A 48-year-old female piano player visited the outpatient clinic with numbness and tingling in her left hand. A diagnosis was made of nerve compression in the carpal tunnel, and the patient underwent an endoscopic nerve release. Two weeks postoperatively the patient complained of a profound weakness in the thumb, with loss of thumb opposition. The sensation to the hand, however, was unaffected. Which of the following nerves was injured during the operation?

**Core Concept**
The question tests the understanding of the anatomical structures involved in wrist and hand function, specifically the nerves responsible for motor and sensory functions. The **median nerve** is a key structure that runs through the **carpal tunnel**, providing sensation to the palm and thumb, as well as motor control to the thenar muscles.

**Why the Correct Answer is Right**
The correct answer is related to the nerve that controls the motor function of the thumb, particularly opposition, without affecting sensation. The **recurrent branch of the median nerve**, also known as the **thenar branch**, is responsible for innervating the thenar muscles, which are crucial for thumb opposition. Injury to this branch would result in weakness of the thumb, specifically in opposition, without affecting sensation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be related to a nerve that controls a different function or area of the hand.
**Option B:** Similarly, this option could be related to a nerve or structure not directly involved in thumb opposition.
**Option D:** This option might relate to a nerve with a different function or distribution in the hand.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the **recurrent branch of the median nerve** is at risk during carpal tunnel release procedures, and injury to this branch can result in significant functional impairment, including loss of thumb opposition.

**Correct Answer:** D. The nerve to the thenar muscles (recurrent branch of the median nerve).

✓ Correct Answer: C. Recurrent branch of median nerve