Final answer:
Dermal thickening is common in the palms and soles of the feet.
Step-by-step explanation:
The extra layer of thickened epidermis is common in the palms and soles (option A). This thicker layer of skin is known as dermal thickening and is responsible for providing extra protection and support to these areas of the body. It is important because it helps to reduce friction and increase grip in these regions.
The skin on the palms and soles is structurally designed to withstand a greater degree of wear and pressure, which is part of the reason for this additional layer, contributing to the overall thickness. Also, conditions like psoriasis might lead to itchy or sore patches of thickened, red skin with silvery scales, and these can often affect areas such as elbows and knees, but the inherent thickening is typical of palms and soles.