A 24-year-old man with known sickle cell disease presents to the hospital with acute severe chest pain, dyspnea, fever (39.2°C), and cough. Chest X-ray shows a new infiltrate in the right lower lobe. His oxygen saturation is 88% on room air. Hemoglobin is 6.8 g/dL (baseline 7.5 g/dL), reticulocyte count 12%, and LDH is markedly elevated. Blood culture is pending. What is the most likely diagnosis?