Self-limited papulosquamous eruption with herald patch and Christmas-tree distribution; likely HHV-6/7 reactivation.
Also known as: pityriasis rosea, PR, herald patch
Overview
A benign, self-limited papulosquamous eruption that begins with a single 'herald patch' followed 1-2 weeks later by a generalized symmetric eruption of oval salmon-colored plaques on the trunk and proximal extremities in a Christmas-tree (fir-tree) pattern.
Epidemiology
Affects 0.5-2% of the population annually. Most common ages 10-35. Slight female predominance. Increased incidence in spring and fall in temperate climates.
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Question 1DermatologyMedium
A 24-year-old woman presents with a 2-week history of a single 3-cm oval, salmon-pink plaque with a fine collarette of scale on her trunk, followed several days ago by numerous smaller oval pink plaques over her back and proximal arms. She recalls a mild sore throat and malaise about a week before the first lesion appeared. The smaller lesions are oriented along skin tension lines in a fir-tree pattern, and her palms, soles, and face are spared. She is diagnosed with pityriasis rosea. Which of the following best explains the findings?
AReactivation of latent human herpesvirus 6 and 7 with cutaneous viremia
BIgE-mediated mast cell degranulation against an inhaled environmental allergen
CDermatophyte invasion of the stratum corneum with centrifugal spread
DAutoreactive T-cell response triggered by recent streptococcal infection
EHematogenous dissemination of Treponema pallidum to the skin
Reveal answer & full explanation
Correct answer: A — Reactivation of latent human herpesvirus 6 and 7 with cutaneous viremia
AReactivation of latent human herpesvirus 6 and 7 with cutaneous viremia✓
BIgE-mediated mast cell degranulation against an inhaled environmental allergen
CDermatophyte invasion of the stratum corneum with centrifugal spread
DAutoreactive T-cell response triggered by recent streptococcal infection
EHematogenous dissemination of Treponema pallidum to the skin
Why Reactivation of latent human herpesvirus 6 and 7 with cutaneous viremia is correct
Strong evidence supports systemic reactivation of HHV-6 and HHV-7, with viremia and localization to the skin driving the eruption.
The viral prodrome (sore throat, malaise) in roughly half of patients, the seasonal clustering, and the very low recurrence rate (under 3%) all point to an infectious, reactivation-based etiology.
The resulting T-cell-mediated interface and spongiotic dermatitis produces the herald patch and the symmetric collarette-scaled plaques in the Christmas-tree distribution.
Why the others are wrong
Hematogenous dissemination of Treponema pallidum to the skin — this is secondary syphilis, the classic pityriasis rosea mimic; it lacks a herald patch, characteristically involves the palms and soles (spared here), and often comes with mucous patches, condylomata lata, and generalized lymphadenopathy. Syphilis serology is still worth checking in a sexually active patient with a PR-like eruption.
Autoreactive T-cell response triggered by recent streptococcal infection — this is the mechanism of guttate psoriasis (post-streptococcal), which causes smaller silvery drop-shaped papules rather than collarette-scaled salmon plaques in a fir-tree pattern.
IgE-mediated mast cell degranulation against an inhaled environmental allergen — this type I hypersensitivity mechanism underlies urticaria and atopic flares, not the papulosquamous eruption of pityriasis rosea.
Dermatophyte invasion of the stratum corneum with centrifugal spread — the herald patch, an oval scaly plaque with peripheral scale, is routinely misread as tinea corporis, but dermatophyte lesions have a raised advancing border with central clearing and are KOH positive; tinea does not follow a viral prodrome with a crop of secondary plaques along cleavage lines in a fir-tree pattern.
Question 2DermatologyMedium
A 24-year-old woman presents with a 2-week history of an itchy rash. She first noticed a single 3-cm oval, salmon-colored plaque on her flank, followed about 10 days later by a generalized eruption of smaller oval pink plaques over her trunk and proximal arms. On examination, the plaques have a fine collarette of scale at the periphery and are oriented along skin tension lines, producing a Christmas-tree pattern across her back. Her face, palms, and soles are spared, and an RPR is nonreactive. Which of the following is the most appropriate initial management?
AOral terbinafine and a topical antifungal cream
BTopical selenium sulfide and oral fluconazole
COral erythromycin four times daily for 14 days
DTopical corticosteroid and oral antihistamine
EOral acyclovir and a topical antiviral cream
Reveal answer & full explanation
Correct answer: D — Topical corticosteroid and oral antihistamine
AOral terbinafine and a topical antifungal cream
BTopical selenium sulfide and oral fluconazole
COral erythromycin four times daily for 14 days
DTopical corticosteroid and oral antihistamine✓
EOral acyclovir and a topical antiviral cream
Why Topical corticosteroid and oral antihistamine is correct
The herald patch, Christmas-tree distribution along Langer lines, peripheral collarette of scale, and sparing of palms/soles with a nonreactive RPR are classic for pityriasis rosea, a benign, self-limited HHV-6/HHV-7-associated eruption.
Initial management is reassurance that it resolves in 6-8 weeks without scarring plus symptomatic care for pruritus with a low- to mid-potency topical corticosteroid and an oral antihistamine, supported by emollients.
Why the others are wrong
Oral terbinafine and a topical antifungal cream - treat tinea corporis; tinea is KOH-positive with an outward-facing scaling border, not the inward collarette and Christmas-tree pattern seen here.
Oral erythromycin four times daily for 14 days - an early small trial suggested benefit, but later randomized trials and systematic reviews have not confirmed it; pityriasis rosea is a viral exanthem, and macrolides are not recommended because they add gastrointestinal adverse effects without reliable benefit.
Oral acyclovir and a topical antiviral cream - reserved for severe, extensive, or highly symptomatic disease started in the first week; antiviral therapy offers only modest benefit and is not used for typical, mild pityriasis rosea.
Topical selenium sulfide and oral fluconazole - treat pityriasis (tinea) versicolor, a superficial Malassezia infection; the shared name and truncal distribution invite confusion, but versicolor produces hypo- or hyperpigmented macules with fine scale and lacks a herald patch, an inner collarette, and a Christmas-tree orientation.
Additional high-yield points
Secondary syphilis is the key mimic and is treated with IM benzathine penicillin G; it is excluded here by sparing of the palms and soles and a nonreactive RPR, and an RPR should be checked in sexually active patients or atypical presentations of a pityriasis rosea-like eruption.
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Strong evidence supports reactivation of human herpesviruses HHV-6 and HHV-7 with viremia and skin localization. Mild prodrome, seasonality, and rare recurrence (<3%) suggest infectious etiology. T-cell mediated inflammation produces interface and spongiotic dermatitis.
Pruritus mild to moderate (25-75%), occasionally severe
Asymptomatic in many patients
Signs / physical exam
Herald patch: 2-10 cm single oval, salmon-pink plaque with collarette of scale at periphery; usually on trunk; precedes generalized eruption by 1-2 weeks (sometimes weeks)
Secondary eruption: numerous smaller (0.5-2 cm) oval pink plaques with peripheral collarette of fine scale
Distribution: trunk and proximal extremities; long axis of lesions parallels skin tension (Langer) lines → 'Christmas tree' or 'fir tree' pattern on back
Spares face, palms, soles (typical) — palmoplantar involvement should prompt syphilis workup
Inverse pityriasis rosea: variant predominantly in axillae and groin; more common in children and skin of color
Classic findings
Herald patch + Christmas-tree distribution on trunk + collarette of scale on peripheral lesions.
Differential diagnosis
Secondary syphilis — MUST rule out — palmoplantar copper macules, lymphadenopathy, condyloma lata, mucous patches; positive RPR/FTA. Check RPR in all sexually active adults with PR-like eruption
Tinea corporis (multiple lesions) — Annular plaques with raised border; KOH positive
Nummular eczema — Coin-shaped pruritic plaques, not in Christmas-tree pattern
Pityriasis lichenoides chronica — Chronic recurrent crops of papules with mica-like scale, longer course
Drug-induced PR-like eruption — More widespread, less typical distribution, longer course; recent new drug
Lichen planus — Pruritic violaceous polygonal papules with Wickham striae
Erythema migrans (Lyme disease) — Expanding annular erythema, usually without scale, at a tick-bite site (multiple lesions in early disseminated disease, often with fever, myalgia, arthralgia); no herald patch, no collarette scale, not aligned with skin lines
Diagnostic workup
Diagnostic criteria
Clinical: herald patch + secondary eruption in characteristic distribution + collarette of scale; self-limited course 6-8 weeks.
Labs
Clinical diagnosis
RPR/VDRL to exclude secondary syphilis in all sexually active adults
flowchart TD
A["Single oval scaly plaque<br/>'Herald patch'"] --> B["1-2 weeks later"]
B --> C["Generalized eruption<br/>Trunk + proximal extremities"]
C --> D["Oval lesions parallel<br/>Langer lines = 'Christmas tree'"]
D --> E{"Sexually active<br/>adult?"}
E -->|Yes| F["Check RPR/VDRL<br/>Rule out 2° syphilis"]
E -->|No| G["Clinical diagnosis"]
F --> G
G --> J{"Pregnant<br/><15 wks?"}
J -->|Yes| K["OB consult; consider acyclovir;<br/>monitor for preterm delivery"]
J -->|No| H["Reassure + symptomatic care<br/>Emollients, low-mid steroid, antihistamine"]
K --> H
H --> I["Self-resolves in 6-8 weeks"]
Pityriasis rosea diagnostic and management algorithm.
Treatment
First-line
Reassurance — disease is self-limited, resolving in 6-8 weeks (range 2 weeks to 3 months) without scarring
Symptomatic care for pruritus: emollients, low-mid potency topical corticosteroid (triamcinolone 0.1%), oral antihistamines (cetirizine, hydroxyzine for sleep)
Oatmeal baths, calamine lotion for itch
Sun exposure or narrowband UVB phototherapy may shorten course for severe pruritus
Severe / extensive / unresponsive
Oral acyclovir 800 mg 5x/day × 7 days — modest evidence for faster resolution and itch relief if started in first week
Short course oral prednisone (rarely needed) — controversial, may prolong course
Pityriasis rosea in pregnancy
Increased risk of preterm delivery and fetal loss if PR occurs in first 15 weeks of gestation (limited data)
Consider obstetric consultation and serial fetal monitoring
Acyclovir has reassuring pregnancy safety data and may be considered for early gestational PR
Second-line / adjunct
Educate patient: condition is NOT contagious to household contacts
Recurrence is rare (<3%) — counsel to return if relapsing or persistent >3 months
Complications
Post-inflammatory hyperpigmentation, especially in skin of color (months to fade)
Pruritus disrupting sleep
Possible adverse pregnancy outcomes (preterm delivery, fetal loss) if early gestation — limited evidence
Diagnostic delay missing secondary syphilis
PANCE pearls
ALWAYS check RPR/VDRL in sexually active adults with PR — secondary syphilis is the great mimicker (the 'great imitator').
Herald patch may be misdiagnosed as tinea — KOH is helpful when in doubt; collarette scale faces inward in PR vs outward border in tinea.
Christmas-tree pattern is best seen on the back with lesions oriented along Langer lines.
Atypical PR (inverse, predominantly facial, palmoplantar involvement, vesicular, hemorrhagic) should prompt biopsy and broader workup.
PR is self-limited — patients want a name and a timeline, not an aggressive treatment.
References
AAFP 2018 — Pityriasis Rosea: Diagnosis and Treatment (Villalon-Gomez JM, Am Fam Physician 2018;97(1):38-44)
Drago HHV — Pityriasis rosea: an update with a critical appraisal of its possible herpesviral etiology (Drago F, Broccolo F, Rebora A, J Am Acad Dermatol 2009;61(2):303-318)
Pregnancy PR — Pregnancy outcome in patients with pityriasis rosea (Drago F, et al., J Am Acad Dermatol 2008;58(5 Suppl 1):S78-S83)
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