Plasma Cell Predominance in Lichen Planus Pigmentosus: A Rare Case in a Young Female.

Lichen planus pigmentosus (LPP), a rare variant of lichen planus, characterized by the insidious onset of dark brown to gray macules. In this report, we present a unique case of LPP in a 30-year-old female patient who developed diffuse hyperpigmented macules with nail dystrophy. Notably, histopathology revealed classic lichenoid interface dermatitis with predominant plasma cell infiltration in the papillary dermis, confirmed by strong CD38 and CD138 immunoreactivity. Laboratory evaluation revealed elevated serum Kappa light chain and thyroid stimulating hormone levels, raising the possibility of immune dysregulation. No evidence of plasma cell malignancy was found after further hematologic evaluation. To our knowledge, this may be the first reported case of plasma cell-rich LPP in a young female with concurrent nail involvement and isolated serum kappa light chain elevation without evidence of clonality. Awareness of this entity is essential for differentiating it from mimickers such as cutaneous plasmacytosis, multicentric Castleman disease, and secondary syphilis. This case highlights the need for heightened awareness of this variant and its possible systemic associations, and underscores the importance of integrating histopathology, immunophenotyping, and laboratory workup to avoid misdiagnosis and guide clinical care.