**Core Concept**
Allergic rhinitis is a common disorder characterized by symptoms such as sneezing, nasal congestion, and itching, resulting from an overreaction of the immune system to inhaled allergens. A definitive diagnosis of allergic rhinitis relies on identifying specific IgE antibodies against the allergen or observing the presence of eosinophils in nasal secretions.

**Why the Correct Answer is Right**
A nasal smear, also known as a nasal smear cytology, involves collecting a sample of nasal secretions to examine under a microscope for the presence of eosinophils, a type of white blood cell associated with allergic reactions. By detecting eosinophils in the nasal smear, clinicians can diagnose allergic rhinitis. This test is particularly useful for differentiating allergic rhinitis from other types of rhinitis, such as infective rhinitis.

**Why Each Wrong Option is Incorrect**
**Option A:** A CT (Computed Tomography) scan is an imaging test that provides detailed cross-sectional images of the nasal passages and sinuses, but it is not used to diagnose allergic rhinitis.
**Option B:** An X-ray is a type of imaging test that uses radiation to produce images of the nasal passages and sinuses, but it is not sensitive enough to diagnose allergic rhinitis.
**Option D:** An MRI (Magnetic Resonance Imaging) scan is an imaging test that provides detailed images of the nasal passages and sinuses, but it is not used to diagnose allergic rhinitis.

**Clinical Pearl / High-Yield Fact**
Eosinophils are a key indicator of allergic reactions, and their presence in nasal secretions is a strong predictor of allergic rhinitis. This knowledge can help clinicians make an accurate diagnosis and initiate appropriate treatment.

**✓ Correct Answer: C. Nasal Smear**