The following anti-infective drugs are suitable as prophylaxis in the stated conditions:

Correct Answer: B

Rationale: Ciprofloxacin (B) is the correct answer as it's widely recommended for prophylaxis in close adult contacts of meningococcal disease caused by Neisseria meningitidis. A single dose effectively reduces nasopharyngeal carriage, preventing invasive disease spread. Co-amoxiclav (A) is appropriate for human and animal bites due to its coverage of anaerobes and skin pathogens like Pasteurella, but B is highlighted here. Flucloxacillin (C) isn't used for diphtheria prophylaxis; erythromycin or penicillin is preferred to eradicate Corynebacterium diphtheriae carriage. Erythromycin (D) is suitable for pertussis prophylaxis in unvaccinated infants exposed to Bordetella pertussis, reducing transmission. Aciclovir (original E) isn't standard for herpes labialis contact prophylaxis. Ciprofloxacin's efficacy, ease of administration, and alignment with public health protocols make it a critical tool in controlling meningococcal outbreaks.

Correct Answer: B

Rationale: Rifampicin accelerates the CYP450-mediated metabolism of warfarin (B), a vitamin K antagonist anticoagulant, reducing its efficacy and requiring dose adjustments to maintain therapeutic INR levels. It also affects corticosteroids (A), like prednisolone, lowering their anti-inflammatory effects. Streptomycin (C), an aminoglycoside, isn't metabolized by CYP450, relying on renal clearance. Digoxin (D) metabolism isn't significantly altered by rifampicin, though P-glycoprotein induction may reduce levels. Oestrogen (original E) metabolism is accelerated, reducing contraceptive efficacy. Rifampicin's potent CYP450 induction, particularly CYP3A4, stems from its activation of the pregnane X receptor, impacting pharmacokinetics of co-administered drugs, critical in TB management where polypharmacy is common.

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: AZT + lamivudine + lopinavir/ritonavir (B) is an accepted first-line HIV therapy, combining two nucleoside reverse transcriptase inhibitors (NRTIs: AZT, lamivudine) with a boosted protease inhibitor (lopinavir/ritonavir), effectively suppressing viral replication and preventing resistance. AZT + ddC (A) lacks a third class, reducing efficacy. Ritonavir + amprenavir + enfuvirtide (C) mixes protease inhibitors and an entry inhibitor, not typical first-line. AZT + lamivudine + nevirapine (D), with an NNRTI, is viable but less common now versus integrase inhibitors. Modern guidelines favor integrase-based regimens (e.g., dolutegravir), but B reflects older, still-accepted triple therapy, balancing potency and resistance barriers.

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.