WBR0189
| Author | [[PageAuthor::Ogheneochuko Ajari, MB.BS, MS [1]]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Microbiology |
| Sub Category | SubCategory::Infectious Disease |
| Prompt | [[Prompt::A 45-year old HIV positive woman presents to the emergency department with complaints of headache, cough, shortness of breath and blurred vision. Physical examination reveals lymphadenopathy, tachypnea, rales and papilledema. A CT scan of the head was normal but CSF obtained revealed encapsulated organisms stained with India ink. Which of the following is correct concerning the causative agent?]] |
| Answer A | AnswerA::It is a dimorphic encapsulated yeast |
| Answer A Explanation | AnswerAExp::It is a dimorphic encapsulated yeast. This is a wrong option because crytococcus neoformans is the only encapsulated fungal pathogen and it is monomorphic. |
| Answer B | AnswerB::It is a monomorphic encapsulated yeast |
| Answer B Explanation | AnswerBExp::Monomorphic encapsulated yeast describes Cryptococcus neoformans and the only one that stains with India ink to produce the features seen in the vignette. |
| Answer C | AnswerC::It is a broad based budding yeast |
| Answer C Explanation | AnswerCExp::Broad based budding yeast is seen in Blastomyces dermatitidis and it is not encapsulated and the India ink is not typically used for its staining. |
| Answer D | AnswerD::It consists of branching septate hyphae |
| Answer D Explanation | AnswerDExp::It consists of branching septate hyphae describes Aspergillus. |
| Answer E | AnswerE::It can also be seen as spaghetti and meatballs on KOH stain |
| Answer E Explanation | AnswerEExp::It can also be seen as spaghetti and meatballs on KOH stain describes Malassezia furfur. |
| Right Answer | RightAnswer::B |
| Explanation | [[Explanation::The patient in this vignette has Cryptococcosis, caused by cryptococcus neoformans. Cryptococcus neoformans grows as a yeast (unicellular) and replicates by budding. C. neoformans makes hyphae during mating, and eventually creates basidiospores at the end of the hyphae before producing spores. Under host-relevant conditions, including low glucose, serum, 5% carbon dioxide, and low iron, among others, the cells produce a characteristic polysaccharide capsule. When grown as a yeast, C. neoformans has a prominent capsule composed mostly of polysaccharides. Microscopically, the India ink stain is used for easy visualization of the capsule in cerebral spinal fluid. The particles of ink pigment do not enter the capsule that surrounds the spherical yeast cell, resulting in a zone of clearance or "halo" around the cells. This allows for quick and easy identification of C. neoformans. Unusual morphological forms are rarely seen. Cryptococcal antigen from cerebrospinal fluid is thought to be the best test for diagnosis of cryptococcal meningitis in terms of sensitivity though it might be unreliable in HIV-positive patients.
The clues in this vignette are HIV, headache, blurred vision, encapsulated organism in CSF and the India ink stain. Also note that Cryptococcus neoformans is monomorphic. |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::Cryptococcus, WBRKeyword::Cryptococcus neoformans, WBRKeyword::HIV, WBRKeyword::Opportunistic infection, WBRKeyword::Fungus, WBRKeyword::Yeast |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |