A 4-month-old infant, not previously in care, presents with a one-week history of fever, tachypnea, and significant respiratory distress. A chest x-ray reveals diffuse bilateral interstitial infiltrates, and oxygen saturation is 85% on room air. A rapid HIV test on the mother is positive. What is the most common opportunistic infection responsible for this clinical picture in an infant with undiagnosed perinatal HIV?