This patient has systemic signs of infection (eg, fever, confusion, tachycardia) and manifestations that localize the infection to the lungs (hypoxia, cough, right lower lobe crackles).  Although these findings are classically associated with community-acquired pneumonia (CAP), the diagnosis of CAP requires a pulmonary infiltrate on chest imaging.

However, patients who are immunosuppressed (eg, prednisone use) are often unable to generate a strong enough cytokine response to recruit significant inflammatory cells to areas of pulmonary infection, which frequently results in minimal or no alveolar infiltrate on initial chest x-ray.  This may be exacerbated by a dehydrated state (eg, dry mucous membranes), which limits fluid extravasation into the lung tissue.  Therefore, immunosuppressed patients with likely CAP who have an initial negative chest x-ray should undergo more sensitive pulmonary imaging with high-resolution CT scan of the chest.  This test better visualizes the pulmonary parenchyma and can typically identify subtle pulmonary infiltrates, thereby confirming the diagnosis of CAP.

(Choice A)  Bronchoscopy with bronchoalveolar lavage can help establish a microbial diagnosis in patients with recalcitrant pulmonary infection.  Because this is an invasive test, it would not be the best next step in this case; additional pulmonary imaging should be performed first.

(Choice B)  CT angiography of the chest can diagnose pulmonary embolism, which can present with normal chest x-ray and pulmonary findings.  However, confusion and several days of fever make this diagnosis less likely.  Although CT angiography of the chest provides some visualization of the lung parenchyma, the contrast bolus is primarily timed to visualize the lung vasculature; a CT scan (vs CT angiography) of the chest provides much better visualization of lung fields and is the preferred test for suspected pneumonia when chest x-ray is unrevealing.

(Choice D)  CT scan of the head is generally indicated in those with suspected CNS infection, stroke, and/or tumor.  Although this patient is confused, the presence of hypoxia, cough, and right lower lobe crackles suggests an infection in the lungs; additional pulmonary imaging would be the best next step.

(Choice E)  Echocardiography can identify congestive heart failure (CHF).  Although chest x-ray can be normal in some patients, CHF exacerbation is usually associated with bilateral (not unilateral) lung findings and lower extremity edema; furthermore, fever would be atypical.  Echocardiography can also identify endocarditis with pulmonary septic emboli; however, pulmonary complications typically occur with right-sided endocarditis, which is seen primarily in those who use intravenous drugs (unlike this patient).  This patient's 2/6 systolic ejection murmur is likely due to his known aortic stenosis.

(Choice F)  Although this patient is confused and has fever, he does not have meningeal signs (eg, neck stiffness) or headache.  Given the presence of hypoxia, right lower lobe crackles, and cough, a pulmonary disease is much more likely than meningitis.  Therefore, additional pulmonary imaging should be obtained before considering an invasive procedure such as lumbar puncture.

Educational objective:
Patients with community-acquired pneumonia usually have a pulmonary infiltrate on initial chest x-ray.  However, those who are immunosuppressed may have normal pulmonary imaging despite signs and symptoms that localize the infection to the lungs.  These patients should undergo additional pulmonary imaging with high-resolution CT scan of the chest to identify pulmonary infiltrate.