A 20-year-old male presents with diffuse chest pain that increases with deep inhalation but decreases when leaning forward. His EKG shows changes in all leads. What is the most probable diagnosis?

The clinical presentation you described is highly indicative of pericarditis. This condition is characterized by inflammation of the pericardial sac surrounding the heart and often presents with symptoms of chest pain that may worsen with deep breaths or coughing, a characteristic known as pleuritic pain. Patients frequently find relief when sitting forward, which distinguishes it from other cardiac-related pain.

The EKG changes noted in all leads further support this diagnosis. In cases of pericarditis, the EKG typically shows widespread ST-segment elevation and PR segment depression as common findings, suggesting the inflammation's effect on electrical activity across the heart.

In contrast, while conditions like pneumothorax and pulmonary embolism can cause acute chest pain and some respiratory distress, they do not typically present with the specific pattern of pain relief upon leaning forward and widespread EKG changes. Aortic dissection generally manifests with excruciating, tearing chest pain and may not show the same EKG changes or relief with position change. Thus, the combination of symptoms and EKG findings aligns best with a diagnosis of pericarditis.