Answer: Scabies caused by the microscopic mite Sarcoptes scabiei.
This mite burrows into the superficial layers of the epidermis and deposits eggs and fecal pellets (scybala). The diagnostic features of this case are:
1. The location within the epidermis
2. Size of mite and presence of toothlike spines on its thin exoskeleton (see below).
3. Adult and egg forms (below)
The best way to diagnose scapies is via skin scrapings (rather than biopsy). Skin scrapings can be obtained through which will produce mites with the best morphology. To obtain skin scrapings, mineral oil can be applied to the possible skin tracts followed by vigorous scraping with a sterile scalpel blade or glass slide. The material obtained should be placed on glass slides and examined for the presence of mites, larvae, and fecal pellets.
Anonymous and Neuronurse mentioned that this parasite resembles Gnathostoma spp, which also have spines on their external surface. However, Gnathostoma are uncommon in the epidermis (usually seen in subcutaneous tissue and viscera) and the spines are much smaller than those seen on the scabies mite. Gnathostoma also has different internal structures including large lateral cords. Good thought!