A 42-year-old woman with a 10-year history of plaque psoriasis now presents with sudden onset of widespread erythema, pustules, and systemic symptoms (fever, malaise). She had recently stopped her oral corticosteroid therapy. What is the most appropriate investigation to confirm the diagnosis of pustular psoriasis?

Explanation

## Pustular Psoriasis — Diagnostic Confirmation ### Clinical Context The patient presents with acute transformation from plaque psoriasis to pustular psoriasis, a severe variant triggered by: - Abrupt withdrawal of systemic corticosteroids (most common trigger) - Infections - Medications (NSAIDs, lithium) - Pregnancy **High-Yield:** Pustular psoriasis is a medical emergency with systemic manifestations (fever, malaise, fluid loss, electrolyte imbalance). ### Histopathological Diagnosis ### Key Point Skin biopsy is the investigation of choice to confirm pustular psoriasis and differentiate it from other pustular disorders. ### Characteristic Histological Findings in Pustular Psoriasis | Feature | Finding | Significance | |---|---|---| | **Pustule location** | Spongiform pustules within stratum corneum (Kogoj pustules) | Pathognomonic for psoriasis | | **Epidermal changes** | Acanthosis with elongated rete ridges | Consistent with psoriasis | | **Dermal changes** | Dilated capillaries, inflammatory infiltrate | Typical psoriatic pattern | | **Absence** | No subcorneal pustules (unlike pemphigus) | Rules out pemphigus vulgaris | | **Absence** | No IgA deposits (unlike dermatitis herpetiformis) | Rules out DH | **Clinical Pearl:** Kogoj pustules (spongiform pustules formed by neutrophils within the epidermis) are characteristic of psoriasis and distinguish it from subcorneal pustular dermatosis or pemphigus. ### Mnemonic for Pustular Psoriasis Triggers **SLIP** — Steroids (withdrawal), Lithium, Infections, Pregnancy ### Why Biopsy Over Other Tests - Blood culture: Rules out secondary infection but does not confirm pustular psoriasis - CBC/LFTs: Assess systemic involvement but not diagnostic - Immunofluorescence: Used for autoimmune blistering disorders (pemphigus, DH), not psoriasis [cite:Harrison 21e Ch 325]