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 69-year-old man with metastatic colon cancer is brought to the emergency department because of shortness of breath, fever, chills, and a productive cough with streaks of blood for the past 5 days. He has a history of emphysema. The patient does not have abdominal pain or headache. He receives chemotherapy with 5-fluorouracil, leucovorin, and oxaliplatin every 6 weeks; his last cycle was 3 weeks ago. His temperature is 38.3°C (101°F), pulse is 112/min, and blood pressure is 100/70 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 83%. A few scattered inspiratory crackles are heard over the right lung. His mucous membranes are dry. Cardiac examination is normal. Laboratory studies show: Hemoglobin 9.3 mg/dL Leukocyte count 700/mm3 Segmented neutrophils 68% Lymphocytes 25% Eosinophils 4% Monocytes 3% Platelet count 104,000/mm3 Serum Glucose 75 mg/dL Urea nitrogen 41 mg/dL Creatinine 2.1 mg/dL Galactomannan antigen Positive Which of the following is the most appropriate initial pharmacotherapy?

Rifampin, isoniazid, pyrazinamide, and ethambutol

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 prophylaxis protocols for USMLE Step 3: High-Yield Pharmacology Lessons

Indications for Prophylaxis - Fungal Gatekeepers

Hematologic Malignancy & HSCT
- Acute Myeloid Leukemia (AML) / Myelodysplastic Syndrome (MDS) induction chemotherapy.
- Allogeneic Hematopoietic Stem Cell Transplant (HSCT) with Graft-vs-Host Disease (GVHD).
- Prolonged neutropenia (ANC <500/mm³ for >7 days).
- Target: Candida, Aspergillus.
Solid Organ Transplant (SOT)
- High-risk liver, lung, or small bowel transplants.
- Risk factors: re-transplantation, surgical leaks, high immunosuppression.
- Target: Candida, Aspergillus.
HIV/AIDS
- Pneumocystis jirovecii (PJP): CD4 <200/mm³.
- Cryptococcus neoformans (secondary prophylaxis): CD4 <100/mm³.

⭐ In AML/MDS patients receiving induction chemotherapy, posaconazole prophylaxis has shown superior efficacy in preventing invasive fungal infections, particularly aspergillosis, compared to fluconazole or itraconazole.

The Antifungal Arsenal - Meet the Players

Fungal cell structure and antifungal drug targets

Polyenes (Amphotericin B, Nystatin)
- Mechanism: Binds to ergosterol → forms pores → cell lysis.
- Use: Broadest spectrum; severe, systemic mycoses.
- ⚠️ Toxicity: Nephrotoxicity, infusion reactions ("shake and bake").
Azoles (-conazoles)
- Mechanism: Inhibit fungal CYP450 (14-α-demethylase) → ↓ ergosterol synthesis.
- Use: Broad; Voriconazole for Aspergillus, Fluconazole for Candida.
Echinocandins (-fungins)
- Mechanism: Inhibit β-(1,3)-D-glucan synthesis → disrupt cell wall.
- Use: Invasive candidiasis (incl. azole-resistant), Aspergillus.
Flucytosine (5-FC)
- Mechanism: Converted to 5-FU → inhibits DNA/RNA synthesis.
- Use: Synergy with Ampho B for cryptococcal meningitis.
- ⚠️ Toxicity: Myelosuppression.

⭐ High-Yield: Azoles are potent inhibitors of human cytochrome P450 enzymes, increasing levels of drugs like warfarin and statins.

Prophylaxis Playbook - The Right Drug, Right Patient

Goal: Prevent invasive fungal infections (IFIs) in high-risk patients.
Selection depends on: patient's immune status, local epidemiology, and expected fungal pathogens.

Antifungal prophylaxis decision tree for high-risk patients

⭐ Exam Favorite: Posaconazole is the preferred agent for prophylaxis in patients with prolonged neutropenia and for allogeneic hematopoietic stem cell transplant (HSCT) recipients with graft-versus-host disease (GVHD) due to its superior activity against molds like Aspergillus.

High‑Yield Points - ⚡ Biggest Takeaways

Neutropenic patients often receive fluconazole or an echinocandin (e.g., caspofungin).

HIV/AIDS patients with CD4 < 100 require fluconazole for Cryptococcus prophylaxis.

Transplant recipients (stem cell, solid organ) need broad coverage, often with posaconazole or voriconazole.

Candida prophylaxis with fluconazole is crucial for high-risk ICU patients.

Remember PJP/PCP prophylaxis (CD4 < 200) is TMP-SMX, not a primary antifungal.

Azoles exhibit significant drug-drug interactions via CYP450 inhibition.

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