Gastroesophageal Reflux Disease (GERD): This condition is a common cause of chest pain in young adults, especially when the pain is centered in the chest and can radiate to the left side. The pain can be exacerbated by eating or lying down, and a trial of antacids or proton pump inhibitors can be diagnostic.

Musculoskeletal Chest Pain: Costochondritis or muscle strain can cause sharp pains in the chest that can move around, including to the left side. This diagnosis is plausible given the normal heart and lung sounds and lack of other symptoms.
Anxiety or Panic Disorder: These conditions can cause chest pain or discomfort that is often centered and can radiate, accompanied by other symptoms like palpitations, shortness of breath, or fear of dying. The normal labs and physical exam support considering a non-cardiac cause like anxiety.
Pericarditis: Although less common, pericarditis can cause sharp, central chest pain that may improve with sitting up and leaning forward. The pain can sometimes be referred to the left side of the chest.

Acute Coronary Syndrome (ACS): Although the patient is young and has normal labs, it's crucial not to miss ACS, as it can occur in anyone, especially if there are risk factors like family history, smoking, or hyperlipidemia. The presentation can sometimes be atypical, especially in younger patients.
Pulmonary Embolism (PE): This is another critical diagnosis not to miss, as it can present with sudden onset of chest pain that can be central and may worsen with deep breathing or movement. Normal lung sounds do not rule out PE.
Aortic Dissection: A tear in the aorta can cause severe, tearing chest pain that can radiate to the back or left side of the chest. It's a rare but life-threatening condition that requires immediate diagnosis and treatment.

Esophageal Spasm: This condition involves abnormal muscle contractions of the esophagus, which can cause chest pain that mimics cardiac pain. It's less common but should be considered if other diagnoses are ruled out.
Pneumothorax: Although the lung sounds are normal, a small pneumothorax could potentially cause chest pain, especially if it's increasing in size. This would be more likely if there was a history of trauma or underlying lung disease.
Mitral Valve Prolapse: This condition can cause chest pain, palpitations, and sometimes shortness of breath. It's more common in women but can occur in men, and the chest pain can be atypical, including being centered or radiating to the left side.