Question 287

Question:

A 31 year old man presents to the emergency department with increasing shortness of breath for the past three days, especially when climbing stairs or walking for a few minutes. He reports having fevers, chills, night sweats, and a non-productive cough. A chest radiograph demonstrates bilateral interstitial infiltrates. Laboratory studies reveal that the patient is HIV positive and has a CD4 count of 150. Additionally, the patient’s oxygen saturation is decreased to 92% on 3L of O₂. What is the most likely causative organism of this acute illness?

Correct Answer: C.  Pneumocystis jiroveci

Patients with CD4 counts below 200 in the setting of HIV infection are considered to have AIDS despite viral titers. Immuno-compromised patients have increased susceptibility to opportunistic infections which includes several of the possible answer choices. However, only Pneumocystis carinii (choice C) fits the clinical picture. PCP is usually found in HIV patients with CD4 counts < 200 and the disease is characterized by constitutional symptoms, fever, night sweats, dyspnea on exertion, nonproductive cough, interstitial pattern on CXR, decreased PaO2, increased A-a gradient, and increased serum LDH.

Cytomegalovirus (choice A) is an opportunistic infection that has clinical manifestations of retinitis, esophagitis, colitis, hepatitis, and neuropathies.

Pneumococcus (choice B) is still the most common cause of pneumonia in immuno-compromised adults; however, it is characteristically a lobar pneumonia with discrete areas of consolidation on CXR as opposed to interstitial as described in this patient.

Tuberculosis (choice D) is unlikely in the setting of a non-productive cough and interstitial patterned infiltrates on CXR.

Histoplasmosis (choice E) has a similar clinical presentation as tuberculosis and would be equally unlikely in the setting of a non-productive cough and interstitial patterned infiltrates on CXR.