--the skin biopsy protocol for measuring small-fiber condition and density involves taking samples from just above the ankle and from the upper thigh, at least, as these are the sites that were "normed" in the original Johns Hopkins protocols.

At good academic neuropathy centers, there might also be a sample taken from the knee area, or even from the upper arm just above the elbow, as these sites were also normed by later research.

But there should be at least the two leg samples taken for exactly the reason en bloc mentions--there needs to be a comparison between distal (ankle) and proximal (thigh) sites in order to determine if there is length-dependency or not.