Denied & requesting internal review. What should my doctor letter include?

Today I got my denial letter with the reason of "you do not have a substantial physical or mental impairment that is continuous or

recurrent" which is not true and goes against all of the proof and information I provided.

To do everything I can to ensure having my application approved during the internal review I'm going to have my doctor write up a letter.

What details should this letter include? I know outlining how my ailments affect my daily living and ability to work and take care of myself. But any other advice would be greatly appreciated.