A 38-year-old man presents with a 6-month history of well-demarcated, erythematous plaques with silvery-white scale on the extensor surfaces of elbows and knees. The lesions are pruritic and worsen in winter. Physical examination reveals Auspitz sign and Koebner phenomenon. Which investigation is most appropriate to confirm the diagnosis?

## Diagnosis of Psoriasis: Role of Skin Biopsy **Key Point:** While psoriasis is primarily a clinical diagnosis based on characteristic morphology and distribution, skin biopsy with histopathology is the most appropriate confirmatory investigation when diagnosis is uncertain or atypical presentations occur. ### Histopathological Features of Psoriasis A punch or shave biopsy from an active lesion demonstrates: 1. **Epidermal changes:** - Acanthosis (epidermal thickening) - Parakeratosis (retention of nuclei in stratum corneum) - Thinning of the suprapapillary epidermis - Elongation of rete ridges 2. **Dermal changes:** - Dilated and tortuous capillaries in dermal papillae - Inflammatory infiltrate (lymphocytes, neutrophils) - Microabscesses in stratum corneum (Munro microabscesses) **High-Yield:** The combination of parakeratosis, acanthosis, and Munro microabscesses is pathognomonic for psoriasis and distinguishes it from other papulosquamous disorders. ### Why Biopsy Is Confirmatory **Clinical Pearl:** Psoriasis diagnosis is usually clinical (based on Auspitz sign, Koebner phenomenon, and characteristic distribution), but biopsy becomes essential in: - Atypical or unusual presentations - Diagnostic uncertainty (e.g., distinguishing from lichen planus or pityriasis rubra pilaris) - Pustular or erythrodermic variants - Nail involvement requiring confirmation **Tip:** The biopsy should be taken from an active lesion margin, not from the center (which may show different histology) or from resolved areas. ### Differential Diagnosis Table | Feature | Psoriasis | Lichen Planus | Pityriasis Rosea | | --- | --- | --- | --- | | **Parakeratosis** | Present | Absent | Focal | | **Munro Microabscesses** | Present | Absent | Absent | | **Hypergranulosis** | Absent | Present | Present | | **Wedge-shaped Infiltrate** | Absent | Present | Absent | **Mnemonic:** **PAMS** = **P**arakeratosis, **A**canthosis, **M**unro microabscesses, **S**uprapapillary thinning — the hallmark tetrad of psoriasis histology.