A rare example of locus minoris resistentiae

A 15-year-old female consulted our outpatient department for an asymptomatic single orange-brown raised lesion over her scalp since birth. Her mother noticed a new growth with finger-like projections over the existing skin lesion for six months. On examination, there was a solitary well-defined hairless orange-brown plaque with filiform growth present over the scalp [Figure 1a]. Dermoscopic findings over the orange-brown plaque include yellowish globules on a papillary grayish-yellow background suggestive of nevus sebaceous. Dermoscopy over the filiform projections revealed hairpin vessels and thrombosed vessels suggestive of the filiform wart [Figure 1b]. Skin biopsy from the filiform projections confirmed the diagnosis of the filiform wart. Biopsy from the underlying orange-brown plaque showed orthokeratosis, acanthosis, and defective hair follicle with accumulation of sebaceous glands suggestive of nevus sebaceous. Hence, a diagnosis of filiform wart overlying a nevus sebaceous was made.

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Figure 1

(a): A solitary well-defined verrucous hairless orange-brown plaque with filiform growth over the vertex of the scalp; (b): Dermoscopy (Dermlite, ×10) yellowish globules on a papillary grayish-yellow background with thrombosed vessels and hairpin vessels suggestive of filiform wart overlying nevus sebaceous.

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Locus minoris resistentiae (LMR) is a site that offers increased vulnerability to the onset of the disease than the rest of the body. Trauma, irradiation, chronic lymph stasis, and herpes scars act as a nidus for various other infectious, inflammatory, and neoplastic conditions. Cutaneous mosaicism serves as a congenital LMR. Others include epidermal nevus, congenital hemangioma, and linear porokeratosis, which can act as a congenital LMR.^[1] This case is highlighted as an infectious lesion over an area of cutaneous mosaicism, a rare example of LMR.