Immunocompromised patient with confusion and fever; CSF specimen cultured. Birdseed agar shows brown colonies; India ink shows encapsulated budding yeast with halo; Urea test is pink. Which organism is suspected?

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The key idea is recognizing cryptococcal meningitis from lab clues that point to Cryptococcus neoformans. In an immunocompromised patient with meningitis, a CSF culture showing a yeast with a prominent capsule is classic. India ink highlights that capsule as a halo around the budding yeast, a hallmark of Cryptococcus. The pigment on birdseed agar—brown colonies—comes from melanin production by Cryptococcus species, especially C. neoformans, which helps differentiate it from other yeasts. A positive urea (urease) test turning pink fits with Cryptococcus, which often exhibits urease activity.

Putting these clues together, the organism most consistent with encapsulated yeast seen with India ink, brown pigment on birdseed due to melanin, and urease positivity is Cryptococcus neoformans. While Cryptococcus gattii can produce similar results, it is more associated with disease in otherwise healthy individuals and different epidemiology; the immunocompromised context plus the classic capsule and melanin indicators strongly point to C. neoformans. Histoplasma capsulatum would typically present as intracellular yeasts within macrophages and lacks the capsule and pigmented melanin on birdseed, and Aspergillus fumigatus is a mold, not a encapsulated yeast seen on India ink.