**Core Concept**
Glomus tumors are rare, vascular neoplasms originating from the glomus body, a specialized arteriovenous anastomosis in the dermis of the skin. These tumors are known to secrete vasoactive substances, leading to various clinical manifestations.

**Why the Correct Answer is Right**
The earliest symptom of a glomus tumor is typically a severe, intense pain, often described as a sharp, stabbing, or burning sensation. This pain is usually triggered by minor trauma, pressure, or temperature changes. The pain is thought to be due to the release of vasoactive substances, such as histamine and prostaglandins, from the tumor, which causes vasodilation and increased blood flow. The tumor's location in the dermis, near sensory nerve endings, also contributes to the intense pain perception.

**Why Each Wrong Option is Incorrect**
**Option A:** While hearing loss can be a symptom of a glomus tumor, it is not the earliest symptom. Hearing loss typically occurs in more advanced cases, when the tumor has grown to invade the middle ear or auditory nerve.

**Option B:** Tinnitus is a symptom of many ear conditions, but it is not the earliest symptom of a glomus tumor. Tinnitus is usually a later manifestation, occurring when the tumor has caused significant damage to the auditory system.

**Option C:** While a mass or swelling can be a symptom of a glomus tumor, it is not the earliest symptom. A mass or swelling typically becomes apparent only after the tumor has grown significantly in size.

**Clinical Pearl / High-Yield Fact**
Glomus tumors are highly vascular and can be difficult to differentiate from other vascular lesions. A key clinical feature is the presence of a "glomus sign," where the tumor blanches with pressure and then rapidly refills with blood, giving it a characteristic "cherry-red" appearance.

**Correct Answer:** D. Pain.