**Core Concept**
Cozen's test is a clinical examination technique used to diagnose lateral epicondylitis, also known as tennis elbow. It involves resisted wrist extension against resistance, which reproduces the pain experienced by patients with this condition. This test is based on the anatomical relationship between the lateral epicondyle and the extensor muscles of the forearm.

**Why the Correct Answer is Right**
The mechanism behind Cozen's test involves the extensor muscles of the forearm, particularly the extensor carpi radialis brevis (ECRB), which originates from the lateral epicondyle. When these muscles contract against resistance, it puts tension on the tendon attachment at the lateral epicondyle, reproducing the pain experienced by patients with lateral epicondylitis. This test is a reliable indicator of the condition, as it targets the specific muscles and tendon involved.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Cozen's test is not used for diagnosing medial epicondylitis, which is also known as golfer's elbow. Medial epicondylitis involves the medial epicondyle and the flexor muscles of the forearm.
**Option B:** This option is incorrect because Cozen's test is not used for diagnosing radial tunnel syndrome. Radial tunnel syndrome involves the radial nerve and is diagnosed using different clinical examination techniques.
**Option C:** This option is incorrect because Cozen's test is not used for diagnosing cubital tunnel syndrome. Cubital tunnel syndrome involves the ulnar nerve and is diagnosed using different clinical examination techniques.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Cozen's test should be performed with the elbow in full extension and the forearm in neutral rotation to accurately diagnose lateral epicondylitis.

**Correct Answer: C. Lateral epicondylitis.**