Got a bad doctor?

Long story short:

80% off my back alone, I put in for my knees, ankles, and feet as a secondary condition and the doctor & VA gave me a diagnosis, service connected but the doctor gave me a negative nexus opinion during my C&P saying these are different body systems therefore do not have a direct mechanism connecting the two.

Do I have a chance at winning my appeal with a Nexus letter from another doctor? Am I going to be able to win this?