Why Does The Doctor

[fruitsalad]

Ask if you are having bowel or bladder problems when you have low back pain?

[237SJ]

I think it`s because there could be a problem with the spinal cord that could cause those areas to be paralysed/not function.

[Buenchico]

Because (among other things) that's where your kidneys are:
http://www.nhs.uk/Conditions/Kidney-infection/Pages/Symptoms.aspx

[slaney]

To make sure that you haven't got something like a large disc prolapse pressing on the nerves that control bowel and bladder function.

[slaney]

...and because there are many bowel and bladder conditions which give you low back pain as well.

[Sqad]

Simple....it is an association with pressure and the lumbosacral spine (lower back)

Buenchico....I don't think the kidneys are in the lower back....but i may be wrong.

[Buenchico]

Correction for Sqad:
That's where kidney pain is often experienced!
http://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Internal_organs.png/864px-Internal_organs.png