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
Wenckebachk phenomenon is defined as- 
Select and explain. 
A. Progressive lengthening of PR interval till a beat is dropped, B. Slurred QRS complex, C. Irregular heart rate & permature ventricular beats, D. Shortened ST intervals
A: Progressive lengthening of PR interval till a beat is dropped
Answer: A: Progressive lengthening of PR interval till a beat is dropped 
Explanation: None