**Core Concept**
The patient's symptoms of hives (urticaria) without new medications, respiratory symptoms, or lip/tongue swelling suggest an allergic reaction. The goal is to identify a medication that can effectively manage pruritis (itching) without causing sedation.

**Why the Correct Answer is Right**
The patient's symptoms are consistent with chronic idiopathic urticaria (CIU), a condition characterized by recurrent episodes of hives lasting more than 6 weeks without a clear etiology. For patients with CIU who want to avoid sedating medications, antihistamines are the preferred treatment. **Fexofenadine** is a non-sedating second-generation antihistamine that can provide effective relief from pruritis without significant sedative effects.

**Why Each Wrong Option is Incorrect**
**Option A:** **Diphenhydramine** is a first-generation antihistamine that can cause significant sedation, making it an undesirable option for this patient.
**Option B:** **Prednisone** is a corticosteroid that can be used to treat urticaria, but it is not the first-line treatment for chronic idiopathic urticaria and is associated with various side effects, such as weight gain and mood changes.
**Option C:** **Montelukast** is a leukotriene receptor antagonist used primarily to treat asthma and allergic rhinitis. It is not typically used to treat urticaria.

**Clinical Pearl / High-Yield Fact**
When treating chronic idiopathic urticaria, it's essential to start with non-sedating antihistamines like fexofenadine or loratadine and consider adding corticosteroids or cyclosporine if symptoms persist.

**Correct Answer:** C. Fexofenadine.