A 22-year-old woman from Mumbai presents with sudden onset of numerous small, red, scaly papules covering her trunk and proximal limbs over the past 2 weeks. She reports a sore throat 3 weeks ago that resolved spontaneously. She denies previous skin disease. On examination, lesions are < 1 cm in diameter, distributed symmetrically on the trunk and flexural surfaces. Nikolsky sign is negative. What is the most likely diagnosis?

## Diagnosis: Guttate Psoriasis **Key Point:** Guttate psoriasis is an acute form of psoriasis characterized by the sudden onset of small (< 1 cm), drop-like papules and plaques, typically triggered by a streptococcal infection (usually pharyngitis) 2–3 weeks prior. It accounts for 2–3% of psoriasis cases. ### Clinical Features Supporting Guttate Psoriasis | Feature | Significance | |---------|-------------| | **Acute onset** | Sudden appearance over days to weeks | | **Small lesions (< 1 cm)** | Drop-like or "guttate" morphology (gutta = drop in Latin) | | **Trunk and proximal limbs** | Typical distribution; flexural involvement possible | | **Preceding streptococcal pharyngitis** | 3 weeks prior — classic trigger | | **No prior skin disease** | First presentation of psoriasis | | **Symmetric distribution** | Characteristic pattern | | **Negative Nikolsky sign** | Rules out pemphigus/pemphigoid | **High-Yield:** The **temporal relationship** between streptococcal pharyngitis (2–3 weeks prior) and acute onset of small papules is the key diagnostic clue for guttate psoriasis. This is the most common trigger for guttate psoriasis in young patients. ### Pathophysiology 1. **Streptococcal infection** (usually Group A Streptococcus) triggers immune response 2. **Cross-reactivity** between streptococcal antigens and keratinocyte antigens 3. **T-cell activation** leads to psoriasiform inflammation 4. **Acute onset** of numerous small lesions ("rain on the skin" appearance) **Clinical Pearl:** Guttate psoriasis can **resolve spontaneously** in 50–70% of cases within weeks to months, or it may evolve into chronic plaque psoriasis. Approximately 10% of patients with guttate psoriasis have a family history of psoriasis. ### Differential Diagnosis | Condition | Key Distinguishing Features | |-----------|----------------------------| | **Guttate psoriasis** | Acute onset; small (< 1 cm) lesions; preceding streptococcal pharyngitis; trunk/proximal limbs | | **Plaque psoriasis** | Chronic course; large well-demarcated plaques; extensor surfaces; silvery scale; no preceding infection | | **Pityriasis rosea** | Herald patch; oval lesions with "Christmas tree" distribution; fine scale; trunk; self-limited; no streptococcal trigger | | **Lichen planus** | Violaceous, flat-topped papules; Wickham striae; oral involvement; pruritic; no preceding infection | **Mnemonic: GUTTATE PSORIASIS TRIGGERS — STRESS** - **S**treptococcal pharyngitis (most common) - **T**rauma - **R**espiratory infections - **E**motional stress - **S**ystemic illness - **S**kin infections ### Management Considerations 1. **Confirm streptococcal infection** — Throat culture or ASO titer if available 2. **Treat streptococcal infection** — Antibiotics (penicillin V or amoxicillin) may prevent progression 3. **Topical corticosteroids** — First-line for localized lesions 4. **Phototherapy** — Consider if extensive involvement 5. **Systemic agents** — Reserved for severe or persistent cases **Warning:** Do NOT confuse guttate psoriasis with pityriasis rosea. Pityriasis rosea has a **herald patch**, **oval lesions** with fine scale in a **Christmas tree distribution**, and is **self-limited**. It is NOT triggered by streptococcal infection. [cite:Fitzpatrick's Dermatology 9e Ch 9]