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Pityriasis rosea and other papulosquamous diseases
Pityriasis rosea is a self-limiting papulosquamous rash linked to reactivation of HHV-6 and HHV-7, starting with a "herald patch" followed by a generalised rash in a "Christmas tree" pattern on the trunk; it clears on its own within 6–8 weeks, but secondary syphilis must be excluded.
Why it matters in the exam
The exam shows a teenager or young adult with a single oval salmon-pink patch on the trunk, followed after one to two weeks by a rash of smaller patches with an inner "collarette" of scale, arranged along skin lines. The question tests the right step: reassurance and treating the itch, and a syphilis test when the rash involves the palms and soles, when there is no herald patch or when there is sexual risk. Also asked are pityriasis rosea in pregnancy (in the first half it may be linked to miscarriage), drugs that cause a similar rash, and other papulosquamous diseases: guttate psoriasis after a sore throat, pityriasis lichenoides with lesions at different stages, pityriasis rubra pilaris (PRP) with "islands of sparing" and orange palms, and parapsoriasis. Remember that high-dose aciclovir shortened the illness in some studies, and that sunlight or NB-UVB eases the itch. The disease is not meaningfully contagious and isolation is not needed. It is important to explain that the disease leaves no scars, but in darker skin dark or light patches may remain for months.
Key points
- Pityriasis rosea is a self-limiting papulosquamous rash linked to HHV-6 and HHV-7.
- It starts with a herald patch, followed by a Christmas-tree rash on the trunk with an inner collarette of scale.
- The disease clears on its own within 6–8 weeks, and treatment is reassurance and symptomatic.
- Secondary syphilis must be excluded when the palms are involved, there is no herald patch or there is sexual risk.
- Other papulosquamous diseases include guttate psoriasis, pityriasis lichenoides and PRP.
Summary table
| Disease | Decisive feature | Test | Treatment |
|---|---|---|---|
| Pityriasis rosea | Herald patch, then a Christmas-tree rash with an inner collarette of scale | Clinical diagnosis; VDRL if in doubt | Reassurance, emollients, antihistamines; clears within 6–8 weeks |
| Secondary syphilis | Rash on the palms and soles, nodes, oral lesions, no herald patch | VDRL or RPR and TPPA | Benzathine penicillin G |
| Guttate psoriasis | Small "drop-like" papules with silvery scale after a sore throat | ASO and throat swab | Topical steroids, NB-UVB |
| Pityriasis lichenoides | Papules at different stages, some necrotic, relapsing course | Biopsy | Tetracycline or erythromycin, phototherapy |
| Pityriasis rubra pilaris (PRP) | Orange-red plaques with islands of normal skin, follicular papules, thick orange palms and soles | Biopsy | Acitretin, methotrexate, biologics |
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Sources
- British Association of Dermatologists patient and clinical guidance on pityriasis rosea, 2021 — Diagnosis and management
- CDC Sexually Transmitted Infections Treatment Guidelines, 2021 — Secondary syphilis
- Fitzpatrick's Dermatology, 9th edition (2019) — Pityriasis rosea, pityriasis lichenoides and pityriasis rubra pilaris
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.