**Core Concept**
The triangular relation of the elbow refers to the precise alignment of the bones and joints in the elbow joint, which is essential for normal movement and function. This relation involves the distal humerus, the proximal radius and ulna, and the surrounding ligaments and muscles.

**Why the Correct Answer is Right**
The correct answer is maintained due to the combined action of the medial collateral ligament (MCL), lateral collateral ligament (LCL), and the annular ligament. These ligaments provide stability to the elbow joint and help maintain the triangular relation. The MCL prevents excessive valgus stress, while the LCL prevents varus stress. The annular ligament encircles the head of the radius, maintaining the radial head's position and preventing subluxation.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because it does not specify a particular ligament or structure that maintains the triangular relation of the elbow. While the biceps tendon does play a role in elbow stability, it is not primarily responsible for maintaining the triangular relation.

**Option B:**
This option is incorrect because the joint capsule is a fibrous sac that surrounds the elbow joint, but it does not specifically maintain the triangular relation.

**Option C:**
This option is incorrect because the medial and lateral epicondyles are bony prominences on the distal humerus, but they do not directly maintain the triangular relation of the elbow.

**Clinical Pearl / High-Yield Fact**
The triangular relation of the elbow is often compromised in conditions like posterolateral rotatory instability (PLRI), where the LCL and annular ligament are injured, leading to instability and potential subluxation of the radial head.

**Correct Answer: A. The medial collateral ligament (MCL) maintains the triangular relation of the elbow.**