**Core Concept**
Felon is a type of infection of the fingertip, characterized by purulent paronychia that involves the pulp space of the fingertip. It is typically caused by bacterial infection, often Staphylococcus aureus.

**Why the Correct Answer is Right**
The most common site of felon is the little finger (pinkie), which is due to several factors, including the anatomical structure of the finger and the frequency of use. The little finger is often subjected to repetitive pressure and trauma, making it more susceptible to infection. Additionally, the small size of the little finger's pulp space allows the infection to spread quickly, leading to a higher incidence of felon in this area.

**Why Each Wrong Option is Incorrect**
**Option A:** The thumb is less commonly affected by felon due to its larger size and less frequent involvement in fine motor activities.
**Option B:** The ring finger is also less commonly affected, although it can still be a site of felon, especially in individuals with manual labor jobs.
**Option C:** The index finger can be a site of felon, but it is less common than the little finger due to its larger size and less frequent involvement in activities that lead to pressure and trauma.

**Clinical Pearl / High-Yield Fact**
It's essential to note that felon can be a serious infection that requires prompt medical attention. Delayed treatment can lead to abscess formation, nerve damage, and even amputation in severe cases.

**Correct Answer:** . Little finger.