Xray revealed narrowing of L5 s1 disc space. Doc said this does not explain pelvic and lower back pain. Insists on blood test instead of mri. Odd?

Deoends. Pelvic pain can but is uncommonly referred from the lumbar spine. Disc space narrowing is very common. Your age makes it more likely there might be another cause to consider which is why the recommended blood work. Thus far i would tend to agree with your doctors work up. The plan could change if the blood work comes back normal.