All Sample Questions

Opportunistic Infection 1

Opportunistic infection


Vignette: A 45-year-old man comes to the clinic with complaints of fever, non-productive cough, and generalized malaise for a week. Chest X-ray reveals diffuse interstitial infiltrates. He has a history of HIV infection and his most recent CD4 count was 50 cells/mm3.

Which of the following is the most likely pathogen causing this patient's symptoms?


A) Streptococcus pneumoniae

B) Mycobacterium tuberculosis

C) Pneumocystis jirovecii

D) Legionella pneumophila

E) Haemophilus influenzae


C) Pneumocystis jirovecii


This patient's presentation is consistent with Pneumocystis pneumonia (PCP), which is caused by Pneumocystis jirovecii. PCP is a common opportunistic infection in HIV-infected individuals when their CD4 count falls below 200 cells/mm3. It presents with fever, nonproductive cough, and diffuse interstitial infiltrates on chest radiograph. Other pathogens like streptococcus pneumoniae (Choice A), Mycobacterium tuberculosis (Choice B), legionella pneumophila (Choice D), and Haemophilus influenzae (Choice E) can also cause pneumonia in immunocompromised individuals, but the patient's low CD4 count and radiographic findings strongly suggest PCP. Prophylaxis with trimethoprim-sulfamethoxazole is recommended when CD4 count falls below 200 cells/mm3 to prevent PCP.


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