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
Constant PR inverval is seen in- 
Select and explain. 
A. First degree block, B. Second degree - Mobiz type I block, C. Second degree - Mobiz type II block, D. Third degree block
A: First degree block
Answer: A: First degree block 
Explanation: constant PR interval is a feature of first degree hea block second and third degree hea block doesnt have constant intervals Ref Harrison 20th edition pg 1534