**Core Concept**
Carpal tunnel syndrome (CTS) is a condition characterized by median nerve compression at the wrist level, leading to symptoms such as numbness, tingling, and pain in the hand, particularly in the thumb, index, and middle fingers. The condition arises due to the entrapment of the median nerve as it passes through the carpal tunnel, a narrow passageway in the wrist formed by bones and ligaments.

**Why the Correct Answer is Right**
The condition that would help determine the diagnosis of CTS is Tinel's sign. Tinel's sign is a clinical test where the examiner gently taps the median nerve at the wrist to elicit a tingling sensation in the hand. This is a positive sign for CTS if the patient experiences numbness, tingling, or paresthesia upon nerve stimulation. The mechanism behind Tinel's sign is the activation of nerve sprouting and regeneration, which can exacerbate the symptoms of CTS.

**Why Each Wrong Option is Incorrect**
* **Option A:** Phalen's test, while a useful clinical assessment for CTS, involves flexion of the wrist to elicit symptoms, not nerve tapping.
* **Option B:** Wrist extension test is not a standard clinical assessment for CTS.
* **Option D:** None of the other options directly assess the median nerve compression characteristic of CTS.

**Clinical Pearl / High-Yield Fact**
A positive Tinel's sign is a high-yield fact for diagnosing CTS, but it is essential to note that a negative Tinel's sign does not rule out CTS. A thorough clinical examination and possibly electromyography (EMG) or nerve conduction studies (NCS) may be required for a definitive diagnosis.

**Correct Answer:** C.