This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.V.S, Subject name: Medicine
[{'content': 'Wenckebachk phenomenon is defined as - \nSelect and explain. \nA. Progressive lengthening of PR, B. Slurred QRS complex, C. Irregular hea rate & permature ventricular beats, D. Shoened ST intervals', 'role': 'user'}, {'content': 'Answer: A \nExplanation: wenkebach phenomenon is the progressive lengthening of PR i nterval seen in mobitz type 1 AV block. Ref Harrison 20th edition pg 1436', 'role': 'assistant'}]