The lesion distribution and erythematous , thickened, plaque-like appearance are indicative of psoriasis.

Psoriasis is a chronic , relapsing , immune-mediated disease estimated to affect 0.5–4.6 % of the population [1, 2]. It affects men and women equally, and is about twice as likely to occur in Caucasians [1, 2]. Onset of psoriatic symptoms is most likely between the ages of 15 and 30 [3]. One large study reported a median age of 28 years [1]. Global epidemiology of psoriasis is varied, with highest incidence in Western Europe, North America, southern Latin America, Australasia, and Asia Pacific [4]. Higher incidence is also associated with higher geographic latitudes (i.e., closer to the North or South Pole) [1, 2].

The exact cause of psoriasis is not known [3]. There is a strong genetic basis for the disease, and its expression is thought to result from contribution by multiple genes. Monozygotic twin studies show a concordance rate of 35–73 %, suggesting there are environmental factors contributing to the pathogenesis of psoriasis [3, 5]. HLA genes on chromosome 6 have been implicated, but the disease is thought to be caused by an interaction of several genes and environmental factors [1–3, 5]. UV exposure may be one environmental factor, supported by the efficacy of UV therapy in treating psoriasis [3].

Onset of psoriasis occurs after predisposed individuals experience a triggering event, such as trauma, infection, allergic drug reaction, or emotional stress [3, 5]. T cell mediated immune responses are activated and proinflammatory cytokines are released. Natural killer cells, TNF-α, and interleukins IL-12, 17, 22, and 23 are frequently involved [3, 5, 6]. These mediators lead to a range of changes on the skin, and nails, and dilation of capillaries causing erythema.

There are several phenotypic variants of psoriasis. They include plaque, inverse, guttate , erythrodermic , and pustular psoriasis . The most common is plaque psoriasis, also called psoriasis vulgaris , and accounts for over 80–90 % of cases [1, 3, 7]. It is characterized by erythematous plaques with well-defined borders, occurring on the extensor surfaces of arms and legs, the lower back, scalp, and neck. Plaques may feature a silvery-white scale. The lesions usually occur symmetrically, and range from coin-sized to large plaques [3]. They are often asymptomatic but can be accompanied by pruritus. Nail changes , pitting and onycholysis (separation of the nail from the nail bed) are eventually seen in about half of patients with chronic psoriasis [1, 3]. Psoriatic arthritis occurs in up to 25 % of patients with psoriasis [3]. This usually manifests as joint swelling, particularly at the distal interphalangeal joints, inflammation of an entire finger or toe (dactylitis), and inflammation of the vertebra (spondylitis). It can precede psoriasis in 20 % of patients [1].

Inverse psoriasis occurs in intertriginous areas where skin touches or rubs together, such as the axilla, inguinal folds, and retroauricular folds. This variant accounts for 2–6 % of psoriatic patients, and is more likely to present in obese people [7]. Lesions usually do not feature the scaling typical of psoriasis, and may be confused for tinea infections.

Guttate psoriasis is usually self-limiting (resolution within 6–12 weeks) and occurs following streptococcal throat infection [1, 3, 7]. It is characterized by an eruption of small erythematous papules, most often on the trunk and extremities. Guttate psoriasis is more common in young adults, and is associated with an increased risk of developing plaque psoriasis [7].

Erythrodermic psoriasis is a variant whose most prominent feature is erythema. It can affect all parts of the body and have up to 100 % cutaneous surface area involvement. It can develop slowly in patients with plaque psoriasis, with lesions coalescing over time, or suddenly and unexpectedly [3]. Patients with this variant are at risk of developing hypothermia and hyperthermia in cold and warm climates respectively, due to body temperature loss through extensive vasodilation.