Final answer:
The most likely diagnosis for an 18-year-old male with acute onset of dyspnoea and chest pain after lifting weights, accompanied by certain physical examination findings, is spontaneous pneumothorax.
Step-by-step explanation:
Based on the symptoms of acute onset of moderate dyspnoea and right-sided chest pain following physical exertion, decreased chest expansion, hyper resonant percussion note, and decreased breath sounds on the right side of the chest without tracheal deviation, the most likely diagnosis for the 18-year-old male is a spontaneous pneumothorax. This condition, often called a collapsed lung, is related to subtle abnormalities in connective tissue and occurs when air enters the pleural space causing the lung to collapse. This condition is different from an acute myocardial infarction (MI), pulmonary edema, and infections like pneumonia, all of which present with different clinical signs and symptoms.