This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: ECG, Subject name: Physiology
PR interval is constant in: 
Select and explain. 
A. Mobitz type I block, B. Mobitz type II block, C. All types of second degree blocks, D. None of the varieties of hea block.
B: Mobitz type II block
Answer: B: Mobitz type II block 
Explanation: - Mobitz type II of the second degree block has a constant PR interval, with every 'nth' beat is dropped; that is, inconsistently one impulse from atria does not reach ventricles. - Mobitz type I of the second degree block shows lengthening PR interval in successive beats, with finally in one beat P wave is not followed by QRS complex. - In a third variety of the second degree block, atria-to-ventricle excitation is a fixed small number ratio (e.g., 3:2). For instance, every third impulse from SA node will fail to reach ventricles.