**Core Concept**
Gout is a form of inflammatory arthritis characterized by deposition of monosodium urate crystals in the joints, leading to tophi formation. Tophi are deposits of monosodium urate crystals that accumulate in the tissues.

**Why the Correct Answer is Right**
Tophi in gout are typically found in the skin, particularly over pressure points such as the elbows, hands, and feet. This is due to the accumulation of monosodium urate crystals in the soft tissues, including the skin. The skin's ability to tolerate the deposition of these crystals makes it a common site for tophi formation. Furthermore, the skin's thickness and the presence of subcutaneous fat provide a suitable environment for tophi to develop.

**Why Each Wrong Option is Incorrect**
* **Option A:** Synovial fluid - While monosodium urate crystals can be found in the synovial fluid of patients with gout, tophi are not typically formed in this location. Tophi are larger deposits that form in the soft tissues.
* **Option B:** Articular Cartilage - Articular cartilage is not a common location for tophi formation. Gout primarily affects the synovial fluid and soft tissues surrounding the joint.
* **Option C:** Joint capsule - The joint capsule is not a typical location for tophi formation. While the joint capsule can be affected by gout, tophi are more commonly found in the skin and other soft tissues.
* **Option D:** Muscle - Muscle is not a typical location for tophi formation. While gout can cause muscle pain and stiffness, tophi are not typically found in muscle tissue.

**Clinical Pearl / High-Yield Fact**
Tophi can be a diagnostic clue for gout, especially in patients with a history of recurrent joint attacks. However, not all patients with gout develop tophi, and their presence does not necessarily confirm the diagnosis.

**Correct Answer: D. Muscle**