Geographic, polygonal, light-brown scales and fluorescent bright-blue margins: Characteristic dermoscopic clues for granular parakeratosis

• Identification of a distinctive dermoscopic feature of granular parakeratosis: large, geographic, polygonal, light-brown scales. • A novel, specific UVFD sign—bright-blue margins—is present in 81.0% of cases of granular parakeratosis, providing a powerful diagnostic marker. • Establishment of a comparative dermoscopic/UVFD framework that accurately differentiates granular parakeratosis from its clinical mimics (inverse psoriasis, tinea cruris, and erythrasma). • Proposes a shift towards non-invasive diagnosis by integrating dermoscopy and UVFD, potentially reducing the need for confirmatory biopsy in granular parakeratosis. Granular parakeratosis (GP) is an uncommon dermatosis characterized by scaly, erythematous to brownish eruptions in intertriginous areas. Diagnosis is challenging and traditionally requires histological confirmation. While dermoscopy and ultraviolet-induced fluorescence dermoscopy (UVFD) offer non-invasive diagnostic approaches, their characteristic findings in GP remain inadequately defined. This study aimed to systematically characterize the dermoscopic and UVFD features of GP, with a secondary aim of evaluating their diagnostic utility in differentiating GP from other intertriginous dermatoses. We retrospectively analyzed 21 patients with clinicopathologically confirmed GP. Dermoscopic and UVFD data from patients with inverse psoriasis (n=10), tinea cruris (n=23), and erythrasma (n=12) were collected for comparison. Parameters included background color, vascular morphology and distribution, scale color and distribution, and UVFD fluorescence. Among 21 GP patients (male: female 13:8; mean age 38.4 ± 15.2 years), 71.4% reported prior exposure to disinfectants containing benzalkonium chloride. Symmetrical, erythematous to brown patches/plaques with variable scaling were observed, predominantly in axillae (61.9%), with pruritus /burning in 71.4%. Dermoscopy revealed large, geographic, polygonal, light-brown scales. Under UVFD, 81% of GP exhibited bright-blue margins. Comparative analysis distinguished GP from inverse psoriasis (uniform red background with regular vessels), tinea cruris (peripheral scaling and distinct vascular patterns), and erythrasma (coral-red fluorescence under UVFD, also seen in 60.0% of inverse psoriasis cases). Dermoscopy and UVFD provide reliable, non-invasive diagnostic clues for the GP. The presence of light-brown geographic scales and fluorescent bright-blue margins offers a practical, non-invasive tool for accurate GP diagnosis.