My 92 year old mother awakes frequently at night with a strong urgency to urinate, but feels she is unable to empty her bladder. What could be the reason for this?

Sharman asks:My mother is 92 and she is awakened at night 4 to 5 times with a very strong urgency to urinate (wish she does). Meanwhile, she tells me that she feels that she is unable to empty her bladder and that it seems as if it turns off and she can't empty it totally.She eats and drinks nothing after 6 pm. She weighs about 93 pounds and has a past history of being anorexic. Other than being