Final answer:
An HIV patient with a CD4 count <200 cells/µL, hypoxia, CXR with interstitial infiltrates, and negative cultures for PCP suggests AIDS with a probable opportunistic lung infection such as PCP. Further investigation with induced sputum, BAL, and possibly PCR is necessary for confirmation. Treatment typically involves Bactrim or alternative antimicrobials.
Step-by-step explanation:
The patient described has a CD4 count less than 200 cells/µL, is hypoxic, and has interstitial infiltrates on chest X-ray (CXR), as well as negative cultures for Pneumocystis pneumonia (PCP). Given these findings, the patient likely has AIDS with an opportunistic lung infection. Despite negative cultures, the clinical picture is highly suggestive of PCP. It is possible that the culture was a false negative, which can occur. The next steps in the workup would include additional diagnostic tests such as induced sputum or bronchoalveolar lavage (BAL) for special stains and PCR to test for Pneumocystis jirovecii, as well as assessing for other potential causes of the patient's symptoms with a broad differential diagnosis, given the significant immunocompromise.
Diagnosis and Treatment of PCP
PCP treatment typically involves the administration of antimicrobial therapy, like Bactrim (trimethoprim-sulfamethoxazole), but alternatives include atovaquone or pentamidine in case of sulfa drug allergies or adverse reactions.