Final answer:
A chest radiograph is the initial imaging of choice to diagnose the cause of a patient's acute shortness of breath and chest pain.
Step-by-step explanation:
The imaging of choice to make the diagnosis for a 25-year-old man presenting with acute onset of shortness of breath and right-sided chest pain that is worse with inspiration is a chest radiograph. Given the clinical signs of increased resonance to percussion, absence of breath sounds on the right anterior apex, and a presentation that is not suggestive of an electrocardiogram (ECG) finding, a chest radiograph is the most appropriate initial imaging modality to assess for potential pathologies such as pneumothorax or pleural effusion that may present with these symptoms. A CT scan might be considered following the radiograph if further details are needed or if the radiograph is inconclusive. However, based on the given scenario, the most essential and immediate imaging to perform is a chest radiograph.