**Core Concept**
The underlying principle being tested is the likelihood of a specific injury occurring due to a common mechanism of trauma. In this case, the mechanism of twisting and falling on the ankle can lead to various injuries, but the most probable cause should be identified based on the symptoms and presentation.

**Why the Correct Answer is Right**
The most probable cause of the boy's symptoms is a ligament sprain or a meniscal tear in the knee joint, specifically the medial meniscus. When the boy twisted his knee on the ankle, he likely caused stress and strain on the medial meniscus, leading to a tear. The medial meniscus is more susceptible to injury due to its location and the mechanism of trauma. The symptoms of swelling, pain, and inability to move the knee are classic signs of a meniscal tear. The boy's attempt to play on despite the initial pain may have exacerbated the injury, leading to further swelling and limited mobility.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the scenario described, as it does not relate to the mechanism of injury or the symptoms presented.
**Option B:** While a bone fracture is a possible injury from a fall, the symptoms described (swelling, pain, and inability to move the knee) are more consistent with a soft tissue injury such as a ligament sprain or meniscal tear rather than a bone fracture.
**Option C:** A muscle strain is a possible injury from a sudden movement, but it is less likely to cause significant swelling and limited mobility in the knee joint.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the "Osgood-Schlatter disease" can sometimes mimic the symptoms of a meniscal tear, especially in adolescents. However, Osgood-Schlatter disease typically presents with a more gradual onset of pain and swelling, and it is often associated with overuse or repetitive stress on the patellar tendon.

**Correct Answer:** C.