**Core Concept:** Allergic rhinitis is an immunoglobulin E (IgE) mediated inflammatory response to allergens, most commonly pollens, dust mites, and moulds. IgE antibodies bind to mast cells and basophils, leading to degranulation and the release of inflammatory mediators, such as histamine, leukotriene, and prostaglandin. These mediators cause the characteristic symptoms of allergic rhinitis: sneezing, itching, rhinorrhea, and ocular symptoms.

**Why the Correct Answer is Right:** The patient's history of seasonal symptoms, worsening with spring and summer, and relief with diphenhydramine (antihistamine) strongly suggest allergic rhinitis as the primary diagnosis. Diphenhydramine, a second-generation H1 antihistamine, is administered as needed for symptom relief.

**Why Each Wrong Option is Incorrect:**

A. **Allergic rhinitis:** Although the patient's history is consistent with allergic rhinitis, the test mentioned is typically used to identify specific allergens (e.g., skin prick test or specific serum IgE assays). These tests help identify the offending allergen(s) rather than confirm the diagnosis itself.

B. **Viral rhinitis:** Viral rhinitis can present similarly to allergic rhinitis, but the patient's response to diphenhydramine supports an allergic rather than viral etiology.

C. **Non-allergic rhinitis:** The patient's seasonal pattern and response to diphenhydramine are consistent with allergic rhinitis, ruling out this option.

D. **Substance P assay:** Substance P is a neuropeptide involved in the sensory processing of pain, itch, and allergic reactions. While substance P levels may be raised in allergic rhinitis, the test mentioned is not commonly used for this diagnosis. The correct test would be a neurokinin-1 receptor antagonist challenge or a substance P radioligand binding assay.

**Clinical Pearl:** Allergic rhinitis is a common condition affecting millions worldwide, with a prevalence of up to 40% in some regions. Early recognition and proper management can improve the patient's quality of life and prevent complications such as sinusitis, asthma, and otitis media.

**Correct Answer:**

**Correct Answer: A.** Allergic rhinitis