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

A 57-year-old florist presents to his family physician with nodular lesions on his right hand and forearm. He explains that he got pricked by a rose thorn on his right "pointer finger" where the first lesions appeared, and the other lesions then began to appear in an ascending manner. The physician prescribed a medication and warned him of gynecomastia as a side effect if taken for long periods of time. Which of the following is the mechanism of action of the medication?

Binds to ergosterol, forming destructive pores in cell membrane

Inhibits formation of beta glucan

A 25-year-old man visits a local clinic while volunteering abroad to rebuild homes after a natural disaster. He reports that he has been experiencing an intermittent rash on his feet for several weeks that is associated with occasional itching and burning. He states that he has been working in wet conditions in work boots and often does not get a chance to remove them until just before going to bed. On physical exam, there is diffuse erythema and maceration of the webspaces between his toes. He starts taking a medication. Two days later, he experiences severe nausea and vomiting after drinking alcohol. Which of the following is the mechanism of action of the drug most likely prescribed in this case?

Inhibition of cell wall synthesis

Inhibition of steroid synthesis

Disruption of fungal cell membrane

A 45-year-old man presents to the emergency department with difficulties swallowing food. He states that he experiences pain when he attempts to swallow his medications or when he drinks water. He reveals that he was diagnosed with HIV infection five years ago. He asserts that he has been taking his antiretroviral regimen, including emtricitabine, rilpivirine, and tenofovir. His temperature is 98°F (37°C), blood pressure is 100/60 mmHg, pulse is 90/min, respirations are 22/min, and oxygen saturation is 99% on room air. His physical exam is notable for a clear oropharynx, no lymphadenopathy, and a normal cardiac and pulmonary exam. No rashes are noted throughout his body. His laboratory results are displayed below: Hemoglobin: 12 g/dL Hematocrit: 37 % Leukocyte count: 8,000/mm^3 with normal differential Platelet count: 160,000/mm^3 Serum: Na+: 138 mEq/L Cl-: 108 mEq/L K+: 3.5 mEq/L HCO3-: 26 mEq/L BUN: 35 mg/dL Glucose: 108 mg/dL Creatinine: 1.1 mg/dL CD4+ count: 90/mm^3 HIV viral load: 59,000 copies/mL What is the best next step in management?

Esophageal endoscopy and biopsy

A 41-year-old man comes to the physician because of a 3-week history of fatigue, cough, and a 4.5-kg (10-lb) weight loss. He does not smoke or drink alcohol. He appears emaciated. A chest x-ray shows a calcified nodule in the left lower lobe and left hilar lymphadenopathy. The physician initiates therapy for the condition and informs him that he will have to return for monthly ophthalmologic examination for the next 2 months. These examinations are most likely to evaluate the patient for an adverse effect of a drug with which of the following mechanisms of action?

Impaired synthesis of cell wall polysaccharides

Impaired synthesis of mycolic acids

Impaired protein synthesis due to binding to 50S ribosomes

Impaired production of hemozoin from heme

Impaired protein synthesis due to binding to 30S ribosomes

A 72-year-old woman with type 2 diabetes mellitus comes to the physician because she is concerned about the appearance of her toenails. Examination shows yellowish discoloration of all toenails on both feet. The edges of the toenails are lifted, and there is subungual debris. Potassium hydroxide preparation of scrapings from the nails shows multiple branching septate hyphae. Treatment with oral terbinafine is begun. Which of the following is the primary mechanism of action of this drug?

Inhibition of squalene epoxidase

Formation of pores in cell membrane

Inhibition of β-glucan synthesis

Interference with mitosis during metaphase

Prevention of lanosterol to ergosterol conversion

Antifungal spectrum of activity — USMLE Lesson

Polyenes - Ampho-terrible Toxicity

Amphotericin B: Ergosterol binding and cation leakage

Mechanism: Bind to ergosterol in the fungal cell membrane, forming pores that lead to leakage of intracellular contents and cell death.

Drug	Spectrum of Activity	Clinical Use	Key Toxicity
Amphotericin B	Broad: Candida, Aspergillus, Cryptococcus, endemic mycoses. (Resistant: C. lusitaniae)	Severe, systemic mycoses.	📌 Ampho-terrible: Nephrotoxicity, infusion reactions (fever, chills - "shake & bake"), electrolyte wasting (↓K⁺, ↓Mg²⁺).
Nystatin	Narrower; primarily Candida species.	Topical for oropharyngeal/cutaneous candidiasis ("swish and swallow/spit").	Too toxic for systemic use; minimal side effects with topical/oral use.

Azoles - Conazoles Get Confusing

Azole antifungals and CYP51 heme sterol 14α-demethylase

Inhibit ergosterol synthesis by blocking fungal cytochrome P450 enzyme 14α-demethylase. Generally fungistatic.

Drug	Candida	C. krusei/glabrata	Aspergillus	Cryptococcus	Zygomycetes	Endemic Mycoses
Fluconazole	++	-	-	+	-	++ (Coccidio)
Itraconazole	+	-	+/-	+	-	++
Voriconazole	++	+	++	+	-	+
Posaconazole	++	+	+	+	+	+

📌 Mnemonic: For Candida/Crypto? Fluconazole. Aspergillus? Voriconazole. Zygomycetes? Posaconazole.

Echinocandins - Fungins Finish Walls

📌 "-fungin" drugs finish fungal cell walls.

Agents: Caspofungin, Micafungin, Anidulafungin.
Mechanism: Inhibit cell wall synthesis by blocking $β(1,3)$-D-glucan synthase.
Spectrum of Activity:
- Excellent vs.: Most Candida (including fluconazole-resistant C. glabrata/krusei) and Aspergillus.
- Ineffective vs.: Cryptococcus, Zygomycetes, and dimorphic fungi.

Fungal Cell Wall Synthesis & Antifungal Targets -D-glucan synthase inhibition in the fungal cell wall)

⭐ Administered only IV, these large lipopeptide molecules are favored for critically ill patients with invasive candidiasis due to their potent efficacy and excellent safety profile.

Miscellaneous Antifungals - The Odd Ones Out

Agent	Mechanism	Spectrum	Key Use
Flucytosine (5-FC)	Inhibits DNA/RNA synthesis	Cryptococcus, Candida	Systemic mycoses
Griseofulvin	Disrupts mitotic spindle	Dermatophytes	Tinea capitis
Terbinafine	Inhibits squalene epoxidase	Dermatophytes	Onychomycosis

High‑Yield Points - ⚡ Biggest Takeaways

Amphotericin B has the broadest spectrum of activity, covering most clinically relevant yeasts, molds (including Mucorales), and endemic fungi.

Voriconazole is the drug of choice for invasive Aspergillosis; it also provides excellent coverage for most Candida species.

Echinocandins are highly active against Candida (including azole-resistant C. glabrata) and Aspergillus but have no activity against Cryptococcus.

Fluconazole has a narrow spectrum, excellent for C. albicans and Cryptococcus maintenance, but misses molds and C. krusei.

Itraconazole is the treatment of choice for non-severe, non-meningeal endemic mycoses like Histoplasmosis and Blastomycosis.

Flucytosine is always used in combination, typically with Amphotericin B for cryptococcal meningitis.

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