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: Unknown
Characteristic feature of the ECG is ? 
Select and explain. 
A. Progressive lengthening of PR interval till a beat is dropped., B. Slurred QRS complex, C. Irregular heart rate and premature ventricular beats, D. Shortened ST interval
A: Progressive lengthening of PR interval till a beat is dropped.
Answer: A: Progressive lengthening of PR interval till a beat is dropped. 
Explanation: Mobitz type I second-degree A V block (A V Wenckebach block) is characterized by progressive PR interval prolongation prior to block of an atrial impulse.