Dermatology: Practical and Conceptual Image Letter | Dermatol Pract Concept. 2025;15(4):5542 1 Inverted Follicular Keratosis of the Nail Unit: A Rare Case Report Hanane Rachadi1, Fadoua Chemsy1, Farida Marnissi2, Soumiya Chiheb1 1 Dermatology and venerology department, Ibn Rochd University hospital, Hassan II University, Casablanca, Morocco 2 Department of Pathology, Ibn Rochd University Hospital, Hassan II University, Casablanca, Morocco Citation: Rachadi H, Chemsy F, Marnissi F, Chiheb S. Inverted follicular keratosis of the nail unit: a rare case report. Dermatol Pract Concept. 2025;15(4):5542. DOI: https://doi.org/10.5826/dpc.1504a5542 Accepted: June 29, 2025; Published: October 2025 Copyright: ©2025 Rachadi et al. This is an open-access article distributed under the terms of the Creative Commons Attribution- NonCommercial License (BY-NC-4.0), https://creativecommons.org/licenses/by-nc/4.0/, which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original authors and source are credited. Funding: None. Competing Interests: None. Authorship: All authors have contributed significantly to this publication. Corresponding Author: Hanane Rachadi, Dermatologist, Professor, Dermatology and venerology department, Ibn Rochd University hospital, Hassan II University, Casablanca, Morocco. E-mail: hananerachadi@gmail.com Case Presentation A sixty-two-year-old female presented with a painless peri- ungual lesion on the left thumb, which had evolved over three months. Clinical examination revealed a nodular pink lesion located beneath the proximal fold of the nail of the left thumb, with a verrucous surface at its distal part. The lesion was painless on palpation and did not bleed upon touch (Figure 1A). An excisional biopsy of the lesion was performed. Histopathological examination showed epithelial proliferation with a predominantly verrucous exophytic and endophytic growth pattern. The epidermis was thickened, ex- hibiting ortho- and parakeratotic hyperkeratosis. It was ma- ture, without significant cytonuclear atypia, and contained rare dyskeratotic bodies. Notably, no koilocyte was observed. The endophytic component invaginated into the dermis as large lobules or small lobules of coiled cells without atypia, with some keratin pearls. The limited visible dermis beneath the endophytic component showed a lymphocytic inflam- matory reaction and neovascularization, consistent with an inverted follicular keratosis (Figures B and C). Postoperative outcomes following excision were uncomplicated, and no re- currence was observed after one-year follow-up. Teaching Point Inverted follicular keratosis (IFK) is a rare benign lesion that typically manifests as a small, solitary, papillomatous papulo-nodule, most commonly in elderly individuals. The exact etiopathogenesis of IFK remains unclear. In 90% of cases, IFK is localized in the head and neck region. Other locations have rarely been documented in the literature, and to our knowledge, the localization in the nail unit has never been reported in the literature. Clinically and dermoscopically, IFK lacks specific dis- tinguishing features, making it challenging to differentiate it from other keratinizing lesions such as viral warts, sebor- rheic keratoses, keratoacanthoma, fibrokeratoma, squamous cell carcinoma, basal cell carcinoma, and melanoma [1]. Thus, the diagnosis of IFK is primarily established through histopathological examination. 2 Image Letter | Dermatol Pract Concept. 2025;15(4):5542 Histologically, IKF can also pose a differential diagnos- tic challenge with trichilemmoma, squamous cell carcinoma, keratoacanthoma, verruca vulgaris, and especially seborrheic keratosis, particularly when it is irritated. In this specific case, the characteristic endophytic growth pattern of IKF al- lowed for distinguishing between these two entities [2]. Surgical excision is the treatment of choice for IFK, al- though topical application of imiquimod 5% cream has also been reported to be effective. Figure 1. A nodular lesion in the proximal fold of the nail of the left thumb, with a verrucous surface at its distal part. B: Histological image (4x): epidermal prolifera- tion with both exophytic and endophytic components covered by orthokeratotic and parakeratotic hyperkeratosis. C: Histological image (10x): endophytic component with whorls and dyskeratosis. References 1. Díez-Montero C, González González D, Pérez Martínez E, Schellini S, Galindo-Ferreiro A. Periocular inverted follicular keratosis: a retrospective series over 17 years. Jpn J Ophthalmol. 2019;63:210–4. DOI: 10.1111/cup.13107. PMID: 30604112. 2. Kenney M, Lester EB, Cook DL. Inverted follicular keratoses of the buttocks. J Cutan Pathol. 2018;45:369–70. DOI: 10.1007 /s10384-018-00650-7. Epub 2019 Jan 2. PMID: 29418015.