**Question:** A 30-year-old male underwent excision of the right radial head. Following surgery, the patient developed inability to extend the fingers and thumb of the right hand. He did not have any sensory deficit. Which one of the following is the most likely cause?

A. Radial Nerve Injury
B. Ulnar Nerve Injury
C. Radial Artery Injury
D. Tendon Injury

**Core Concept:** Radial nerve, ulnar nerve, and radial artery are important structures in the upper limb that can lead to motor deficits following injury. Tendon injuries, on the other hand, usually result in loss of passive extension and passive flexion.

**Why the Correct Answer is Right:** The correct answer is **D** for tendon injury. The radial nerve and ulnar nerve injuries would typically result in a loss of sensation or a combination of motor and sensory deficits, not just finger and thumb extension deficit. Radial artery injury would cause symptoms such as discoloration, swelling, and coldness of the affected extremity, which is not mentioned in the patient's symptoms.

**Why Each Wrong Option is Incorrect:**

A. Radial Nerve Injury: A radial nerve injury would typically cause a loss of sensation on the dorsum of the hand and forearm, and would not cause a selective loss of finger and thumb extension.

B. Ulnar Nerve Injury: A ulnar nerve injury would cause a loss of sensation on the medial aspect of the hand and fingers, not the dorsum of the hand and forearm. Additionally, ulnar nerve injuries are more likely to cause sensory deficits than motor deficits.

C. Radial Artery Injury: A radial artery injury would cause symptoms such as discoloration, swelling, and coldness of the affected extremity, which is not mentioned in the patient's symptoms. Radial artery injuries are also less likely to cause a selective motor deficit of finger and thumb extension than a tendon injury.

D. Tendon Injury: Tendon injuries often result in a selective loss of passive extension and passive flexion in the affected limb. In this case, the patient has a selective loss of finger and thumb extension, which supports the diagnosis of tendon injury.

**Clinical Pearl:** Motor deficit can result from radial nerve, ulnar nerve, or tendon injuries. However, in this case, the selective loss of finger and thumb extension is suggestive of a tendon injury. Tendons are responsible for passive movement, and their injury would cause a loss of passive extension and passive flexion in the affected limb. In contrast, nerve injuries would typically cause a loss of active movement, sensory deficit, and sometimes pain.