Acute Coronary Syndrome (ACS): This is the most likely diagnosis due to the high prevalence of coronary artery disease and the typical presentation of chest pressure or discomfort associated with ACS.

Gastroesophageal Reflux Disease (GERD): Chest pressure can be a symptom of GERD, especially if it worsens with eating or lying down.
Musculoskeletal Pain: Chest wall pain or costochondritis can cause chest pressure, often related to muscle strain or inflammation of the cartilage that connects the ribs to the breastbone.
Anxiety or Panic Disorder: These conditions can manifest with chest pressure or discomfort, often accompanied by other symptoms like palpitations, shortness of breath, or fear of dying.
Pulmonary Embolism: While less common than ACS, pulmonary embolism can cause sudden onset of chest pressure, especially if it is large or involves multiple segments of the lung.

Aortic Dissection: A life-threatening condition where the inner layer of the aorta tears, causing severe chest pain that can be described as pressure. It is crucial to consider this diagnosis due to its high mortality rate if not promptly treated.
Pneumothorax: A collapsed lung can cause sudden chest pressure, especially if it is a tension pneumothorax, which is a medical emergency.
Pericarditis: Inflammation of the pericardium, the sac surrounding the heart, can cause chest pressure, often sharp and improving with leaning forward.

Esophageal Spasm: Abnormal contractions of the esophagus can cause chest pressure, often accompanied by difficulty swallowing.
Hypertrophic Cardiomyopathy: A condition where the heart muscle becomes abnormally thick, which can lead to chest pressure or pain, especially during exertion.
Mitral Valve Prolapse: A condition where the mitral valve does not close properly, potentially causing chest discomfort or pressure, though it is less commonly associated with chest pressure compared to other symptoms like palpitations.