Terbinafine:

Correct Answer: A

Rationale: Terbinafine is indicated in dermatophyte nail infections (A), such as onychomycosis, due to its fungicidal action against Trichophyton and other dermatophytes, accumulating in keratin-rich tissues for prolonged effect. Topical forms treat ringworm (B), like tinea pedis or cruris, effectively. A single dose isn't usually effective (C); oral therapy requires weeks to months. It's not a potent CYP450 inducer (D), though it inhibits CYP2D6, affecting some drugs. It may exacerbate psoriasis (original E). Terbinafine's allylamine mechanism disrupts ergosterol synthesis early, offering high cure rates in superficial fungal infections, with minimal systemic toxicity compared to azoles.

Correct Answer: B

Rationale: Melphalan (B) is associated with profound and prolonged myelosuppression, an alkylating agent causing dose-limiting neutropenia and thrombocytopenia, common in multiple myeloma treatment. Chlorambucil (A) causes milder, reversible suppression. BCNU (C), or carmustine, also causes severe, delayed myelosuppression, notable in brain tumors. Bleomycin (D) primarily causes pulmonary toxicity, not myelosuppression. Vincristine (original E) affects nerves, not marrow. Melphalan's DNA cross-linking halts cell division, effective in hematologic malignancies, but its marrow toxicity requires growth factor support or dose delays, balancing efficacy with infection risk.

Correct Answer: C

Rationale: Recombinant human erythropoietin (EPO) treats anemia of chronic renal failure (C), stimulating red blood cell production in kidneys unable to produce endogenous EPO, improving quality of life in dialysis patients. It's not used for iron-deficient anemia (A); iron supplementation is needed. Sickle cell anemia (B) relies on hydroxyurea, not EPO. It treats AZT-related anemia (D) in HIV, countering marrow suppression. It's ineffective in clozapine-induced agranulocytosis (original E). EPO's targeted stimulation of erythropoiesis, monitored via hemoglobin levels, reduces transfusion needs, though hypertension and thrombosis risks require oversight.

Correct Answer: D

Rationale: Protonation (D) is the correct answer, as it describes the process where a weak acid gains a proton (H⁺) in a low pH environment, becoming un-ionized, less water-soluble, and more lipid-soluble. This enhances membrane permeation, as seen with aspirin in the stomach (pH ~2). Distribution (A) is movement into tissues, not a solubility change. Elimination (B) is excretion, unrelated to solubility shifts. First-pass effect (C) is hepatic metabolism post-absorption, not a physical process. Permeation (original E) is movement across membranes, not the solubility change itself. Protonation leverages the Henderson-Hasselbalch equation, where a weak acid's pKa (e.g., aspirin 3.5) relative to pH dictates ionization, impacting absorption and distribution in acidic compartments like the stomach.

Correct Answer: C

Rationale: Steroid hormone action involves diffusion into the cytoplasm and binding to an intracellular receptor (C), as lipophilic steroids (e.g., cortisol) cross membranes, bind nuclear receptors, and regulate gene expression. Option A describes tyrosine kinase receptors (e.g., insulin). Option B fits G-protein-coupled receptors (e.g., epinephrine). Option D involves JAK-STAT pathways (e.g., cytokines), not steroids. Option E (original) about ion channels applies to neurotransmitters. This genomic mechanism, slow but sustained, contrasts with rapid membrane receptor signaling, altering protein synthesis for effects like anti-inflammation, critical in endocrinology and pharmacology.