The following drugs are effective in staphylococcal infections:

Correct Answer: D

Rationale: Flucloxacillin (D) is the correct answer as it's highly effective against staphylococcal infections, particularly those caused by Staphylococcus aureus, including beta-lactamase-producing strains. Its stability against these enzymes makes it a first-line choice for skin, soft tissue, and bone infections. Ampicillin (A) is less effective against staphylococci due to its susceptibility to beta-lactamases, targeting mainly Gram-negative bacteria. Co-amoxiclav (B), combining amoxicillin with clavulanic acid, is effective against resistant staphylococci, broadening coverage to anaerobes and other pathogens. Fusidic acid (C) is also effective, often used topically or in combination for systemic staphylococcal infections like osteomyelitis. Metronidazole (original E) targets anaerobes, not staphylococci. Flucloxacillin's narrow-spectrum activity and resistance to beta-lactamases make it ideal for MSSA (methicillin-sensitive S. aureus), balancing efficacy with reduced resistance development.

Correct Answer: B

Rationale: Amphotericin is effective in systemic Candida spp. infections (B), such as candidemia or invasive candidiasis, due to its broad-spectrum antifungal activity, binding ergosterol in fungal membranes to cause cell death. It's also effective in local Candida infections (A), like oral thrush, but systemic use is more critical. It's nephrotoxic (C), damaging renal tubules, often dose-limiting, requiring hydration and monitoring. It causes hypokalemia (D), from distal tubule effects, necessitating potassium supplementation. It's effective in cryptococcosis (original E is incorrect), notably meningitis. Amphotericin's efficacy in severe fungal infections is unmatched, but its toxicity profile, mitigated by lipid formulations, demands careful administration, balancing therapeutic benefit with renal and electrolyte disturbances.

Correct Answer: C

Rationale: Ribavirin must be taken up into cells and phosphorylated to be effective (C), converting to its triphosphate form to inhibit viral RNA synthesis, critical for its antiviral action. It's not indicated in CMV retinitis (A); ganciclovir or foscarnet are used. It inhibits viral RNA methyltransferase (B), part of its broad mechanism against RNA viruses. It's given via aerosol for RSV bronchiolitis (D), especially in infants. It's combined with interferon alfa for hepatitis C (original E). Ribavirin's efficacy in hepatitis C and severe RSV stems from its interference with viral replication and mRNA capping, though anemia is a significant side effect requiring monitoring.

Correct Answer: A

Rationale: Quinine sulfate is the drug of choice in chloroquine-resistant falciparum malaria (A), used IV or orally for severe Plasmodium falciparum infections, acting as a blood schizonticide to reduce parasitemia. It's available IV and orally (B), critical for flexibility in severe cases. It doesn't eradicate hepatic P. vivax parasites (C); primaquine is needed for hypnozoites. It's not contraindicated in renal failure (D), though dose adjustment is advised. Large doses cause tinnitus (original E), part of cinchonism. Quinine's efficacy in multidrug-resistant malaria, via heme polymerization disruption, remains vital, though cardiotoxicity (QT prolongation) requires ECG monitoring.

Correct Answer: A

Rationale: Methotrexate inhibits dihydrofolate reductase (A), blocking tetrahydrofolate synthesis, critical for purine and pyrimidine production, halting cancer and immune cell proliferation in leukemia and rheumatoid arthritis. Folinic acid (leucovorin) rescue after high doses (B) reduces toxicity by bypassing the block. It's primarily renally excreted (C is incorrect), not biliary. Chronic use causes cirrhosis (D), a cumulative hepatotoxicity risk. It's first-line for choriocarcinoma (original E). Methotrexate's antimetabolite action is reversible, but mucositis, myelosuppression, and liver fibrosis necessitate monitoring and supportive care.