**Question:** Adult patient with multiple scaly macules over chest and back, test(s) which can diagnose this condition is(are):

A. Skin biopsy
B. Serum vitamin D level
C. Direct immunofluorescence
D. Skin prick test

**Correct Answer:** .

**Core Concept:** Psoriasis is a chronic immune-mediated inflammatory skin disorder characterized by the overproduction of skin cells, resulting in the formation of red, scaly patches typically on exposed areas like the chest, back, and joints. This condition is associated with increased white blood cell count, elevated inflammatory markers, and genetic predisposition.

**Why the Correct Answer is Right:**

1. Skin biopsy (Option A): This is a crucial diagnostic tool for psoriasis, as it allows for histopathological examination of skin lesions to confirm the presence of hyperkeratosis (thickened epidermis), parakeratosis (thickened cornified layer), and inflammatory infiltrate, which are characteristic features of psoriasis.

2. Direct immunofluorescence (Option C): This test is used to detect immune complex deposits in the skin lesions, which can be helpful in differentiating between psoriasis and other dermatological conditions like lichen planus or pemphigus vulgaris. However, it is not typically the primary diagnostic test for psoriasis in routine clinical practice.

3. Serum vitamin D level (Option B): Although low vitamin D levels have been associated with psoriasis, it is not a definitive diagnostic test for the condition. Improving vitamin D levels may help in managing psoriasis symptoms but does not confirm the diagnosis.

4. Skin prick test (Option D): This test is used to identify specific allergens or antigens in cases of hypersensitivity reactions or atopic dermatitis. It is not a standard diagnostic tool for psoriasis.

**Why Each Wrong Option is Incorrect:**

1. Serum vitamin D level (Option B): While vitamin D deficiency is common in psoriasis patients, measuring serum vitamin D levels does not directly diagnose psoriasis. A low vitamin D level may indicate poor skin barrier function due to the thickened epidermis in psoriasis.

2. Skin prick test (Option D): Psoriasis is an immune-mediated inflammatory disorder, not an allergic reaction. A skin prick test is used to identify specific allergens or antigens, which is not relevant to diagnosing psoriasis.

3. Direct immunofluorescence (Option C): While direct immunofluorescence can help differentiate psoriasis from other dermatological conditions like lichen planus or pemphigus, it is not the primary diagnostic test for psoriasis, which is typically diagnosed based on clinical presentation and histopathology.

4. Serum vitamin D level (Option B): Measuring serum vitamin D levels is not a definitive diagnostic tool for psoriasis. The presence of psoriasis is more likely when a patient presents with characteristic skin lesions, and the histopathological examination is required for a definitive diagnosis.

**Clinical Pearls:**

1. **Psoriasis affects approximately