There are three classical pathologies with which you should be familiar. The first is tabes dorsalis. Tabes (tay bees) is a form of tertiary syphilis in which a spirochaete invades cell bodies of the larger myelinated fibers. These neurons degenerate and there is a conspicuous lack of fibers in the posterior columns. The patient would have difficulty with discriminative touch, including stereognosis and graphesthesia. The sense of vibration and limb position would be impaired as well. Non-localized touch remains intact. see figure