Parapsoriasis is a general term for a group of papulosquamous skin diseases characterized by chronicity, absence of conscious symptoms and erythematous scaling. The current classifications are: small plaque parapsoriasis, large plaque parapsoriasis, chronic mossy furunculosis, and acute acneiform mossy furunculosis. These diseases are prone to coexist or overlap with each other and share the common feature of being T-cell clonal proliferative diseases, with macular parapsoriasis associated with cutaneous T-cell lymphoma. Etiology and pathogenesis The etiology is unknown. It is currently considered to be a lymphoproliferative disease of skin-associated lymphoid tissue, a clonal dermatitis intermediate between chronic dermatitis and mycosis fungoides. Evidence includes: clonal proliferation of T cells can be detected in lesions and peripheral blood of patients with paronychia; there are structural and chromosomal abnormalities in peripheral blood individual nuclei but lack of genetic or epigenetic alterations like mycosis fungoides; most paronychia is clinically benign and even resolves spontaneously in some patients, but some patients can develop mycosis fungoides. Some of the patients with psoriasis vulgaris may be associated with toxoplasma, EBV, cytomegalovirus, parvovirus B19, and human immunodeficiency virus infections. It has been reported that the epidermal keratin-forming cells of children with mossy furunculosis patients originating from their mothers are 10 times more than those of healthy controls. Clinical manifestations 1. Small plaque type parapsoriasis: Preferred in middle-aged and elderly people, with peak incidence of 50-60 years. The lesions are round or oval scattered patches or very thin plaques, yellowish brown or salmon-like yellow-red, covered with fine, moderate adherent scales, mainly located on the trunk. In some patients, the lesions are elongated fingerprint-like patches distributed along the skin lines. The lesions are less than 5 cm in diameter and often persist for several years, with few patients progressing to mycosis fungoides. 2.Large plaque type parapsoriasis: It occurs in middle-aged and elderly people, and the peak incidence is also 50-60 years old. The incidence is higher in men than in women, but the difference between men and women is lower than that of SPP. The lesions are oval or irregular patches or very thin plaques, pale red or salmon-like yellow-red, covered with fine scales and clearly defined or indistinct from the surrounding normal skin. They are clinically asymptomatic or mildly pruritic. The lesions vary in size, but most are larger than 5 cm in diameter, usually 10 cm or larger. The lesions are often stable in size, and their number often increases gradually. The lesions are mainly located on the lower part of the trunk and the flexors of the limbs, but other areas may be involved. The surface of the lesions may be mildly wrinkled, showing “cigarette paper” changes, and due to the atrophy of the epidermis, capillary dilation and skin heterochromia may also appear. The lesions often persist for several years. LPP is a rare subtype of LPP, and the lesions are scaly patches and papules with a reticular distribution, which eventually develop into skin heterochromia-like changes. 10% of patients with LPP progress to mycosis fungoides every 10 years, and almost all patients with reticular parapsoriasis can progress to mycosis fungoides. 3. Chronic mossy furunculosis: It often occurs in children and young individuals, but it can also occur in other age groups. The ratio of male to female is 1.5-3:1. The lesions are multiple red scaly papules that appear repeatedly in batches and can fade on their own in a few weeks or months or last for several years. They are mainly located on the trunk and proximal extremities. After the lesions improve, hypopigmentation or hyperpigmentation is left behind, but no scarring. 4, acute acne-like mossy furunculosis: often occurs in children and young individuals, but can also occur in other age groups. The ratio of male to female is 1.5-3:1. The lesions are multiple red papules, crust, blisters, pustules or vesicles occur on their surface before they recede on their own. They are mainly located on the trunk and proximal extremities. PLEVA may be accompanied by symptoms such as low-grade fever, malaise, headache, arthralgia, etc. A few may develop high fever and gastrointestinal and central nervous system changes.