You are answering a USMLE-style medical question. Question: A 64-year-old man presents with severe respiratory distress. This has onset over the past 3 weeks, progressively worsening. Past history involves congestive heart failure (CHF). Due to concerns of concurrent pneumonia, sputum studies are ordered. Microscopic examination of his sputum demonstrates heart failure cells (hemosiderin-containing macrophages). Which of the following cellular organelles is significant in the formation of these cells? Options: A) Endoplasmic reticulum (ER) B) Ribosomes C) Lysosomes D) Mitochondria Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.