Lichen planus pigmentosus inversus presenting with clinical features mimicking acanthosis nigricans.

Lichen planus pigmentosus (LPP) is recognized as a rare variant of lichen planus, characterized by dermal hyperpigmentation. Specifically, a particular intertriginous variant of LPP is known as lichen planus pigmentosus inversus (LPPI). In our case, the patient presented with symmetric, hyperpigmented dark brown patches mainly in axillary areas, closely resembling the features of acanthosis nigricans (AN). The differential diagnosis considered included LPPI, AN, post-inflammatory hyperpigmentation related to contact dermatitis, symmetrical drug-related intertriginous and flexural exanthema, fixed drug eruption, and erythema dyschromicum perstans (EDP). Histopathological examination revealed the absence of hyperkeratosis and papillomatosis, typically associated with AN. Dermoscopy revealed diffuse brownish hue along with dots and globules of inconsistent size, which suggests dermal pigmentary incontinence and the likelihood of LPPI. This case illustrates the challenge in differentiating LPPI from similar flexural hyperpigmentation disorders based on the comprehensive approach including thorough history taking, clinical manifestations, histopathological analysis, and dermoscopic examination.