This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Emergency Clinical Scenario, Subject name: Medicine
A 56-year-old man presents in the casualty with severe chest pain and difficulty in breathing. His ECG was taken immediately. The below ECG suggest the following diagnosis: 
Select and explain. 
A. Ventricular fibrillation, B. Acute pulmonary embolism, C. Second degree hea block, D. Atrial fibrillation
D: Atrial fibrillation
Answer: D: Atrial fibrillation 
Explanation: Atrial fibrillation. More or less normal QRS complexes Variable RR intervals and Absent P waves Absence of P waves rules out the possibility of ventricular fibrillation or second degree hea block. Although clinical features mentioned may point to a diagnosis of pulmonary embolism, ECG features are not contributory. ECG of pulmonary embolism: