**Core Concept**
Allergic rhinitis is an IgE-mediated hypersensitivity reaction to inhaled allergens, leading to symptoms such as sneezing, nasal congestion, and rhinorrhea. The diagnosis of allergic rhinitis relies on a combination of clinical history, physical examination, and laboratory tests.

**Why the Correct Answer is Right**
The most useful test in diagnosing allergic rhinitis is the radioallergosorbent test (RAST), also known as the enzyme-linked immunosorbent assay (ELISA). This test measures the level of specific IgE antibodies against common allergens such as pollen, dust mites, and pet dander. The RAST/ELISA test is a blood test that can confirm the presence of allergic rhinitis by detecting elevated levels of IgE antibodies against specific allergens. The test is particularly useful when the diagnosis is uncertain or when the patient has atypical symptoms.

**Why Each Wrong Option is Incorrect**
* **Option A:** Nasal endoscopy is a useful tool for visualizing the nasal passages and detecting anatomical abnormalities, but it is not a primary diagnostic test for allergic rhinitis.
* **Option B:** Skin prick testing (SPT) is a useful diagnostic tool for allergic rhinitis, but it is not the most sensitive or specific test. SPT is a more immediate test that can cause an allergic reaction, whereas RAST/ELISA is a more accurate test for diagnosing allergies.
* **Option C:** Complete blood count (CBC) is a basic blood test that can help identify systemic symptoms of allergic reactions, but it is not specific for diagnosing allergic rhinitis.

**Clinical Pearl / High-Yield Fact**
The key to diagnosing allergic rhinitis is to identify the specific allergen responsible for the patient's symptoms. A thorough history of exposure to potential allergens, combined with laboratory tests such as RAST/ELISA, can help confirm the diagnosis and guide treatment.

**Correct Answer: B. Skin prick testing is a useful diagnostic tool, but RAST/ELISA is the most useful test in diagnosing allergic rhinitis.**