What is the most likely cause of sharp chest pain and cyanosis in an 18-year-old male after weight lifting?

The most likely cause of sharp chest pain and cyanosis in an 18-year-old male after weight lifting is a spontaneous pneumothorax. This condition can occur when air leaks into the space between the lung and the chest wall, often due to the rupture of a small bleb on the lung surface. Engaging in activities like weight lifting increases intra-thoracic pressure, which can contribute to the rupture of these blebs, leading to sudden sharp chest pain and potential respiratory distress.

Cyanosis, which is a bluish discoloration of the skin due to inadequate oxygenation, aligns with the acute respiratory complications that can arise from a pneumothorax. As the air enters the pleural space, lung capacity is compromised, resulting in difficulty breathing and reduced oxygen exchange, which can manifest as cyanosis.

In contrast, myocardial infarction and pulmonary embolism are less common causes of chest pain in a young, otherwise healthy individual. Pneumonia typically presents with different symptoms, such as a productive cough and fever, rather than sudden sharp pain and cyanosis. Therefore, spontaneous pneumothorax is the most fitting diagnosis given the context of recent vigorous physical activity and the specific symptoms described.