- At 25°C, the culture of the causative organism would most likely demonstrate which of the following forms?
Answer: Mold form with arthroconidia
The geographic location (Southwestern US) and symptoms are highly suggestive of coccidioidomycosis ('valley fever'), caused by Coccidioides immitis. This is a dimorphic fungus. In the environment and on culture at room temperature (25°C), it grows as a mold consisting of hyphae that fragment into barrel-shaped arthroconidia. In the host's body (at 37°C), it exists as large spherules containing endospores. Cigar-shaped yeasts are characteristic of Sporothrix schenckii in its yeast form. Pseudohyphae are seen with Candida.
- The silver stain is expected to reveal which of the following characteristic structures?
Answer: Saucer-shaped cysts
This patient has a classic presentation of Pneumocystis pneumonia (PCP), caused by Pneumocystis jirovecii, in the setting of advanced AIDS. Diagnosis is typically made by identifying the organism in respiratory specimens. Methenamine silver stain is used to visualize the cyst form of P. jirovecii, which appears as characteristic oval, indented, or saucer-shaped structures. Cigar-shaped yeasts are characteristic of Sporothrix schenckii. Arthroconidia are the infectious form of Coccidioides. Germ tubes are formed by Candida albicans.
- The severe neurologic manifestations of this infection are primarily attributed to which pathogenic mechanism?
Answer: Sequestration of infected erythrocytes in cerebral microvasculature
The patient has severe, complicated malaria, most likely due to Plasmodium falciparum, which is known for high parasitemia and severe manifestations. A key virulence mechanism of P. falciparum is the expression of proteins on the surface of infected erythrocytes that cause them to adhere to the endothelial lining of capillaries. This sequestration in the microvasculature of vital organs, including the brain, leads to obstruction, ischemia, and inflammation, causing complications like cerebral malaria. The other options describe mechanisms for different pathogens: capsule (Cryptococcus), neurotoxin (Clostridium tetani), and granulomas (Mycobacterium tuberculosis or Taenia solium).
- This patient's condition is most likely caused by a parasite transmitted by which of the following vectors?
Answer: Reduviid bug
The clinical presentation of dilated cardiomyopathy and megaesophagus in a patient from South America (Bolivia) is highly characteristic of chronic Chagas disease, caused by the protozoan Trypanosoma cruzi. This parasite is transmitted to humans through the feces of the reduviid bug (also known as the 'kissing bug'). The sandfly transmits Leishmania, the tsetse fly transmits Trypanosoma brucei (African sleeping sickness), and the Aedes mosquito transmits viruses like dengue and Zika.
- Which of the following organisms is the most likely cause of this patient's condition?
Answer: Aspergillus fumigatus
This patient's presentation with a fungal ball (aspergilloma) within a pre-existing lung cavity (from prior tuberculosis) is classic for Aspergillus fumigatus infection. The microscopic finding of septate hyphae with acute-angle (45-degree) branching is characteristic of Aspergillus. Mucor species have nonseptate hyphae with right-angle branching. Pneumocystis jirovecii is a yeast-like fungus that causes diffuse pneumonia, not fungal balls, in immunocompromised hosts. Blastomyces dermatitidis is a dimorphic fungus that appears as broad-based budding yeast in tissue samples.
- Which of the following microscopic findings is most likely to be seen in the biopsy specimen?
Answer: Large yeasts with a single, broad-based bud
The patient's geographic location (Great Lakes region, Wisconsin), combined with pulmonary disease and characteristic wart-like (verrucous) skin lesions, points to blastomycosis. The causative agent, Blastomyces dermatitidis, is a dimorphic fungus. In tissue specimens, it is identified by its characteristic large, round yeast form with a single, broad-based bud. Small yeasts within macrophages are seen in histoplasmosis. Encapsulated yeasts are seen in cryptococcosis. Spherules are seen in coccidioidomycosis.
- Which of the following microscopic findings is most characteristic of the causative organism?
Answer: Pseudohyphae and budding yeasts
The patient has oral candidiasis (thrush), a common opportunistic infection in individuals with advanced HIV. The causative agent, Candida albicans, is a dimorphic fungus that can exist as both a budding yeast and in a filamentous form. On microscopic examination of clinical specimens, it is characteristically seen as a combination of budding yeasts and pseudohyphae. Spherules with endospores are characteristic of Coccidioides. Broad-based budding yeasts are seen with Blastomyces. Nonseptate hyphae with right-angle branching are characteristic of Mucorales.
- Chronic infection with this organism is a major risk factor for which of the following malignancies?
Answer: Squamous cell carcinoma of the bladder
The patient's history, symptoms, and the finding of eggs with a terminal spine are characteristic of urinary schistosomiasis, caused by Schistosoma haematobium. The adult worms live in the venous plexus of the bladder and release eggs that become trapped in the bladder wall. The resulting chronic inflammation, granuloma formation, and fibrosis are a well-established risk factor for developing squamous cell carcinoma of the bladder. Transitional cell carcinoma is the most common type of bladder cancer in the Western world, but it is not the type associated with schistosomiasis. Hepatocellular carcinoma is associated with Schistosoma mansoni and japonicum (due to liver fibrosis), and cholangiocarcinoma is associated with liver flukes like Clonorchis sinensis.
- The pathogenesis of this patient's diarrhea is primarily due to which of the following mechanisms?
Answer: Adherence to the small intestinal wall causing villous atrophy and malabsorption
The patient's symptoms and history are classic for giardiasis, caused by Giardia lamblia. The primary pathogenic mechanism of Giardia involves the attachment of trophozoites to the epithelial surface of the small intestine. This attachment, along with host immune responses, leads to inflammation, villous blunting (atrophy), and crypt hyperplasia, resulting in malabsorption of fats and carbohydrates and causing the characteristic foul-smelling, fatty diarrhea (steatorrhea). Invasion of colonic mucosa with ulcer formation is characteristic of Entamoeba histolytica. Toxin-mediated increases in cGMP or cAMP are mechanisms of certain bacterial diarrheas (e.g., ETEC, Cholera).
- Which feature of this parasite's life cycle best explains the development of overwhelming infection in this immunocompromised patient?
Answer: Presence of an autoinfection cycle
This patient has Strongyloides hyperinfection syndrome, a life-threatening condition triggered by immunosuppression (in this case, corticosteroids). Strongyloides stercoralis is unique among intestinal nematodes because of its autoinfection cycle. Non-infective rhabditiform larvae can develop into infective filariform larvae within the host's intestine. These larvae can then penetrate the intestinal mucosa or perianal skin to re-enter circulation and establish a new infection, leading to a massive increase in worm burden and dissemination when host immunity is suppressed. Dormant cysts in muscle are characteristic of Trichinella. An intermediate snail host is required for Schistosoma. Transmission via egg ingestion is typical for Ascaris or Enterobius.
- Acquisition of a primary infection with Toxoplasma gondii during pregnancy poses the greatest risk of congenital transmission. Which of the following is a primary mode of transmission for this organism to humans?
Answer: Ingestion of oocysts from cat feces or tissue cysts from undercooked meat
Toxoplasma gondii is primarily transmitted to humans through two routes: ingestion of tissue cysts found in undercooked or raw meat (especially pork, lamb, and venison) and ingestion of oocysts shed in the feces of infected cats, which can contaminate soil, water, or unwashed vegetables. The other options describe transmission routes for different pathogens: Histoplasma (inhalation of spores), Plasmodium (mosquito bite), and Strongyloides or hookworm (skin penetration by larvae).
- This patient's condition most likely resulted from which of the following exposures?
Answer: Ingestion of eggs from food contaminated with human feces
The patient's presentation with seizures and characteristic brain lesions on MRI is diagnostic of neurocysticercosis, which is caused by the larval stage of the pork tapeworm, Taenia solium. Humans develop cysticercosis by ingesting T. solium eggs, which are passed in the feces of a human who is harboring the adult tapeworm. This typically occurs via the fecal-oral route, often through contaminated food or water. Ingesting larvae in undercooked pork leads to intestinal taeniasis (the adult tapeworm), not cysticercosis. Skin penetration by cercariae is the route of infection for Schistosoma. Mosquito bites transmit malaria.
- Microscopic examination of a fresh stool sample would most likely reveal which of the following findings?
Answer: Trophozoites containing ingested red blood cells
This clinical picture of bloody diarrhea (amebic dysentery) and a liver abscess ('anchovy paste' consistency) is classic for amebiasis, caused by Entamoeba histolytica. A pathognomonic finding on microscopic examination of a fresh stool specimen is the presence of trophozoites that have ingested erythrocytes (erythrophagocytosis). Pear-shaped trophozoites are characteristic of Giardia. Acid-fast oocysts in stool are seen with Cryptosporidium. Rhabditiform larvae are seen with Strongyloides.
- This special stain is most likely to reveal which of the following?
Answer: Encapsulated yeasts
The clinical presentation of meningitis in a patient with a very low CD4 count is highly suggestive of Cryptococcus neoformans infection. C. neoformans is a yeast characterized by a thick polysaccharide capsule. The India ink stain is a classic method used to visualize this capsule in CSF; the ink particles are excluded by the capsule, creating a clear halo around the yeast cells. Gram-negative diplococci suggest Neisseria meningitidis. Acid-fast bacilli suggest Mycobacterium tuberculosis. Trophozoites with ingested RBCs are characteristic of Entamoeba histolytica in stool samples from patients with amebic dysentery.
- The hypopigmentation seen in this condition is caused by the organism's production of an acid that inhibits which of the following enzymes involved in melanin synthesis?
Answer: Tyrosinase
The patient has tinea versicolor, a superficial fungal infection caused by Malassezia furfur. The characteristic hypopigmentation occurs because the organism produces azelaic acid, a dicarboxylic acid that inhibits tyrosinase. Tyrosinase is the key enzyme in the metabolic pathway that converts tyrosine to melanin. Inhibition of this enzyme leads to decreased melanin production in the affected skin. Phenylalanine hydroxylase deficiency causes phenylketonuria. Homogentisate oxidase deficiency causes alkaptonuria. Lysyl oxidase is involved in collagen cross-linking.
- A biopsy of the lesion is taken for microscopic examination. Which of the following findings is most expected?
Answer: Broad, nonseptate hyphae with right-angle branching
This patient's presentation with diabetic ketoacidosis (DKA) and a black necrotic eschar in the nasal cavity is pathognomonic for rhinocerebral mucormycosis. The causative agents are fungi of the order Mucorales, such as Rhizopus and Mucor species. Histopathologic examination of infected tissue characteristically reveals broad, ribbon-like, nonseptate (or sparsely septate) hyphae that branch at wide, often 90-degree, angles. Acute-angle branching is seen with Aspergillus. Pseudohyphae are seen with Candida. Small yeasts in macrophages are seen with Histoplasma.
- This presentation is characteristic of an infection caused by a dimorphic fungus that grows as which of the following forms in human tissue at 37°C?
Answer: Cigar-shaped yeast
This is a classic case of lymphocutaneous sporotrichosis, often called 'rose gardener's disease', caused by Sporothrix schenckii. The organism is introduced into the skin via trauma (e.g., a thorn prick) and spreads along the lymphatic channels. S. schenckii is a dimorphic fungus. In human tissue (at 37°C), it grows as a yeast, which is characteristically elongated or cigar-shaped. In the environment (at 25°C), it grows as a mold with branching, septate hyphae. Spherules are seen with Coccidioides. Broad-based budding is characteristic of Blastomyces.
- Which of the following is the most appropriate method to confirm the suspected diagnosis?
Answer: Application of transparent tape to the perianal skin
The clinical presentation is classic for pinworm infection, caused by Enterobius vermicularis. The adult female worm migrates to the perianal region at night to lay her eggs, which causes intense pruritus. The most reliable and standard diagnostic method is the 'Scotch tape test' or paddle test. Transparent adhesive tape is pressed against the perianal skin in the morning before bathing, and the tape is then examined under a microscope for the characteristic eggs. The eggs are not typically found in stool, so a stool O&P is not the preferred test. While eosinophilia can be present with helminth infections, it is not diagnostic. Serology is not used for diagnosis.
- Microscopic examination of the lavage fluid is most likely to reveal which of the following?
Answer: Small, oval yeasts within macrophages
The patient's history of cave exploration (spelunking) in the Ohio/Mississippi River Valley region (Kentucky) is a major risk factor for histoplasmosis, caused by Histoplasma capsulatum. This dimorphic fungus is found in soil contaminated with bat or bird droppings. In tissue samples, such as from a bronchoalveolar lavage, H. capsulatum characteristically appears as small, oval yeasts located intracellularly within macrophages. Spherules are characteristic of Coccidioides. Broad-based budding yeasts are characteristic of Blastomyces. Septate hyphae are characteristic of Aspergillus.
- Microscopic examination of the aspirate is most likely to reveal which of the following?
Answer: Macrophages containing amastigotes
The patient's symptoms (fever, weight loss), physical findings (splenomegaly), lab results (pancytopenia), and geographic exposure (Middle East) are classic for visceral leishmaniasis (kala-azar), caused by Leishmania donovani. The diagnosis is confirmed by identifying the parasite in tissue samples. The form of Leishmania found within host cells is the amastigote, a small, oval, non-motile form that multiplies within macrophages of the reticuloendothelial system (e.g., bone marrow, spleen, liver). Ring forms are seen in malaria. Trypomastigotes are seen in Chagas disease or African trypanosomiasis. Spherules are seen in coccidioidomycosis.
