**Core Concept**
Adson's test is a clinical examination used to assess for thoracic outlet syndrome (TOS), specifically to evaluate for compression of the subclavian artery by the scalene muscles. This test is based on the principle that compression of the subclavian artery can cause a decrease in the radial pulse.

**Why the Correct Answer is Right**
During Adson's test, the patient's arm is abducted to 90 degrees and internally rotated. The examiner then applies pressure over the scalene muscles while palpating the radial pulse. A positive test is indicated by a decrease or disappearance of the radial pulse. This occurs due to compression of the subclavian artery by the scalene muscles, which can lead to decreased blood flow to the arm. The scalene muscles are innervated by the brachial plexus, and their compression can lead to symptoms of TOS.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as Adson's test is not used to diagnose conditions affecting the ulnar nerve.
**Option B:** This option is incorrect as Adson's test is not used to diagnose conditions affecting the median nerve.
**Option C:** This option is incorrect as Adson's test is not used to diagnose conditions affecting the axillary artery.

**Clinical Pearl / High-Yield Fact**
A positive Adson's test can also be associated with a decrease in the blood pressure in the affected arm. This is due to the compression of the subclavian artery, which can lead to a decrease in the blood flow to the arm.

**Correct Answer:** C.