A 38-year-old man presents with a 6-month history of well-demarcated, erythematous plaques with silvery-white scale over the extensor surfaces of elbows and knees. He also reports nail pitting and joint pain in the morning. What is the investigation of choice to confirm the diagnosis of psoriasis?

Explanation

## Diagnosis of Psoriasis ### Clinical Presentation Recognition The patient presents with classic features of plaque psoriasis: - Well-demarcated erythematous plaques with silvery-white scale - Extensor surface involvement (elbows, knees) - Nail pitting (onycholysis) - Morning joint stiffness (suggestive of psoriatic arthritis) ### Histopathological Confirmation **Key Point:** Skin biopsy is the gold standard investigation for confirming psoriasis when the clinical diagnosis is uncertain. ### Characteristic Histological Findings | Histological Feature | Pathognomonic Finding | |---|---| | Epidermis | Parakeratosis (retained nuclei in stratum corneum) | | Epidermis | Acanthosis (epidermal thickening) | | Epidermis | Thinning of suprapapillary epidermis | | Dermis | Dilated and tortuous capillaries in dermal papillae | | Dermis | Inflammatory infiltrate (lymphocytes) | | Absence | Spongiosis (rules out eczema) | **High-Yield:** The combination of parakeratosis + acanthosis + dilated capillaries in dermal papillae is virtually diagnostic of psoriasis. ### Clinical Pearl While psoriasis is often diagnosed clinically based on morphology and distribution, biopsy is indicated when: - Diagnosis is unclear - Atypical presentation - Differentiation from other papulosquamous disorders (lichen planus, pityriasis rubra pilaris) ### Auspitz Sign Scratching the scale reveals pinpoint bleeding due to dilated capillaries in the dermal papillae — this clinical sign correlates with the histological finding. [cite:Robbins 10e Ch 25]