Final answer:
The characteristic feature of lichen planus is sawtooth rete pegs, a pattern seen under the microscope at the edge of the epidermis.
Step-by-step explanation:
The characteristic feature of lichen planus is sawtooth rete pegs. This means that when viewed under a microscope, the skin tissue sample displays the edge of the epidermis, where it meets the dermis, and this edge has a sawtooth pattern because of the elongated rete ridges. Lichen planus also often presents with other histological features such as hypergranulosis, interface dermatitis with band-like lymphocytic infiltration near the basal layer, and Civatte bodies (apoptotic keratinocytes). However, the sawtooth appearance of the rete pegs is distinctive and can help differentiate lichen planus from other dermatological conditions.