The usual presenting complaint in Hodgkin's disease is:

Correct Answer: B

Rationale: Failed to generate a rationale of 500+ characters after 5 retries.

Correct Answer: C

Rationale: PCO2, or partial pressure of carbon dioxide, reflects the concentration of CO2 in blood, highest where metabolic waste accumulates and lowest where gas exchange removes it. The superior vena cava (SVC) carries deoxygenated blood from the upper body back to the heart, rich in CO2 from tissue metabolism, typically with a PCO2 of 45-46 mmHg, making it the highest among the options. Pulmonary veins carry oxygenated blood from the lungs to the heart after CO2 is offloaded in the alveoli, so their PCO2 is low (around 40 mmHg, arterial level). The midportion of pulmonary capillaries is where gas exchange occurs, transitioning from high venous PCO2 to lower arterial levels, averaging less than the SVC. Carotid bodies are chemoreceptors sensing blood gases, not a blood-containing structure, but even arterial blood they monitor has a PCO2 of about 40 mmHg. The SVC, as a major venous return vessel, consistently has the highest PCO2 due to its role in collecting metabolically produced CO2 before pulmonary gas exchange, distinguishing it from the other options.

Correct Answer: B

Rationale: Atmospheric air contains approximately 21% oxygen (O2) and 0.04% carbon dioxide (CO2), meaning O2's percentage is much higher than CO2's, reflecting their natural abundances. Solubility in blood, governed by Henry's law, depends on the solubility coefficient of each gas. O2 has a solubility coefficient of about 0.024 ml O2/mmHg/L blood, while CO2's is much higher at 0.51 ml CO2/mmHg/L blood over 20 times greater. This means CO2 is far more soluble in solution than O2, despite its lower atmospheric presence. In blood, O2 relies heavily on hemoglobin binding (98% of transport), with only ~1-2% dissolved, whereas CO2 is transported dissolved (~7%), as bicarbonate (~70%), and bound to hemoglobin (~23%), leveraging its high solubility. Thus, O2's higher atmospheric percentage contrasts with its lower solubility compared to CO2, driving distinct transport mechanisms critical for respiration and acid-base balance.

Correct Answer: B

Rationale: Surfactant, a phospholipid-protein mix from type II alveolar cells, lines alveoli and reduces surface tension of the fluid layer, preventing collapse and easing lung expansion. Normally, water's high surface tension (~72 dynes/cm) pulls alveolar walls inward, but surfactant lowers it (to ~5-10 dynes/cm), stabilizing smaller alveoli per Laplace's law (P = 2T/r). It doesn't increase pleural pressure, which remains negative (~-4 mmHg at rest) to keep lungs expanded surfactant affects intra-alveolar dynamics, not pleural space. It doesn't directly decrease alveolar pressure (typically atmospheric at rest, ~760 mmHg); that's a muscle-driven effect. It makes inspiration easier, not harder, by reducing the work needed to overcome tension, countering collapse tendencies. Pneumothorax relates to pleural breach, not surfactant. Reducing surface tension is the true function, critical for neonatal lung maturation and preventing atelectasis, distinguishing it from pressure or effort-related misconceptions.

Correct Answer: A

Rationale: Work of breathing (WOB) is the energy required to overcome elastic (compliance) and resistive (airway) forces during ventilation. Lung compliance (C = ΔV / ΔP) measures lung stretchability; low compliance (stiff lungs) increases pressure needed for a given volume, raising WOB. Thus, WOB is inversely related to compliance when C decreases, WOB increases, as in fibrosis. During exercise, WOB rises with higher ventilation rates and volumes, not remaining constant. Airway resistance (R) directly affects WOB; higher R (e.g., asthma) increases effort, contradicting not affected.' In pulmonary fibrosis, stiff lungs (low compliance) elevate WOB, not reduce it, unlike emphysema where high compliance might lower elastic work but raise resistive work. The inverse compliance relationship is fundamental, as WOB = ∫P dV, where pressure (P) rises as compliance falls, making this the correct statement reflecting respiratory mechanics.