A 67-year-old man presents to his family physician’s office for a routine visit and to discuss a growth on his toenail that has been gradually enlarging for a month. He has a history of diabetes mellitus, hyperlipidemia, and hypertension and is on metformin, atorvastatin, and lisinopril. He admits to smoking 2 packs of cigarettes daily for the past 45 years. His blood pressure reading today is 132/88 mm Hg, heart rate is 78/min, respiration rate is 12/min and his temperature is 37.1°C (98.8°F). On exam, the patient appears alert and in no apparent distress. Capillary refill is 3 seconds. Diminished dull and sharp sensations are present bilaterally in the lower extremities distal to the mid-tibial region. An image of the patient’s toenail is provided. A potassium hydroxide (KOH) preparation of a nail clipping sample confirms the presence of hyphae. Which of the following treatment options will be most effective for this condition?

Betamethasone + vitamin D analog

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

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 64-year-old woman presents to the clinic with a history of 3 fractures in the past year with the last one being last month. Her bone-density screening from last year reported a T-score of -3.1 and she was diagnosed with osteoporosis. She was advised to quit smoking and was asked to adapt to a healthy lifestyle to which she complied. She was also given calcium and vitamin D supplements. After a detailed discussion with the patient, the physician decides to start her on weekly alendronate. Which of the following statements best describes this patient’s new therapy?

The patient must stay upright for at least 30 minutes after taking this medication

It should be stopped after 10 years due to the risk of esophageal cancer

It is typically used as a second-line therapy for her condition after raloxifene

It can cause hot flashes, flu-like symptoms, and peripheral edema

It must be taken with the first meal of the day due to the significant risk of GI upset

Topical antifungal agents for USMLE Step 1: High-Yield Pharmacology Lessons

Mechanisms of Action - How They Fight Fungi

Topical antifungals primarily disrupt the fungal cell membrane by targeting ergosterol, a key component absent in human cells.

Azoles (e.g., Miconazole, Clotrimazole):
- Inhibit lanosterol 14-α-demethylase, blocking the conversion of lanosterol to ergosterol.
Allylamines (Terbinafine) & Benzylamines (Butenafine):
- Inhibit squalene epoxidase, leading to the toxic accumulation of squalene.
Polyenes (Nystatin):
- Bind directly to ergosterol, forming pores in the membrane, causing leakage and cell death.
Other Mechanisms:
- Ciclopirox: Chelates trivalent cations (e.g., $Fe^{3+}$), inhibiting essential enzymes.
- Tavaborole: Inhibits leucyl-tRNA synthetase, blocking fungal protein synthesis.

⭐ While azoles are typically fungistatic (inhibit growth), allylamines and polyenes are generally fungicidal (kill fungi), a key difference for clinical application.

Azole action on ergosterol synthesis and cell membrane

The Antifungal Arsenal - Key Drug Classes

Azoles (Imidazoles): Inhibit fungal P450 enzyme 14-α-demethylase, blocking ergosterol synthesis.
- Examples: Clotrimazole, Miconazole for tinea and cutaneous candidiasis. Ketoconazole for seborrheic dermatitis.
Polyenes: Bind directly to ergosterol, creating pores in the fungal cell membrane, leading to cell lysis.
- Example: Nystatin, primarily for Candida species.
Allylamines: Inhibit squalene epoxidase, a key enzyme in the ergosterol pathway, leading to toxic squalene accumulation.
- Examples: Terbinafine, Naftifine. Highly effective against dermatophytes.

⭐ Nystatin is not absorbed from the GI tract, making it ideal and safe for treating oral and esophageal candidiasis ("swish and swallow/spit").

Clinical Use - Matching Drug to Bug

Dermatophytes (Trichophyton, Microsporum, Epidermophyton)
- Clinical: Tinea corporis (ringworm), tinea pedis (athlete's foot), tinea cruris (jock itch).
- Agents: Allylamines (Terbinafine, Naftifine) are highly effective. Azoles (Clotrimazole, Miconazole) are also common first-line choices. Butenafine is another option.
Candida albicans
- Clinical: Intertrigo (skin folds), diaper dermatitis, mild oropharyngeal candidiasis (thrush).
- Agents: Nystatin suspension/cream (a polyene, specific for Candida). Topical Azoles are also effective.
Malassezia spp. (formerly Pityrosporum)
- Clinical: Tinea versicolor (hypo/hyperpigmented macules), seborrheic dermatitis.
- Agents: Selenium sulfide, Ketoconazole shampoo, zinc pyrithione.

Malassezia globosa "spaghetti and meatballs" KOH prep

⭐ Terbinafine is fungicidal (inhibits squalene epoxidase), while azoles are fungistatic (inhibit 14-α-demethylase). This mechanistic difference is a classic, high-yield exam question.

Adverse Effects - The Not-So-Fun Side

Common (Local): Generally well-tolerated.
- Irritation, burning, or stinging
- Erythema (redness)
- Pruritus (itching)
Less Common:
- Allergic contact dermatitis: A true hypersensitivity requiring cessation.
- Hypopigmentation with azoles.

⭐ Tinea Incognito: Be cautious with combination creams containing corticosteroids; they can mask inflammation and alter the appearance of a fungal infection, leading to a delayed or incorrect diagnosis.

Topical antifungals primarily treat superficial dermatophyte (tinea) and Candida infections.

Azoles (clotrimazole, miconazole) inhibit ergosterol synthesis by blocking the enzyme 14-alpha-demethylase.

Terbinafine, an allylamine, is fungicidal against dermatophytes by inhibiting squalene epoxidase.

Nystatin, a polyene, binds to ergosterol to form pores in the fungal cell membrane; it is only effective against Candida.

Ketoconazole shampoo is uniquely used for seborrheic dermatitis and tinea versicolor.

Adverse effects are generally mild and limited to local skin irritation.

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