This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Physiology
Atrioventricular nodal delay is due to 
Select and explain. 
A. Less gap junctions, B. More tight junctions, C. Intercalated discs, D. Prolonged refractory period
A: Less gap junctions
Answer: A: Less gap junctions 
Explanation: Cause of the Slow Conduction. The slow conduction in the transitional, nodal, and penetrating A-V bundle fibers is caused mainly by diminished numbers of gap junctions between successive cells in the conducting pathways, so there is great resistance to conduction of excitatory ions from one conducting fiber to the next. Therefore, it is easy to see why each succeeding cell is slow to be excited.Ref: Guyton and Hall textbook of medical physiology 13th edition. Page:125