الفهرس 
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Abstract
Psoriasis is a common chronic inflammatory immune-mediated skin disease with great negative impact on patients’ quality of life. It is characterized by altered proliferation and differentiation of keratinocytes, vascular remodeling and inflammation in the skin.
Modifiable risk factors for psoriasis, namely smoking and obesity, may influence clinical severity, comorbidities, or response to treatment, and may even be responsible for an increased mortality among psoriatic patients as compared with the general population. Emerging evidences corroborated the association with several comorbidities. These evidences, together with the detection of inflammatory markers, lead to the understanding of psoriasis as a complex disease.
The etiology of psoriasis remains unclear, although there is evidence for genetic predisposition, the role of the immune system in psoriasis causation is also a major topic of research. Recent studies have revealed the pivotal role of Vitamin D in the pathogenesis of psoriasis.
Vitamin D is of particular interest to dermatologists for two important reasons: it is synthesized in the skin upon exposure to UV light, and it is an important treatment option for psoriasis and other skin diseases. The relation between 25(OH) D and psoriasis has been studied since the 1930s. Combination of the mechanisms of reduced cellular proliferation, increased cellular differentiation, and immunomodulation may explain the effects of vitamin D and its derivatives on psoriasis.
Although PsA in clinical series represents up to 25% of all patients, population-based estimates suggest that no more than 5–10% of
psoriatic patient’s present sero-negative arthritis as an associated feature.
PsA is an immunologically triggered, chronic inflammatory arthropathy, within the framework of psoriasis, which can have a lasting influence on the quality of life of affected individuals. PsA may be confused with other forms of arthritis, such as rheumatoid arthritis. However, skin lesions, nail problems and specific patterns of inflammation observed in PsA allow clinicians to differentiate it from other forms of inflammatory arthritis. Early diagnosis is essential in order to institute adequate therapy.