What Causes Severe Pain In Knee Along With Tenderness At The Back Of Leg?

My husband has had sever pain which started in his knee, and 6 days later, after MRi, Ct scan, X-ray, and blood work the doctors do not know what is wrong...his muscles behind his knee and thru the back of his leg are tender now as well, likely from not moving it....confusing and frustrated. Anything and everything seems unlikely, but is has to be something...