**Core Concept**
Myositis ossificans is a condition characterized by the formation of bone tissue within skeletal muscle, typically following trauma or injury. This process involves the aberrant differentiation of mesenchymal cells into osteoblasts, leading to the deposition of calcified cartilage and bone.

**Why the Correct Answer is Right**
Myositis ossificans is most commonly seen in the muscles surrounding joints, particularly in areas with high mobility and stress. The condition often occurs after a traumatic injury, such as a fall or a sports-related injury, where the muscle fibers are damaged, and the body attempts to repair the area by forming bone tissue. The involved muscles may include the quadriceps, hamstring, and hip flexor muscles. The process of myositis ossificans involves the activation of osteogenic cells, such as mesenchymal stem cells, which differentiate into osteoblasts and start depositing bone matrix.

**Why Each Wrong Option is Incorrect**
**Option A:** Myositis ossificans can occur in other areas, but it is less common in the muscles of the back.
**Option B:** Myositis ossificans is not typically associated with autoimmune conditions, such as rheumatoid arthritis.
**Option C:** Myositis ossificans can occur in children, but it is more common in young adults who engage in high-impact activities.

**Clinical Pearl / High-Yield Fact**
Myositis ossificans can be challenging to diagnose, as the symptoms may mimic other conditions, such as muscle strain or tendonitis. Imaging studies, such as X-rays or MRI, are often necessary to confirm the diagnosis.

**Correct Answer:** C.