Cryptococcus: "Soap bubbles in brain." India ink stain shows capsule. Meningitis in AIDS (CD4 <100).
Cryptococcus neoformans — encapsulated yeast from pigeon droppings; causes meningitis in AIDS patients with CD4 <100
The encapsulated yeast causing the most common AIDS-related meningitis
Found in pigeon droppings and soil. Large polysaccharide capsule = antiphagocytic. India ink: clear halo around yeast. Latex agglutination: detects capsular antigen in CSF (most sensitive). CSF: India ink+, very high opening pressure ("soap bubbles" on MRI). Treat: amphotericin B + flucytosine (induction 2 weeks), then fluconazole maintenance (lifelong in AIDS).
🍄 Fungi · AIDS
PCP: AIDS-defining illness at CD4 <200. "Bat-wing infiltrates." TMP-SMX treats AND prevents.
Pneumocystis jirovecii Pneumonia — now classified as fungus; AIDS-defining illness when CD4 <200; bilateral interstitial infiltrates
The AIDS-defining lung infection — bilateral infiltrates + high LDH
Cannot be cultured — diagnose by BAL + GMS (Gomori methenamine silver) stain showing cysts. Presentation: progressive dyspnea, dry cough, fever, hypoxia. CXR: bilateral interstitial infiltrates (bat wings). High LDH. Treat: TMP-SMX high dose × 21 days. Add steroids if PaO₂ <70 mmHg. Prophylaxis with TMP-SMX when CD4 <200.
The two-host life cycle that explains malaria symptoms and drug targets
P. falciparum: most severe — cytoadherence, cerebral malaria, no hypnozoites. P. vivax/ovale: tertian fever (48 hr), hypnozoites in liver → relapse — add primaquine. P. malariae: quartan (72 hr). Blood stage treatment: chloroquine (sensitive), artemisinin (resistant). Cerebral malaria: IV artesunate.
Giardia lamblia — fecal-oral via contaminated water; trophozoites attach to small intestine mucosa → malabsorption; treat with metronidazole
Most common intestinal parasitic infection in the US — foul-smelling fatty diarrhea
Cysts in contaminated water (streams, lakes — resistant to chlorine). Trophozoites attach to small intestine via ventral disc → malabsorption. Symptoms: foul-smelling fatty diarrhea (steatorrhea), bloating, no blood/mucus. Diagnosis: stool O&P, antigen test. Treat: metronidazole or tinidazole.
🍄 Fungi · Dimorphic
Dimorphic fungi: "Mold in cold, yeast in heat." Temperature-dependent — mold at room temp, yeast at body temp.
Dimorphic Fungi — grow as mold at room temperature (25°C) and switch to yeast at body temperature (37°C)
Fungi that change form to evade immune defenses at body temperature
Histoplasma capsulatum: Ohio/Mississippi river valleys, bird/bat droppings. Blastomyces: North America, Great Lakes, skin lesions + lung. Coccidioides: Southwest US (California, Arizona), spherules. Paracoccidioides: South America. All: inhale spores → convert to yeast at 37°C → pneumonia. Treat: itraconazole (mild-moderate), amphotericin B (severe/CNS).
Scabies — Sarcoptes scabiei mite burrows in stratum corneum; Type IV hypersensitivity reaction causes intense itching; treat with permethrin
The mite that causes intense itching — and why you must treat the whole household
Sarcoptes scabiei burrows in stratum corneum — especially between fingers, wrists, genitals. Intense pruritus (worse at night) — delayed-type hypersensitivity to mite proteins. Diagnosis: scraping + mineral oil → mites/eggs under microscope. Treat: topical permethrin (entire body from neck down); treat all household contacts simultaneously; wash all bedding.
🎓 Common Exam Questions
Q: What is the NTC helminth classification and give two clinical examples per category with treatments?
A: Nematodes (roundworms): Ascaris lumbricoides (fecal-oral, Loeffler syndrome during lung migration, intestinal obstruction), Hookworm (percutaneous penetration, iron-deficiency anemia), Strongyloides (autoinfection in immunocompromised — hyperinfection syndrome), Pinworm/Enterobius (scotch tape test), Trichinella (undercooked pork → muscle cysts). Treat with albendazole or mebendazole. Trematodes (flukes): Schistosoma (freshwater snail, S. haematobium → hematuria, S. mansoni → portal hypertension), Liver flukes (Clonorchis — raw fish, cholangiocarcinoma risk). Treat with praziquantel. Cestodes (tapeworms): Taenia solium (undercooked pork, neurocysticercosis — ring-enhancing lesions), Echinococcus granulosus (dogs, hydatid cysts in liver). Treat with praziquantel or surgery.
Q: Describe PCP — who gets it, presentation, diagnosis, and treatment.
A: Pneumocystis jirovecii Pneumonia — AIDS-defining illness when CD4 <200. P. jirovecii was once thought to be a protozoan but is now classified as a fungus. Cannot be cultured in vitro — diagnose by BAL (bronchoalveolar lavage) with GMS (Gomori methenamine silver) stain showing cup-shaped cysts or DFA staining. Presentation: progressive dyspnea over days-weeks, non-productive cough, fever, hypoxia (often out of proportion to CXR findings). CXR: bilateral diffuse interstitial infiltrates (bat-wing pattern). Labs: elevated LDH (marker of disease severity and response). Treatment: TMP-SMX (high dose) × 21 days. Add corticosteroids if PaO₂ <70 mmHg or A-a gradient >35 (reduces inflammation and improves survival). Prophylaxis: TMP-SMX 1 DS tablet daily when CD4 <200.
Q: What are the dimorphic fungi and where are they geographically endemic?
A: All dimorphic fungi: mold at 25°C (room temp), yeast at 37°C (body temp). Histoplasma capsulatum: Ohio and Mississippi River valleys; bird and bat droppings (cleaning chicken coops, spelunking). Causes pneumonia resembling TB; disseminates in AIDS. Blastomyces dermatitidis: North America, Great Lakes region; causes lung + skin (verrucous lesions) + bone infection. Coccidioides immitis: Southwest US (California, Arizona desert); spherules (not yeast) with endospores. Valley fever — flu-like illness, erythema nodosum. Most dangerous in HIV/pregnancy. Paracoccidioides brasiliensis: South America; "pilot's wheel" yeast appearance. All treated with itraconazole (mild-moderate) or amphotericin B (severe/disseminated).
Q: Describe the Plasmodium malaria life cycle and what distinguishes the four main species.
A: Anopheles mosquito injects sporozoites → liver (exoerythrocytic, clinically silent 1–4 weeks). Merozoites released → infect RBCs → trophozoites → schizonts → RBC rupture releases merozoites → fever spike coincides with rupture. P. falciparum: no dormant stage (no hypnozoites), infects all RBC ages, cytoadherence to capillaries → cerebral malaria, blackwater fever, most deadly. P. vivax/ovale: tertian fever (rupture every 48 hr), prefer reticulocytes, form hypnozoites in liver → relapse months later → must add primaquine to eradicate liver stage. P. malariae: quartan fever (72 hr cycle), prefers older RBCs, nephrotic syndrome complication. Treatment: chloroquine (chloroquine-sensitive areas), artemisinin-based combination therapy (ACT for falciparum), IV artesunate (severe/cerebral).
Q: What are the Toxoplasma transmission routes and clinical dangers in each at-risk population?
A: Transmission: (1) Cat feces — definitive host; oocysts sporulate in environment; ingestion contaminates food/water. (2) Undercooked meat (lamb, pork) containing tissue cysts. Healthy adults: usually asymptomatic, may have mono-like illness. At-risk populations: Pregnant women (primary infection during pregnancy) → congenital toxoplasmosis: classic triad = chorioretinitis + hydrocephalus + intracranial calcifications (periventricular); may also cause spontaneous abortion. AIDS patients (CD4 <100): reactivation of latent cysts → Toxoplasma encephalitis; ring-enhancing lesions on MRI with surrounding edema; multiple lesions usually at gray-white junction. Treatment: pyrimethamine + sulfadiazine + leucovorin (folinic acid prevents bone marrow suppression). Prophylaxis: TMP-SMX (also covers PCP) when CD4 <100.