You are answering a USMLE-style medical question. Question: A 70-year-old man with chronic heart failure presents to the emergency department due to difficulty in breathing. The patient is a known hypertensive for 20 years maintained on amlodipine and telmisartan. The physician notes that he is also being given a diuretic that blocks the Na+ channels in the cortical collecting tubule. Which drug is being referred to in this case? Options: A) Furosemide B) Hydrochlorothiazide C) Triamterene D) Acetazolamide Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.