**Core Concept**
Dupuytren's contracture is a chronic fibrosing condition of the palmar fascia, leading to flexion deformity of the fingers. The disease involves the formation of nodules and cords in the palmar fascia, which eventually lead to contracture of the fingers.

**Why the Correct Answer is Right**
The earliest finger to be involved in Dupuytren's contracture is the little finger (fifth finger). This is because the palmar fascia is more prone to fibrosis and contracture in this area due to its anatomical location and the distribution of the palmar fascial fibers. The disease often starts as a nodule or a thickening in the palmar fascia of the little finger, which gradually progresses to contracture.

**Why Each Wrong Option is Incorrect**
**Option A:** The ring finger is also commonly affected by Dupuytren's contracture, but it is not the earliest finger to be involved. The disease often progresses from the little finger to the ring finger.
**Option B:** The thumb is not typically involved in Dupuytren's contracture, as the palmar fascia of the thumb is less prone to fibrosis and contracture.
**Option C:** The index finger can be involved in Dupuytren's contracture, but it is less commonly affected than the little finger and ring finger.

**Clinical Pearl / High-Yield Fact**
The presence of Dupuytren's contracture is often associated with other conditions such as Peyronie's disease, plantar fibromatosis, and Ledderhose disease, which are all characterized by fibrotic changes in various tissues.

**Correct Answer: D. The little finger (fifth finger).**