Ventilation-perfusion mismatch and diffusion impairment

Steps:

• In healthy lungs, alveolar PO2 exceeds venous PO2, allowing oxygen diffusion into blood until equilibrium near alveolar levels.
• Lung disease can cause collapsed alveoli (atelectasis), reducing ventilation and preventing full oxygenation of blood.
• Slow diffusion across thickened alveolar-capillary membranes, as in fibrosis, limits oxygen transfer, lowering pulmonary venous PO2.
• Arterial PO2 being lower than alveolar is normal due to mixed venous blood, not a disease explanation for the discrepancy.

Why C is correct:

• Per Fick's law of diffusion, impaired ventilation (option 1) and reduced diffusion rate (option 3) directly lower oxygen equilibration in blood, matching the observed venous-alveolar PO2 gradient.

Why the others are wrong:

• A includes option 2, which describes normal physiology, not a pathological cause.
• B omits diffusion impairment and relies on normal arterial-alveolar difference.
• D excludes ventilation issues, focusing only on normal and diffusion factors.

Final answer: C