A 45-year-old man with HIV (CD4 45/µL) from Bangalore presents with severe headache, neck stiffness, and altered sensorium. CSF shows elevated opening pressure, lymphocytic pleocytosis, low glucose, and India ink preparation reveals large encapsulated yeasts. Which property of the capsule is primarily responsible for virulence?
- A Melanin synthesis in the cell wall neutralising reactive oxygen species
- B Polysaccharide capsule (glucuronoxylomannan, GXM) inhibiting phagocytosis and complement activation ✓
- C Production of phospholipase B degrading pulmonary surfactant
- D Mannitol production as osmotic protectant in the CSF
Explanation
Cryptococcus neoformans virulence is multifactorial, but the polysaccharide capsule composed primarily of glucuronoxylomannan (GXM) and galactoxylomannan (GalXM) is the dominant virulence factor. GXM impairs phagocytosis by inhibiting neutrophil migration, interfering with complement C3b deposition and opsonisation, inducing T-cell anergy by causing APC dysfunction (inhibiting IL-12 production), and stimulating immunosuppressive IL-10. India ink reveals the capsule as a clear halo around the yeast. Melanin (produced by laccase using dopamine/catecholamines in the CNS) scavenges free radicals, explaining CNS tropism. Phospholipase B aids pulmonary establishment but the capsule is the primary immune-evasion virulence factor.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.