Final answer:
The likely cause of the patient's symptoms is acute pericarditis, marked by substernal chest pain that radiates to the left shoulder and is worsened by inspiration and coughing.
Step-by-step explanation:
The likely cause of the patient's symptoms is acute pericarditis. Acute pericarditis is marked by chest pain, difficulty breathing, and a dry cough. The pain is typically substernal and can radiate to the left shoulder. The fact that the pain is worsened by inspiration and coughing is also characteristic of acute pericarditis.