A 28-year-old woman with HIV (CD4 count 150 cells/µL) develops acute-onset dyspnea, dry cough, and chest discomfort. Chest X-ray shows bilateral interstitial infiltrates. Arterial blood gas reveals PaO₂ 65 mmHg on room air and elevated A–a gradient (48 mmHg). Which single finding best distinguishes Pneumocystis jirovecii pneumonia (PCP) from cytomegalovirus (CMV) pneumonitis in this patient?

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