A patient with HIV-AIDS being treated with combined antiviral drugs and immunosuppressive agents, develops an opportunistic infection caused by Pneumocystis carinii. Which of the following drugs most likely be used in the treatment of pulmonary infection caused by P. carinii?

Correct Answer: C

Rationale: The correct answer is C: Trimethoprim/Sulfamethoxazole. This combination is the first-line treatment for Pneumocystis carinii pneumonia in patients with HIV-AIDS due to its efficacy against the organism. Trimethoprim inhibits dihydrofolate reductase, while sulfamethoxazole inhibits dihydropteroate synthase, both essential enzymes for folate synthesis in bacteria and protozoa like P. carinii. Metronidazole (A) is used for anaerobic infections, Carbenicillin (B) is a penicillinase-resistant penicillin used for Gram-negative infections, and Penicillin G (D) is a narrow-spectrum antibiotic mainly used for Gram-positive infections.

Correct Answer: A

Rationale: Acyclovir is an antiviral medication that specifically targets viruses that belong to the herpes virus family by inhibiting their replication. Herpes simplex Type 1 virus is most susceptible to acyclovir because it has a thymidine kinase enzyme that activates acyclovir into its active form, which then interferes with viral DNA synthesis. Herpes simplex Type 2 virus also has this enzyme, but it is not as effective against acyclovir compared to Type 1. Varicella-zoster virus and Epstein-Barr virus do not have the thymidine kinase enzyme necessary for acyclovir activation, making them less susceptible to the drug.

Correct Answer: B

Rationale: The correct answer is B, Tinidazole, for oral treatment of trichomonas vaginitis. Tinidazole is a nitroimidazole antibiotic that is effective against trichomonas. It has good oral bioavailability and is commonly used for this infection. Diiodohydroxyquin (A) is not indicated for trichomonas vaginitis. Clotrimazole (C) and Ketoconazole (D) are antifungal agents used for yeast infections, not trichomonas. Tinidazole is the most appropriate choice due to its effectiveness against trichomonas and its oral administration route.

Correct Answer: A

Rationale: Erythromycin is primarily bacteriostatic by inhibiting protein synthesis in bacteria. At higher concentrations, it can disrupt bacterial cell membranes, making it bactericidal. Tetracycline inhibits protein synthesis, Chloramphenicol blocks peptide bond formation, and Ampicillin disrupts bacterial cell wall synthesis. Therefore, A is correct because of its dual mechanism of action.

Correct Answer: A

Rationale: Alprostadil is a prostaglandin E1 analog that works by binding to specific receptors and activating adenylate cyclase, leading to increased levels of cAMP. This results in smooth muscle relaxation and vasodilation. Therefore, choice A is correct. Choices B and C are incorrect as alprostadil does not directly affect cGMP levels or PDE-5 activity. Choice D is also incorrect as alprostadil does not block nitric oxide, but rather enhances its effects through the cAMP pathway.