Psoriasis is a chronic inflammatory dermatosis in which the skin cells multiply rapidily thereby forming scales that are dry and itchy. 
The causes of psoriasis is unknown but it is thought to be due to immune system disorder sparked off by genetics, environmental triggers such as infections, stress, cold or habits such as smoking or drinking alcohol.
The initial thinking was that it occurs only among the Caucasians (people who reside in temperate regions) but recently the belief is beginning to change since we are also seeing people that have psoriasis commonly in our outpatient dermatology clinic all over Nigeria (200/100,000 dermatological patients seen averagely in a year).
Psoriasis present as a rash that has reddish hue or purplish with bumpy appearance. These may get thicker and larger with a flat top scale that easily fall of and sometimes bleed after scratching due to itch. Different parts of the body can be involved. Mostly, the trunk, upper/lower extremities (back of the elbow, knees), the scalp, nails, joints or axilla and groin in inverse psoriasis. Atleast up to 80-90% of psoriasis are mild. However, depending on the parts of the body involved it can affect the individual’s quality of life.
The physical and emotional effects of psoriasis are significantly similar to those who have cancer, heart disease, diabetes or depression. This is worse when the hand, face or genitals are involved.
Psoriasis can be mistaken as:
1.    Eczema (itchy oozing or crusty lichenified eruption)
2.    Seborrheic dermatitis (mostly when the scalp is involved) biopsy maybe needed to distinguish the two).
3.    Ring worm (Tinea corporis) = roundish raised edges with central thining not itchy.
Others include: Tinea vesicolour or Pityriasis rubra Pilaris.
The best way to make your diagnosis is skin biopsy were hyperkeratosis, hypogranulosis, micro abscess of munro would be seen.
Treatment is dependent on the level of severity. It is usually a step ladder approach using  topical emollient, corticosteroid, tars or dithranol, Systemic steroids, PUVA and extreme cases may need methotrexate or biologics....