Musculoskeletal Chest Pain: Given the normal EKG, troponins, and D-dimer, musculoskeletal pain is a common cause of chest pain, especially if the pain is reproducible with movement or palpation. The patient's history of smoking could contribute to musculoskeletal issues due to decreased lung function and potential for chronic cough.

Gastroesophageal Reflux Disease (GERD): GERD can cause chest pain that may be mistaken for cardiac issues. Smoking can exacerbate GERD symptoms.
Pulmonary Embolism (PE) with Normal D-dimer: Although the D-dimer is normal, it's essential to consider PE, especially in a smoker, as it can sometimes present with a normal D-dimer, particularly if the test is done too early after the onset of symptoms.
Chronic Obstructive Pulmonary Disease (COPD) Exacerbation: Smokers are at high risk for COPD, and an exacerbation could cause chest pain or discomfort.

Aortic Dissection: Although less likely with normal EKG and troponins, aortic dissection is a life-threatening condition that requires immediate attention. Smoking is a risk factor for aortic diseases.
Pneumonia: Pneumonia can cause chest pain and might not always present with typical symptoms like fever or cough, especially in smokers who may have chronic lung changes.
Cardiac Ischemia with Normal Troponins: Early in the course of a myocardial infarction or with intermittent ischemia, troponins might be normal. Given the patient's smoking history, cardiac ischemia should be considered.

Pneumothorax: While less common, pneumothorax can cause sudden onset of chest pain and should be considered, especially in smokers who are at higher risk for lung diseases.
Esophageal Spasm or Rupture: These conditions are less common but can cause severe chest pain and should be considered if other diagnoses are ruled out.
Tietze's Syndrome: An inflammatory condition affecting the cartilages of the ribs, which can cause chest pain, is less common but should be considered in the differential diagnosis of musculoskeletal chest pain.