A 28-year-old woman presents with a 2-week history of a solitary, asymptomatic, scaly patch on her left upper back. She reports mild pruritus. One week later, she develops a widespread eruption of small, oval, scaly papules and plaques on her trunk and proximal limbs, distributed along Langer's lines. The lesions are salmon-pink in colour. She denies fever, arthralgia, or recent upper respiratory infection. Serological testing for syphilis is negative. What is the most likely diagnosis?

Explanation

## Diagnosis: Pityriasis Rosea ### Clinical Presentation **Key Point:** Pityriasis rosea is a self-limited, inflammatory papulosquamous disorder characterized by a distinctive two-phase presentation. ### Pathognomonic Features | Feature | Finding in This Case | Significance | |---------|----------------------|---------------| | **Herald patch** | Solitary scaly patch preceding eruption by 1–2 weeks | Pathognomonic; appears on trunk/proximal limbs | | **Secondary eruption** | Widespread oval papules/plaques on trunk and proximal limbs | Occurs 1–2 weeks after herald patch | | **Distribution** | Along Langer's lines (cleavage lines of skin) | Creates "Christmas tree" pattern on back | | **Colour** | Salmon-pink lesions | Characteristic; distinguishes from psoriasis | | **Pruritus** | Mild to moderate | Usually non-severe | | **Systemic symptoms** | Absent in this case | Fever/arthralgia uncommon; no respiratory prodrome | | **Syphilis serology** | Negative | Rules out secondary syphilis | ### Aetiology & Pathogenesis **High-Yield:** Pityriasis rosea is likely caused by **human herpesvirus 7 (HHV-7)** or HHV-6, though the exact mechanism remains unclear. It is **not contagious**. ### Clinical Course 1. **Herald patch phase** (week 1–2): Single, well-demarcated, scaly plaque, often mistaken for tinea or psoriasis. 2. **Exanthem phase** (week 2–8): Generalized eruption of smaller lesions following skin tension lines. 3. **Resolution**: Spontaneous regression over 4–8 weeks without scarring or post-inflammatory hyperpigmentation (in fair skin); may leave transient dyspigmentation in darker skin types. ### Diagnostic Confirmation **Clinical Pearl:** Diagnosis is primarily **clinical**. No specific laboratory test is required. Negative syphilis serology helps exclude secondary syphilis, which can mimic pityriasis rosea. **Mnemonic: "SHARP" for Pityriasis Rosea features:** - **S**calp/trunk involvement - **H**erald patch (single initial lesion) - **A**long Langer's lines (distribution) - **R**ose/salmon colour - **P**apulosquamous morphology ### Management - **Reassurance**: Self-limiting condition; most resolve spontaneously in 4–8 weeks. - **Symptomatic treatment**: Emollients, topical corticosteroids (mild-to-moderate potency) for pruritus. - **Systemic therapy**: Rarely needed; oral antihistamines for severe itch. Some evidence for **oral erythromycin** (1 g/day × 2 weeks) or **UVB phototherapy** in severe cases. ### Why Negative Syphilis Serology Matters **Warning:** Secondary syphilis can present with a similar papulosquamous eruption and may also show a herald lesion (primary chancre). However, syphilis typically presents with **systemic symptoms** (fever, lymphadenopathy, constitutional signs) and **positive RPR/VDRL and treponemal serology**. Negative serology confidently excludes syphilis here. [cite:Fitzpatrick's Dermatology 9e Ch 150]