Dermatology · Papulosquamous Eruptions
The facts most likely to be tested
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The initial clinical presentation is a solitary, large, salmon-colored herald patch that precedes the generalized eruption.
The secondary generalized eruption follows a classic Christmas tree distribution along the Langer lines of the trunk.
Lesions typically exhibit a characteristic collarette of scale at the periphery of the erythematous plaques.
Pityriasis rosea is strongly associated with reactivation of Human Herpesvirus 6 (HHV-6) or Human Herpesvirus 7 (HHV-7).
The condition is self-limiting and typically resolves spontaneously within 6 to 8 weeks without specific intervention.
Differential diagnosis must always include secondary syphilis, which requires a RPR or VDRL test if the patient has palmar or plantar involvement.
Treatment is primarily supportive with topical corticosteroids or oral antihistamines used only for symptomatic relief of pruritus.
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A 22-year-old male presents to the clinic complaining of a rash on his back that appeared 2 weeks after a single, larger patch on his chest. Physical examination reveals a generalized eruption of small, salmon-colored papules and plaques arranged in a Christmas tree distribution on the trunk. Each lesion displays a fine collarette of scale. The patient denies fever, malaise, or recent medication changes. His palms and soles are spared.
What is the most likely diagnosis?
Pityriasis rosea
The diagnosis is based on the classic sequence of a herald patch followed by a Christmas tree distribution of scaly plaques, which is highly characteristic of pityriasis rosea.
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Etiology / Epidemiology
Common in adolescents and young adults; likely associated with HHV-7 or HHV-6 reactivation.
Clinical Manifestations
Starts with a herald patch followed by a Christmas tree distribution of smaller, scaly plaques.
Diagnosis
Diagnosis is clinical; no specific lab tests are required for typical presentations.
Treatment
Self-limiting; supportive care with topical steroids or antihistamines for pruritus.
Prognosis
Spontaneous resolution within 6 to 8 weeks; no long-term sequelae.
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Epidemiology & Etiology
Primarily affects individuals aged 10–35 years. While the exact trigger is debated, it is strongly linked to HHV-7 and HHV-6 reactivation. It is non-contagious and typically occurs in isolated cases or small clusters.
Pertinent Anatomy
The eruption follows Langer's lines (skin tension lines) on the trunk. This anatomical distribution creates the characteristic Christmas tree pattern on the back.
Pathophysiology
Viral reactivation leads to a cell-mediated immune response. The initial herald patch represents the primary site of viral activity, followed by a secondary generalized eruption as the immune system responds to viral antigens.
Clinical Manifestations
The herald patch is a solitary, salmon-colored oval plaque with a trailing scale. This is followed 1–2 weeks later by a generalized eruption of smaller, oval, scaly papules. Red flags include lesions on palms/soles or failure to resolve after 12 weeks, which should prompt biopsy to rule out secondary syphilis.
Diagnosis
Diagnosis is clinical. If the presentation is atypical or involves palms/soles, order an RPR or VDRL to exclude secondary syphilis. A skin biopsy is reserved for diagnostic uncertainty.
Treatment
No curative therapy is required as the condition is self-limiting. Use topical corticosteroids or oral antihistamines for symptomatic relief of pruritus. Avoid systemic steroids as they are unnecessary for this benign, transient condition.
Prognosis
Lesions typically resolve spontaneously within 6 to 8 weeks. Post-inflammatory hypopigmentation or hyperpigmentation may occur, especially in darker skin tones, but usually fades over time.
Differential Diagnosis
Secondary Syphilis: involves palms and soles
Tinea Corporis: central clearing and fungal hyphae on KOH
Guttate Psoriasis: thicker, silvery scale, often post-streptococcal
Nummular Eczema: coin-shaped, intensely pruritic, no herald patch
Drug Eruption: history of new medication initiation