The drug acts in the collecting tubules:

Correct Answer: C

Rationale: The correct answer is C: Potassium-sparing diuretics. Potassium-sparing diuretics act in the collecting tubules by blocking the sodium-potassium exchange, leading to decreased sodium reabsorption and potassium retention. This mechanism helps to promote diuresis while preventing potassium loss. Loop diuretics (A) act in the loop of Henle, thiazide diuretics (B) act in the distal convoluted tubule, and carbonic anhydrase inhibitors (D) act in the proximal convoluted tubule. Therefore, they are not directly involved in the collecting tubules like potassium-sparing diuretics.

Correct Answer: B

Rationale: The correct answer is B: Metronidazole. Metronidazole is a nitroimidazole derivative commonly used as an antibacterial and antiprotozoal drug. It works by disrupting the DNA structure of bacteria and parasites, leading to their death. Clavulanic acid (A) is a beta-lactamase inhibitor, not an antibacterial drug. Nitrofurantoin (C) is a nitrofuran derivative used for urinary tract infections, not a nitroimidazole derivative. Doxycycline (D) is a tetracycline antibiotic, not a nitroimidazole derivative. Therefore, the correct choice is B as it is the only nitroimidazole derivative among the options provided.

Correct Answer: A

Rationale: The correct answer is A: Zidovudine. Zidovudine is a nucleoside reverse transcriptase inhibitor that works by inhibiting the viral reverse transcriptase enzyme, essential for viral replication in HIV-infected cells. This action helps to slow down the progression of HIV infection. Vidarabine is an antiviral drug used to treat herpes simplex and varicella-zoster viruses, not HIV. Rimantadine is an antiviral drug used to treat and prevent influenza A virus infections by inhibiting viral replication, but it does not inhibit viral reverse transcriptase. Gancyclovir is an antiviral drug used to treat cytomegalovirus infections by inhibiting viral DNA polymerase, not reverse transcriptase.

Correct Answer: B

Rationale: The correct answer is B: Postural hypotension. Nitroglycerin and isosorbide mononitrate are vasodilators that can cause a sudden drop in blood pressure when standing up, leading to postural hypotension. This is especially risky in patients with exertional angina, as it can exacerbate symptoms. A: Cough and wheezing are not common side effects of nitroglycerin or isosorbide mononitrate. C: Reflex bradycardia is unlikely as these medications primarily affect blood vessels, not the heart rate. D: Methemoglobinemia is a rare side effect associated with certain medications but is not commonly seen with nitroglycerin or isosorbide mononitrate.

Correct Answer: B

Rationale: The correct answer is B: Reduction of angiotensin-mediated vasoconstriction in the kidney. Captopril is an ACE inhibitor that works by blocking the conversion of angiotensin I to angiotensin II. In this patient with systolic heart failure, reducing angiotensin II levels will lead to vasodilation and decreased vasoconstriction in the kidney, improving renal blood flow and reducing fluid retention. This helps to alleviate symptoms of heart failure and improve overall cardiac function. Incorrect choices: A: Increased cardiac contractility - Captopril does not directly increase cardiac contractility. C: Reduction of preload and afterload - While ACE inhibitors can reduce preload and afterload, the primary mechanism of captopril is through the renin-angiotensin-aldosterone system. D: Stimulation of epinephrine release from adrenergic nerves - Captopril does not stimulate epinephrine release; it actually inhibits the