**Core Concept**
The patient's presentation of an itchy annular plaque on the face suggests a skin condition that may be related to a specific allergen or irritant. This scenario requires a test that can help identify the underlying cause of the skin reaction, which is likely an allergic contact dermatitis.

**Why the Correct Answer is Right**
The correct answer is a patch test, which involves applying small amounts of potential allergens to the skin to assess for a localized skin reaction. This test is particularly useful for diagnosing allergic contact dermatitis, a common condition characterized by an itchy, annular (ring-shaped) plaque on the skin. The patch test helps identify the specific allergen responsible for the reaction, guiding treatment and avoiding future exposure.

**Why Each Wrong Option is Incorrect**
**Option A:** A biopsy is a histopathological examination of a skin sample, which may provide information on the microscopic appearance of the skin but does not directly identify the allergen responsible for the reaction.

**Option B:** A complete blood count (CBC) is a panel of tests that measure various components of the blood, including red and white blood cells, and is not relevant to diagnosing skin conditions like allergic contact dermatitis.

**Option C:** A skin scraping or potassium hydroxide (KOH) preparation is used to diagnose fungal infections, such as ringworm, but is not indicated in this case where the patient presents with an itchy annular plaque due to an allergic reaction.

**Clinical Pearl / High-Yield Fact**
When diagnosing skin conditions, it's essential to consider the patient's occupation, hobbies, and exposure to potential allergens, as these may provide valuable clues to the underlying cause of the reaction.

**Correct Answer: C. Patch test.**