A 58-year-old man from Delhi presents to the emergency department with acute onset dyspnea and chest pain. He has a history of severe COPD and was straining during a coughing fit 2 hours ago. On examination, he is tachypneic (RR 28/min) and hypoxic (SpO₂ 82% on room air). Chest X-ray shows a left-sided diaphragmatic hernia with bowel loops visible in the thoracic cavity. The hernia is compressing the left lung base. Which of the following structures most likely herniated through the defect, given the acute traumatic mechanism?