**Core Concept**
Glomus tumors are rare, benign neoplasms arising from the glomus body, a specialized vascular structure found in the dermis of the skin. These tumors are characterized by their rich vascular supply and are often associated with pain and other symptoms due to their location.

**Why the Correct Answer is Right**
The earliest symptom of glomus tumor is typically **pain**. This pain is usually localized to the site of the tumor and can be described as a sharp, stabbing, or aching sensation. The pain is thought to be caused by the tumor's proximity to nerve endings and its ability to compress or irritate these nerves. Glomus tumors often arise in the subungual region of the finger or toe, where they can cause significant pain and discomfort.

**Why Each Wrong Option is Incorrect**
* **Option A:** While glomus tumors can cause swelling, this is not typically the earliest symptom. Swelling may occur as the tumor grows, but pain is usually the initial complaint.
* **Option B:** Glomus tumors are not typically associated with fever or systemic illness. While they may cause local pain and discomfort, they do not usually cause a systemic response.
* **Option C:** While glomus tumors can cause redness and warmth to the affected area, these symptoms are not typically the earliest presentation. Pain is usually the initial symptom.

**Clinical Pearl / High-Yield Fact**
Glomus tumors are often referred to as "red, painful, and pulsating" lesions. This mnemonic can help clinicians to remember the characteristic presentation of these tumors.

**Correct Answer: B. Pain**