This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
A 70-year old male patient presented to the emergency department with pain in epigastrium and difficulty in breathing for 6 hours. On examination, his heart rate was 56 beats per minute and the blood pressure was 106/60 mm Hg. Chest examination was normal. The patient has been taking omeprazole for gastroesophageal reflux disease for last 6 months. What should be the intial investigation - 
Select and explain. 
A. An ECG, B. An upper GI endoscopy, C. Urgent ultrasound of the abdomen, D. An x-ray chest
A: An ECG
Answer: A: An ECG 
Explanation: None