger individual diameters.

Correct Answer: B

Rationale: The percentage of CO2 in a structure reflects its PCO2, tied to metabolic production and gas exchange. Pulmonary arteries carry deoxygenated blood from the right heart to the lungs, with a PCO2 of ~45-46 mmHg (venous blood), the highest among options, as it's loaded with CO2 from systemic tissues. Alveolar air has a PCO2 of ~40 mmHg, equilibrated with arterial blood after CO2 diffuses out during respiration. Pulmonary veins, post-gas exchange, carry oxygenated blood with a PCO2 of ~40 mmHg, matching arterial levels. Interstitial fluid's PCO2 varies but approximates venous blood (~45 mmHg) or slightly less, depending on local metabolism, though it's not a standard respiratory measure. Systemic arteries, not listed, also have ~40 mmHg. Pulmonary arteries stand out with the highest CO2 due to their role in transporting metabolically produced CO2 to the lungs for excretion, before alveolar ventilation lowers it, making them the site of peak CO2 concentration.

Correct Answer: C

Rationale: Functional residual capacity (FRC) is the lung volume after a normal expiration (~2.5-3 L), the resting state where lung inward recoil balances chest wall outward recoil. It's the lung's resting volume, but also reflects the thorax's state, though these aren't mutually exclusive options. The key true statement is that at FRC, intra-alveolar pressure equals atmospheric pressure (~760 mmHg), as no airflow occurs (P = 0 gradient), and muscles are relaxed. Intrapleural pressure (IPP) at FRC is negative (~-4 mmHg, 756 mmHg), not more than atmospheric (760 mmHg), due to recoil forces keeping lungs expanded rising above atmospheric only in pathology (e.g., pneumothorax). Lung compliance varies with volume, not lowest at FRC, which is a mid-range point. The equality of alveolar and atmospheric pressure at FRC is a fundamental respiratory principle, ensuring stability at rest, making it the standout true statement.

Correct Answer: D

Rationale: When inspiratory muscles (diaphragm, external intercostals) relax, as after a normal expiration, the lungs reach functional residual capacity (FRC, ~2.5-3 L), the resting volume where lung inward recoil balances chest wall outward recoil. Vital capacity (VC, ~4-5 L) is the maximum exhailable volume after maximal inhalation, requiring active inspiration, not relaxation. Residual volume (RV, ~1-1.5 L) is the air left after maximal expiration, beyond relaxed expiration. Minimal volume' isn't a standard term but might imply RV or zero (collapsed lungs, not natural). FRC is the equilibrium state at rest, with intra-alveolar pressure equaling atmospheric (~760 mmHg), no airflow, and muscles inactive, distinguishing it as the volume post-relaxation, critical for baseline gas exchange and respiratory mechanics.