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Microbiology Flashcards

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  1. A 35-year-old HIV-positive patient with a CD4 count of 50 cells/μL develops white plaques on the tongue that can be scraped off, revealing an erythematous base. KOH preparation shows budding yeast with pseudohyphae. Which antifungal drug is most appropriate for systemic treatment?

    Answer: Fluconazole

    Fluconazole is the drug of choice for oropharyngeal and esophageal candidiasis in immunocompromised patients; it inhibits ergosterol synthesis by blocking fungal CYP450 (lanosterol 14α-demethylase).

  2. A 50-year-old Ohio River valley farmer develops pulmonary nodules and skin lesions with verrucous borders. Biopsy shows broad-based budding yeast. Which organism is responsible?

    Answer: Blastomyces dermatitidis

    Blastomyces dermatitidis causes blastomycosis characterized by broad-based budding yeast on histology and verrucous skin lesions; it is endemic to the Ohio and Mississippi River valleys and Great Lakes region.

  3. A 45-year-old immunocompromised patient develops fever and a pulmonary infiltrate that fails to respond to antibiotics. Bronchoalveolar lavage stains show septate hyphae with 45-degree angle branching invading blood vessels. Which organism is most likely?

    Answer: Aspergillus fumigatus

    Aspergillus fumigatus produces septate hyphae that branch at 45-degree angles and has a propensity for vascular invasion causing infarction; Mucor shows wide, non-septate hyphae at 90-degree angles.

  4. A traveler returns from sub-Saharan Africa with cyclic fever every 48 hours, severe anemia, and splenomegaly. Thick blood smear shows ring forms and banana-shaped gametocytes. Which organism is responsible?

    Answer: Plasmodium falciparum

    Plasmodium falciparum is identified by multiple ring forms per RBC, banana-shaped gametocytes, and a 48-hour (tertian) fever cycle; it causes the most severe malaria and can infect RBCs of all ages.

  5. A 28-year-old woman develops a frothy, malodorous vaginal discharge with a pH of 5.5. Wet mount shows motile, pear-shaped organisms with flagella. Which drug is used for treatment?

    Answer: Metronidazole

    Trichomonas vaginalis is a flagellated protozoan treated with metronidazole (or tinidazole); the drug forms free radical intermediates that damage DNA in anaerobic organisms.

  6. A patient with Chagas disease develops dilated cardiomyopathy years after the initial infection. The causative protozoan is transmitted by the reduviid bug. Which organism is responsible?

    Answer: Trypanosoma cruzi

    Trypanosoma cruzi causes Chagas disease; the organism is transmitted via feces of the reduviid (kissing) bug and can cause chronic cardiomyopathy and megacolon decades later.

  7. A patient develops seizures and ring-enhancing cystic lesions in the brain. MRI shows a scolex (protoscolex) within a cyst. He recently immigrated from a region where pigs are raised. Which organism is responsible?

    Answer: Taenia solium (cysticercosis)

    Neurocysticercosis is caused by ingestion of Taenia solium eggs (not the cysticerci); larvae migrate to the CNS and form cysts containing a scolex, visualized on MRI.