**Core Concept**
The **glomus tumor** is a rare, usually benign neoplasm arising from the **glomus body**, a specialized form of blood vessel involved in thermoregulation. **Glomus bodies** are most densely concentrated in the dermis of the skin, particularly in the fingers and toes.

**Why the Correct Answer is Right**
Given the concentration of **glomus bodies** in the dermis, especially in the distal portions of the fingers and toes, it follows that the most common site for **glomus tumors** would be in these areas, particularly under the fingernails.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because, while **glomus tumors** can occur in various locations, the most common site is not typically described as being in the fingers without specifying under the nail.
**Option B:** Incorrect as this is not a typical location for **glomus tumors**.
**Option C:** Incorrect because, although **glomus tumors** can be found in the toes, they are less common than in the fingers.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **glomus tumors** classically present with a triad of symptoms: intense pain, tenderness, and temperature sensitivity, often localized to a specific area under the nail.

**Correct Answer:** D. Under the fingernail.