Costochondritis: This condition, also known as Tietze's syndrome, is characterized by inflammation of the cartilage that connects the ribs to the breastbone (sternum). It is a common cause of chest pain in young adults, especially when the pain worsens with breathing and movement, such as leaning forward. The pain is typically sharp and localized to the chest wall.

Precordial Catch Syndrome: This condition causes sharp, stabbing chest pain that can worsen with deep breathing or movement. It is often seen in adolescents and young adults and is believed to be related to the musculoskeletal structures of the chest wall.
Musculoskeletal Strain: Strain or injury to the muscles or ligaments of the chest wall can cause pain that worsens with movement or deep breathing.
Pulmonary Embolism (less common in this age group but possible): Although less likely in a 19-year-old without significant risk factors, pulmonary embolism can cause chest pain that worsens with deep breathing. However, it would typically be accompanied by other symptoms such as shortness of breath or cough.

Myocardial Infarction (MI): Although rare in young adults, MI can occur, especially in those with underlying heart conditions or risk factors such as hypercholesterolemia, hypertension, or family history of early heart disease. Chest pain that worsens with exertion or deep breathing could be indicative of an MI.
Pneumothorax: A collapsed lung can cause sudden, severe chest pain that worsens with deep breathing. It is a medical emergency that requires immediate attention.
Pericarditis: Inflammation of the pericardium, the sac surrounding the heart, can cause chest pain that worsens with leaning forward and improves with sitting up and leaning back. It can be a complication of viral infections or other conditions.

Spontaneous Coronary Artery Dissection (SCAD): A rare condition where there is a tear in the coronary artery, which can cause a heart attack. It is more common in women and can occur without traditional risk factors.
Mitral Valve Prolapse: A condition where the two valve flaps of the mitral valve do not close smoothly or evenly but instead bulge (prolapse) upward into the left atrium when the heart contracts. It can cause chest pain, although this is less common.
Pulmonary Hypertension: High blood pressure in the arteries that supply blood to the lungs. It can cause chest pain, especially during exercise, due to the increased strain on the right side of the heart.