Pleuritic Chest Pain (e.g., Viral Pleurisy or Pleuritis): The patient's complaint of chest discomfort worsening with deep breaths is classic for pleuritic chest pain, which involves inflammation of the pleura. This condition is often caused by viral infections and can present with sharp, stabbing pain that worsens with breathing.

Musculoskeletal Chest Pain: This is a common cause of chest discomfort, especially in younger patients. It can be due to strained muscles or costochondritis (inflammation of the cartilage that connects the ribs to the breastbone), which can also worsen with deep breathing or movement.
Pneumonia: Although less likely in a young adult without other symptoms like fever or cough, pneumonia can cause pleuritic chest pain if the infection involves the pleura. The pain would worsen with deep breathing.
Pulmonary Embolism (PE): While less common in a young adult without risk factors, PE can cause sudden onset of pleuritic chest pain. However, it typically presents with other symptoms like shortness of breath or tachycardia.

Myocardial Infarction (MI): Although rare in young adults, MI can occur, especially with risk factors like family history, smoking, or drug use. It's crucial to consider MI in any patient with chest discomfort, as missing this diagnosis can be fatal.
Aortic Dissection: This is a life-threatening condition where there is a tear in the aorta's inner layer. It can cause severe, tearing chest pain that may radiate to the back and can worsen with deep breathing due to movement of the aortic tear.
Pneumothorax: A collapsed lung can cause sudden onset of pleuritic chest pain and shortness of breath. It's more common in tall, thin males or those with lung disease.

Pulmonary Vasculitis: Conditions like Wegener's granulomatosis can cause inflammation of the blood vessels in the lungs, leading to chest pain that worsens with deep breathing.
Diaphragmatic Hernia: A rare condition where part of the stomach or intestine bulges into the chest cavity, which can cause chest discomfort that worsens with breathing or eating.
Esophageal Rupture or Perforation: Although rare, this condition can cause severe chest pain that worsens with breathing or swallowing, often accompanied by vomiting or fever.