Ans. A. Pityriasis rosea. (Ref. Harrison's 18lh/pg. 367)In Pityriasis rosea, initially, there is a single 2- to 6-cm annular salmon-colored patch (herald patch) with a peripheral rim of scale, followed in days to weeks by a generalized eruption involving the trunk and proximal extremities.Pityriasis rosea:# It is an acute, self-limiting skin condition. A primary plaque ('herald patch') is followed by a distinctive, generalized itchy rash 1-2 weeks later. The rash lasts for approximately 2-6 weeks. Lesions are typically oval, dull pink or tawny and appear in a 'Christmas tree' distribution, usually on the trunk and the upper arms and legs.# It is thought that human herpes viruses 6 and 7 may play a role in etiopathogenesis. Some drugs, eg bismuth, barbiturates, captopril, gold, metronidazole, D-penicillamine and isotretinoin occasionally cause a drug-induced pityriasis rosea.# Pityriasis rosea is a self-limiting disease, and treatment is supportive.# Exposure to sunlight is helpful.# Topical zinc oxide and calamine lotion are useful for pruritus.# Pruritus can also be treated with topical corticosteroids, oral antihistamines or antipruritic lotions.# Oral erythromycin may be effective in treating the rash and decreasing the itch.- Pityriasis alba is an uncommon feature of atopic dermatitis that presentsin children and adolescents. Light- coloured skin spots also occur in pityriasis alba (slightly scaly, on skin exposed to the light), Gibert's pityriasis rosea, pityriasis versicolor (Gr. "pityron" = bran; refers to the light skin scaliness). Pityriasis versicolor is a very common skin infection with a fungus: Pityrosporum ovale (yeast stage) or Malassezia furfur (mycelium stage). This lipophilic fungus forms the tyrosinase inhibitor azelaic acid from sebaceous fats, a substance which inhibits melanin synthesis. This explains the white appearance of the skin spots. Account must be taken of the fact that depigmented skin spots can also be caused by damage to the melanocytes (pigment cells) after an ordinary infection, wound or bum (post-inflammatory hypopigmentation), due to vitiligo (the texture of the skin with this condition is otherwise normal), as a residual lesion in endemic treponematosis and syphilis (the differential diagnosis is often difficult here). It is important to know that people with leprosy often have a false positive VDRL (screening for syphilis). TPHA permits differentiation.Pityriasis rubra pilaris (also known as "Devergie's disease,"or "Lichen ruber acuminatus," and "Lichen ruber pilaris"refers to a group of chronic disorders characterized by reddish orange, scaling plaques and keratotic follicular papules. Symptoms may include reddish-orange patches (Latin: rubra) on the skin, severe flaking (Latin: pityriasis), uncomfortable itching, thickening of the skin on the feet and hands, and thickened bumps around hair follicles.