Amoxicillin:

Correct Answer: B

Rationale: Amoxicillin is effective against many strains of Haemophilus influenzae (B), making it a first-line choice for infections like otitis media and pneumonia caused by this pathogen. Unlike benzylpenicillin, it has a broader spectrum, but it is susceptible to beta-lactamases (A), contrary to the statement, unless combined with clavulanic acid (as in co-amoxiclav). It's effective in many community-acquired urinary tract infections (C) caused by Escherichia coli, so that option is incorrect. Drug-related skin rashes can appear after dosing stops (D), especially in delayed hypersensitivity, and are more common in infectious mononucleosis (original E), but B is the key correct statement. Amoxicillin's efficacy against H. influenzae relies on its beta-lactam structure inhibiting cell wall synthesis, though resistance from beta-lactamase production necessitates combination therapies in some cases.

Correct Answer: B

Rationale: Dapsone is indicated in multibacillary leprosy (B), a cornerstone of multidrug therapy with rifampicin and clofazimine, targeting Mycobacterium leprae effectively due to its bacteriostatic action on folate synthesis. It's not used for amoebiasis (A), where metronidazole is preferred. It treats dermatitis herpetiformis (C), a gluten-sensitive skin condition, reducing inflammation. It's acetylated in the liver (D), with fast/slow acetylator status affecting its metabolism. It has cross-sensitivity with sulfonamides (original E), relevant for allergy screening. Dapsone's utility in leprosy leverages its long half-life and synergy with other drugs, though hemolysis (especially in G6PD deficiency) and methemoglobinemia are key adverse effects requiring monitoring.

Correct Answer: A

Rationale: Foscarnet is indicated in CMV retinitis in AIDS patients (A), a key treatment for this vision-threatening infection, directly inhibiting viral DNA polymerase without needing activation, unlike aciclovir. It treats aciclovir-resistant HSV (B), particularly in immunocompromised patients. It's nephrotoxic (C), requiring hydration and monitoring due to renal tubular damage. It causes fits (D), linked to electrolyte imbalances like hypocalcemia. It's not usually oral/topical (original E is incorrect), given IV. Foscarnet's broad antiviral activity, including against ganciclovir-resistant CMV, makes it vital in advanced HIV, though its toxicity profile demands careful management.

Correct Answer: B

Rationale: Intravenous co-trimoxazole (B) is a first-line treatment for Pneumocystis jirovecii pneumonia (PCP) in HIV patients, combining trimethoprim and sulfamethoxazole to inhibit folate synthesis, highly effective against this opportunistic pathogen. Aztreonam (A) targets Gram-negative bacteria, not PCP. Rifabutin (C) treats mycobacterial infections, not PCP. Pentamidine (D) is an alternative for PCP, used IV or inhaled when co-trimoxazole is contraindicated. Glucocorticoids (original E) are adjunctive if PaO2 <60 mmHg. Co-trimoxazole's synergy, prophylaxis role, and IV option for severe cases make it standard, though rash and hyperkalemia are common, manageable side effects.

Correct Answer: B

Rationale: Cyclophosphamide is an alkylating agent (B), cross-linking DNA to halt cancer cell division, widely used in lymphomas, leukemias, and solid tumors. It's typically combined with other agents (A) in regimens like CHOP for synergy. It causes granulocytopenia (C), a dose-limiting toxicity increasing infection risk. It causes nausea and vomiting (D), moderately to highly emetogenic. Alopecia (original E) is common. Its activation by CYP450 to active metabolites like phosphoramide mustard enhances efficacy, but hemorrhagic cystitis (prevented by mesna) and cardiotoxicity at high doses require careful monitoring.