A 70-year-old female patient is admitted with pneumonia. She is exhibiting shortness of breath, a cough producing yellow sputum, and decreased breath sounds in the left lower lobe. A chest X-ray shows consolidation in the left lung. Her arterial blood gas (ABG) results indicate hypoxemia. What is the primary pathophysiological mechanism of V/Q mismatch in this patient with pneumonia?

A. In pneumonia, alveoli become filled with fluid, pus, and inflammatory cells, which impairs ventilation while perfusion to the affected alveoli continues. This leads to a ventilation-perfusion (V/Q) mismatch, where blood flows through poorly ventilated areas, causing hypoxemia. The V/Q ratio is effectively decreased in affected lung segments, which is the primary mechanism for low oxygen levels in pneumonia. B. In healthy lungs, ventilation and perfusion are well matched, resulting in efficient gas exchange. In pneumonia, alveolar consolidation disrupts this balance, so this scenario does not explain the patient’s hypoxemia. C. This describes a situation of dead space ventilation, such as in pulmonary embolism, where alveoli are ventilated but not perfused. This does not apply to pneumonia, where the problem is primarily blocked or impaired ventilation in consolidated areas. D. Decreased perfusion with normal ventilation also describes a form of dead space, not the hypoxemia seen in pneumonia. In pneumonia, perfusion generally continues while ventilation is impaired, causing the mismatch.