A 58-year-old woman has fainted while on the toilet. She states she has been constipated lately and had difficulty with bowel movements. She was having a difficult time when she suddenly "passed out." Her husband heard the noise and found her awake but on the floor. She is now awake and alert. The most probable cause of her syncope is:

A. excessive vagal tone
B. hypovolemia
C. micturition syncope
D. tussive syncope

Answer: A. excessive vagal tone