Correct!
Answer: 3. Nocardia asteroides brain abscess
The brain MRI shows a ring-enhancing lesion related to the right cerebellopontine angle, consistent with a brain abscess (Figure 4).
Figure 4: Axial contrast-enhanced T1-weighted images show a ring enhancing lesion in the right cerebropontine angle (arrows), consistent with a brain abscess.
While tumor can present in this fashion, brain abscess formation caused by Nocardia asteroides is a known complication in patients with pulmonary alveolar proteinosis and is therefore the most likely diagnosis among the options presented. Strokes usually present with a different pattern on MRI. Toxoplasmosis typically presents with lesions in the basal ganglia, often in patients with AIDS and severely depressed CD4 counts, rather than as a posterior fossa abscess. Both Mycobacterium tuberculosis and certain fungal organisms, notably Cryptococcus neoformans and Coccidioidomycosis imitis, may produce basilar meningitis, but this process usually results in more widespread dural enhancement than is seen on these images.)
Diagnosis: Pulmonary alveolar proteinosis with Nocardia asteroides brain abscess
Which of the following regarding pulmonary alveolar proteinosis is false?