## **Core Concept**
The interscalene block is a regional anesthesia technique used to anesthetize the shoulder and upper arm region. It involves injecting local anesthetic between the scalene muscles in the neck to block the brachial plexus. This technique is commonly used for surgeries of the shoulder and proximal humerus.

## **Why the Correct Answer is Right**
The correct answer, **Horner's syndrome**, is a common complication of interscalene block due to the proximity of the stellate ganglion to the brachial plexus at the level of the interscalene groove. The local anesthetic can spread and block the sympathetic nerves, leading to Horner's syndrome, which is characterized by ptosis, miosis, and anhidrosis.

## **Why Each Wrong Option is Incorrect**
* **Option A:** Phrenic nerve block is a possible complication but not the most common. It can lead to hemidiaphragmatic paresis but is usually transient.
* **Option B:** Recurrent laryngeal nerve block can occur but is less common compared to Horner's syndrome. It can cause vocal cord paralysis.
* **Option C:** This option is not provided, but based on the context, we can infer that other complications like pneumothorax or vascular puncture are less common than Horner's syndrome.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that Horner's syndrome, although common, is generally a benign and transient side effect of interscalene blocks. However, patients should be informed about its possibility and its implications.

## **Correct Answer:** D. Horner's syndrome.