**Core Concept**
Gout is a form of inflammatory arthritis characterized by sudden, severe attacks of pain, swelling, redness, and tenderness in one or more joints, most often the big toe. The condition occurs due to the deposition of monosodium urate crystals within the joint space.

**Why the Correct Answer is Right**
The correct answer is the metatarsophalangeal joint of the big toe (hallux). This joint is most commonly affected due to its unique anatomy and the way it functions. The joint's location and the pressure exerted on it during gait make it more susceptible to the deposition of monosodium urate crystals. The inflammation caused by these crystals leads to the characteristic symptoms of gout.

**Why Each Wrong Option is Incorrect**
* **Option A:** The knee joint is not the most commonly involved joint in gout, although it can be affected, especially in advanced cases.
* **Option B:** The elbow joint is not typically associated with gout, and its involvement would be unusual.
* **Option C:** The ankle joint can be involved in gout, but it is not the most commonly affected joint.

**Clinical Pearl / High-Yield Fact**
The "first MTP joint" is the most common site for gouty arthritis, and its involvement is often used as a diagnostic clue. Remembering the acronym "MTP" can help you associate the condition with the metatarsophalangeal joint of the big toe.

**Correct Answer:** D.