**Core Concept**
The radial head's articulation with the capitulum of the humerus is a synovial joint that provides stability and allows for rotational movements of the forearm. The joint's stability is maintained by a combination of anatomical, ligamentous, and bony structures.

**Why the Correct Answer is Right**
The main prevention of the forcible separation of the head of the radius from the capitulum of the humerus is due to the presence of the annular ligament. This ligament encircles the head of the radius, providing a fibrous capsule that maintains the joint's stability. The annular ligament is attached to the margins of the radial notch of the ulna and the neck of the radius, thus preventing excessive movement and separation of the joint.

**Why Each Wrong Option is Incorrect**
**Option A:** The medial collateral ligament of the elbow joint is not primarily responsible for maintaining the stability of the radial head-capitulum joint.
**Option B:** The lateral collateral ligament plays a role in stabilizing the elbow joint, but it is not directly involved in preventing the separation of the radial head from the capitulum.
**Option C:** The interosseous membrane between the radius and ulna provides stability to the forearm, but it is not directly responsible for maintaining the radial head-capitulum joint's stability.

**Clinical Pearl / High-Yield Fact**
The annular ligament's role in maintaining the stability of the radial head-capitulum joint is crucial, and its injury or disruption can lead to radial head subluxation or dislocation.

**Correct Answer:** A. The annular ligament.