**Core Concept**
The patient's presentation of an itchy annular plaque on the face suggests a dermatological condition that requires a diagnostic test to confirm the underlying pathology. The test should ideally target the pathophysiological mechanism responsible for the symptoms.

**Why the Correct Answer is Right**
The correct answer is likely a **Patch Test**. A patch test involves applying small amounts of potential allergens to the skin and observing for any reaction. This test is particularly useful in diagnosing contact dermatitis, a common cause of itchy annular plaques. Contact dermatitis occurs when the skin comes into contact with an allergen or irritant, triggering an immune response that leads to inflammation and itching. The patch test helps identify the specific allergen responsible for the reaction.

**Why Each Wrong Option is Incorrect**
* **Option A:** A biopsy may be necessary in some cases, but it is not the first-line diagnostic test for itchy annular plaques.
* **Option B:** A skin scraping may be used to diagnose fungal or parasitic infections, but it is not typically used for itchy annular plaques.
* **Option C:** A complete blood count (CBC) may be ordered to rule out systemic diseases, but it is not a specific diagnostic test for itchy annular plaques.

**Clinical Pearl / High-Yield Fact**
When considering a diagnosis for an itchy annular plaque, remember the "N" in NBUVB, which stands for Narrowband Ultraviolet B light therapy, a treatment for psoriasis, a common cause of itchy annular plaques.

**Correct Answer: C. Complete Blood Count (CBC).