Metronidazole is used to treat:

Correct Answer: A

Rationale: Metronidazole is used to treat trichomonal infections (A), caused by Trichomonas vaginalis, as a first-line therapy due to its efficacy against anaerobic protozoa, disrupting DNA synthesis. It's also effective in amoebic dysentery (B) from Entamoeba histolytica, targeting liver abscesses and intestinal disease. It treats giardiasis (C), caused by Giardia lamblia, with high cure rates. It's used in gas gangrene (original E), caused by Clostridium perfringens, often with surgery, but not tetanus (D), where antitoxin and penicillin are standard. Metronidazole's selective toxicity to anaerobes and protozoa, sparing aerobic bacteria, stems from its activation in low-oxygen environments, making it invaluable in mixed infections, though its metallic taste and disulfiram-like reaction with alcohol are notable side effects.

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.

Correct Answer: B

Rationale: Aqueous hydrolysis is the correct answer because it is not a mechanism of drug permeation across biological membranes. Permeation refers to the process by which drugs move across cell membranes to reach their site of action, and it includes aqueous diffusion (A), where drugs pass through aqueous channels or pores; lipid diffusion (C), where lipophilic drugs dissolve into the lipid bilayer; and pinocytosis or endocytosis (D), where cells engulf drugs in vesicles. Special carrier transport (original E) involves specific proteins facilitating drug movement. Aqueous hydrolysis (B), however, is a chemical degradation process where a drug reacts with water, breaking chemical bonds (e.g., in esters or amides), not a physical movement across membranes. This distinction is critical in pharmacology, as permeation mechanisms determine bioavailability and tissue distribution, while hydrolysis affects drug stability, often in the gastrointestinal tract or plasma, not membrane crossing.