A 7-year-old boy from Tamil Nadu presents to the emergency department with severe chest pain, fever (38.5°C), and acute dyspnea. His mother reports he has had recurrent painful crises since age 2 years. On examination, he is tachypneic (RR 32/min), oxygen saturation is 88% on room air, and there is dullness to percussion over the left lung base. Chest X-ray shows a new infiltrate in the left lower lobe. His hemoglobin is 7.2 g/dL, reticulocyte count is 18%, and blood smear shows sickle cells and target cells. What is the most likely acute complication occurring in this patient?