You are taking care of a patient with renal failure secondary to anti-fungal therapy. The patient is a 66-year-old male being treated for cryptococcal meningitis. This drug has a variety of known side effects including acute febrile reactions to infusions, anemia, hypokalemia and hypomagnesemia. What is the mechanism of action of this drug?

Pore formation secondary to ergosterol binding

Inhibition of squalene epoxidase

Disruption of microtubule formation

Inhibition of 1,3-beta-glucan synthase

A 45-year-old HIV-positive male presents to his primary care physician complaining of decreased libido. He reports that he has been unable to maintain an erection for the past two weeks. He has never encountered this problem before. He was hospitalized four weeks ago for cryptococcal meningitis and has been on long-term antifungal therapy since then. His CD4 count is 400 cells/mm^3 and viral load is 5,000 copies/ml. He was previously non-compliant with HAART but since his recent infection, he has been more consistent with its use. His past medical history is also notable for hypertension, major depressive disorder, and alcohol abuse. He takes lisinopril and sertraline. His temperature is 98.6°F (37°C), blood pressure is 120/85 mmHg, pulse is 80/min, and respirations are 18/min. The physician advises the patient that side effects like decreased libido may manifest due to a drug with which of the following mechanisms of action?

Inhibition of ergosterol synthesis

Inhibition of beta-glucan synthesis

Formation of pores in cell membrane

Disruption of microtubule formation

Inhibition of pyrimidine synthesis

You are treating a neonate with meningitis using ampicillin and a second antibiotic, X, that is known to cause ototoxicity. What is the mechanism of antibiotic X?

It binds the 30S ribosomal subunit and inhibits formation of the initiation complex

It binds the 50S ribosomal subunit and inhibits formation of the initiation complex

It binds the 30S ribosomal subunit and reversibly inhibits translocation

It binds the 50S ribosomal subunit and inhibits peptidyltransferase

It binds the 50S ribosomal subunit and reversibly inhibits translocation

A 34-year-old woman presents with confusion, drowsiness, and headache. The patient’s husband says her symptoms began 2 days ago and have progressively worsened with an acute deterioration of her mental status 2 hours ago. The patient describes the headaches as severe, localized to the frontal and periorbital regions, and worse in the morning. Review of symptoms is significant for a mild, low-grade fever, fatigue, and nausea for the past week. Past medical history is significant for HIV infection for which she is not currently receiving therapy. Her CD4+ T cell count last month was 250/mm3. The blood pressure is 140/85 mm Hg, the pulse rate is 90/min, and the temperature is 37.7°C (100.0°F). On physical examination, the patient is conscious but drowsy. Papilledema is present. No pain is elicited with extension of the leg at the knee joint. The remainder of the physical examination is negative. Laboratory findings, including panculture, are ordered. A noncontrast CT scan of the head is negative and is followed by a lumbar puncture. CSF analysis is significant for: Opening pressure 250 mm H2O (70-180 mm H2O) Glucose 30 mg/dL (40-70 mg/dL) Protein 100 mg/dL (<40 mg/dL) Cell count 20/mm3 (0-5/mm3) Which of the following additional findings would most likely be found in this patient?

CSF India ink stain shows encapsulated yeast cells

Gram-positive diplococci are present on microscopy

CSF shows a positive acid-fast bacillus stain

Multiple ring-enhancing lesions are seen on a CT scan

CSF shows gram negative diplococci

A 58-year-old man presents with a high-grade fever, throbbing left-sided headache, vision loss, and left orbital pain. He says that his symptoms started acutely 2 days ago with painful left-sided mid-facial swelling and a rash, which progressively worsened. Today, he woke up with complete vision loss in his left eye. His past medical history is significant for type 2 diabetes mellitus, diagnosed 5 years ago. He was started on an oral hypoglycemic agent which he discontinued after a year. His temperature is 38.9°C (102.0°F), blood pressure is 120/80 mm Hg, pulse is 120/min, and respiratory rate is 20/min. On examination, there is purulent discharge from the left eye and swelling of the left half of his face including the orbit. Oral examination reveals extensive necrosis of the palate with a black necrotic eschar and purulent discharge. Ophthalmic examination is significant for left-sided ptosis, proptosis, and an absence of the pupillary light reflex. Laboratory findings are significant for a blood glucose level of 388 mg/dL and a white blood cell count of 19,000 cells/mm³. Urinary ketone bodies are positive. Fungal elements are found on a KOH mount of the discharge. Which of the following statements best describes the organism responsible for this patient’s condition?

Histopathological examination shows non-septate branching hyphae

It appears as a narrow-based budding yeast with a thick capsule

It has budding and filamentous forms

Histopathological examination shows acute angle branching hyphae

Antifungal agents for USMLE Step 3: High-Yield Microbiology Lessons

Antifungal Targets - The Fungal Fortress

Fungal cell structure & antifungal drug targets

Cell Membrane (Ergosterol)
- Polyenes (Amphotericin B): Bind to ergosterol, forming membrane pores causing lethal ion leakage.
- Azoles (-conazoles): Inhibit lanosterol 14-α-demethylase, blocking ergosterol production.
- Allylamines (Terbinafine): Inhibit squalene epoxidase, an early step in ergosterol synthesis.
Cell Wall (β-Glucan)
- Echinocandins (-fungins): Inhibit β-(1,3)-D-glucan synthase, disrupting cell wall integrity and causing osmotic stress.
Nucleic Acid Synthesis
- Flucytosine (5-FC): Converted by fungal cytosine deaminase to 5-FU, which blocks DNA and RNA synthesis.

⭐ Amphotericin B is reserved for severe, life-threatening systemic mycoses. Key toxicities are nephrotoxicity and infusion reactions ("shake and bake"), managed with hydration and premedication.

Nucleic Acid Synthesis - Fungal DNA Disrupters

Flucytosine (5-FC)
- Mechanism: A pyrimidine analog. Fungal cytosine deaminase converts 5-FC → 5-fluorouracil (5-FU). This inhibits thymidylate synthase, disrupting fungal DNA synthesis. It also incorporates into fungal RNA, disrupting protein synthesis.
- Clinical Use: Narrow spectrum. Used in combination with Amphotericin B for severe systemic mycoses (e.g., cryptococcal meningitis).
- Adverse Effects: Bone marrow suppression (anemia, leukopenia, thrombocytopenia) due to conversion to 5-FU by gut bacteria.

⭐ Synergy with Amphotericin B: Amphotericin B creates pores in the fungal cell membrane, which increases the entry of flucytosine, leading to a synergistic antifungal effect.

Antifungal drug mechanisms of action in fungal cells

Clinical Spectrum - Match the Drug to Bug

Drug Class	Key Drugs	Primary Fungal Targets
Polyenes	Amphotericin B	Broad Spectrum: Candida, Aspergillus, Cryptococcus, Mucorales, Dimorphs (e.g., Histo). Used for severe, systemic infections.
	Nystatin	Candida only (topical/oral rinse for thrush). Not absorbed systemically.
Azoles	Fluconazole	Candida (except krusei), Cryptococcus. Excellent for CNS infections.
	Itraconazole	Dimorphic fungi (Blastomyces, Histoplasma, Coccidioides), Aspergillus.
	Voriconazole	Drug of Choice for Invasive Aspergillosis. Also covers Candida.
	Isavuconazole	Invasive Aspergillus and Mucorales.
Echinocandins	Caspofungin	Invasive Candida (especially azole-resistant), Aspergillus. Poor CNS penetration.
Allylamines	Terbinafine	Dermatophytes (especially onychomycosis).
Antimetabolites	Flucytosine	Synergy with Ampho B for cryptococcal meningitis.

High‑Yield Points - ⚡ Biggest Takeaways

Amphotericin B and Nystatin bind ergosterol, creating membrane pores. Key toxicities include nephrotoxicity and infusion reactions.

Azoles inhibit a fungal P450 enzyme to block ergosterol synthesis, leading to significant drug-drug interactions.

Echinocandins target β-glucan synthesis, disrupting the fungal cell wall. They are used for Candida and Aspergillus.

Flucytosine blocks DNA and RNA synthesis, causing bone marrow suppression.

Terbinafine inhibits squalene epoxidase, but watch for hepatotoxicity.

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