Case presentation

A 49-year-old female patient presented to the outpatient department of dermatology with an asymptomatic nodular lesion that measured approximately 1.5 cm and had a keratinous center on the lower extensor aspect of the lower leg (Figure 1).

Figure 1: Clinical appearance of trichilemmal keratosisA well-circumscribed hyperkeratotic nodule with a central keratin plug on the distal extensor aspect of the lower leg.

The lesion had gradually enlarged before presentation and was not associated with any preceding trauma or local procedures. On physical examination, a firm nodular lesion with a central keratinous plug was noted. The clinical differential diagnosis included keratoacanthoma and calcinosis cutis. An excision was performed. The postoperative course was uneventful, with satisfactory healing and no evidence of recurrence during the available clinical follow-up. However, the exact duration of follow-up was not documented in the medical records. Histological examination revealed a crateriform stratified squamous epithelium with a keratotic center extending through the reticular dermis (Figure 2).

Figure 2: Histological appearance of trichilemmal keratosis showing crateriform stratified squamous epithelium with a keratotic center, (H&E, x2.5)H&E: hematoxylin and eosin

The squamous epithelium showed subtle lobulation with abrupt trichilemmal-type keratinization and a limited residual granular layer, in addition to peripheral palisading of the outer layer (Figures 3, 4).

Figure 3: Medium view showing squamous epithelium with subtle lobulation and abrupt trichilemmal-type keratinization, (H&E, x100)H&E: hematoxylin and eosin

Figure 4: High power showing trichilemmal keratinization (H&E, x250)Arrow: abrupt keratinization and absence of granular layer; Asterisk: peripheral palisadingH&E: hematoxylin and eosin

The crater contained dense eosinophilic keratinous material with ‘horn-like’ exophytic extension beyond the crater’s opening.