A 45-year-old woman with a 10-year history of plaque psoriasis now presents with acute onset of widespread erythema, pustules on an erythematous base, and systemic symptoms (fever, chills, malaise). She denies recent infection or medication changes. Which investigation would be most appropriate to confirm the diagnosis of pustular psoriasis?

Explanation

## Pustular Psoriasis: Diagnosis and Histopathology **Key Point:** Pustular psoriasis is a severe variant of psoriasis characterized by sterile pustules on an erythematous base. Skin biopsy is the gold standard investigation to confirm this diagnosis and rule out infectious pustular dermatoses. ### Clinical Presentation of Pustular Psoriasis Pustular psoriasis presents as: - Widespread erythema with sterile pustules (not infected) - Systemic symptoms: fever, malaise, chills, arthralgias - May be localized (acanthosis palmaris/plantaris) or generalized (acute generalized pustular psoriasis, AGPP) - Often triggered by infection, medication withdrawal (especially systemic corticosteroids), or pregnancy ### Histopathological Features **High-Yield:** The diagnostic hallmark is **spongiform pustules** (intraepidermal collections of neutrophils within the stratum corneum and stratum spinosum): 1. **Epidermal findings:** - Spongiform pustules (neutrophilic microabscesses) - Acanthosis - Parakeratosis - Thinned suprapapillary epidermis 2. **Dermal findings:** - Dilated capillaries - Inflammatory infiltrate - Absence of organisms on special stains (rules out infection) **Clinical Pearl:** The pustules are **sterile** — bacterial culture yields no growth. This distinguishes pustular psoriasis from bacterial pustulosis or folliculitis. ### Why Biopsy Confirms Diagnosis ```mermaid flowchart TD A[Acute pustular eruption with fever]:::outcome --> B{Clinical diagnosis clear?}:::decision B -->|Yes, typical history| C[Skin biopsy to confirm]:::action B -->|No, atypical| C C --> D{Histology shows spongiform pustules?}:::decision D -->|Yes| E[Pustular psoriasis confirmed]:::outcome D -->|No| F[Consider alternative diagnosis]:::outcome F --> G[Bacterial culture, fungal culture, or viral PCR]:::action ``` **Mnemonic:** **SPONGIFORM** = **S**terile **P**ustules **O**n **N**eutrophilic **G**athering in **I**ntraepidermal **F**oci **O**f **R**etained **M**orphology — the defining feature of pustular psoriasis histology. ### Differential Diagnosis: Pustular Lesions | Feature | Pustular Psoriasis | Acute Generalized Exanthematous Pustulosis (AGEP) | Bacterial Folliculitis | Candidiasis | | --- | --- | --- | --- | --- | | **Spongiform Pustules** | Present | Present | Absent | Absent | | **Parakeratosis** | Present | Absent | Absent | Absent | | **Systemic Symptoms** | Marked | Marked | Mild | Mild | | **Bacterial/Fungal Culture** | Sterile | Sterile | Positive | Positive | | **Trigger** | Infection, steroids, pregnancy | Drug reaction | Poor hygiene | Immunosuppression | **Tip:** When pustular eruption is acute and systemic symptoms are present, biopsy is essential to confirm sterile pustules and exclude infectious etiologies before initiating systemic corticosteroids (which can worsen infection).