Question: A 33-year old driver complained of mildly pruritic, generalized erythematous eruption first noted at his left inner arm for the past 6 weeks. More extensive involvement of scaly erythematous maculopapular rash scattered at his trunk and four limbs in the past 4 weeks. He otherwise enjoyed good health. There were no systemic upset, no sore-throat, nor any herbs or medications intake prior to the onset of skin rash.

Answer: C. Pityriasis rosea

Clinically, the patient presented with an erythematous papulosquamous skin rash which can have many differential diagnoses. Common conditions like eczema and fungal infection should be included though the mild pruritus makes these diagnoses less likely. Pityriasis rosea is not a rare skin disease worldwide. It is an acute, self-limiting disease lasting for 6 to 8 weeks. The first manifestation of the disease is usually the appearance of the herald patch, which is larger and is usually situated on the upper arm as in this case. It is a sharply defined, round or oval plaque soon covered by fine scales. It rapidly reaches its maximum size of 5 cm or larger. After an interval of one week or so, the generalized eruption appears as crops from days to weeks. The centre tends to clear with a marginal collarette of scale attached peripherally. The long axes of the lesions follow the line of cleavage parallel to the ribs in a so called Christmas-tree pattern. Clinical recognition of the pattern is not difficult though sometimes there may be atypical forms. It can be asymptomatic but can sometimes be variably pruritic. The most important differential diagnosis is secondary syphilis. Generalized lymphadenopathy with some systemic upset should alert one to exclude this condition by taking a screening test for syphilis e.g. EIA-syphilis with VDRL test. No herald patch will be found in secondary syphilis. Skin lesions of guttate psoriasis are more papular and persistent. Silvery scaling may be demonstrated by scraping the oval patches by a wooden spatula. The characteristic marginal collarette scaling in pityriasis rosea will not be present. The rash in subacute cutaneous lupus erythematosus is usually of larger size, and in the form of annular polycyclic lesions. It will not present with a Christmas-tree pattern and can be found around the neck, outer aspects of the arms and face in addition to the trunk. The blood immune markers, anti-Ro and anti-La antibodies are usually positive.