**Core Concept**
The question tests the understanding of the common side effects associated with neuraxial opioid administration, which is a critical aspect of regional anesthesia. Neuraxial opioids are used to provide postoperative pain relief by targeting opioid receptors in the spinal cord.

**Why the Correct Answer is Right**
Pruritus, or itching, is the most common side effect of neuraxial opioid administration. This occurs due to the activation of opioid receptors in the spinal cord, particularly the mu receptors, which are involved in the regulation of histamine release. The release of histamine leads to the sensation of itching. The incidence of pruritus can be as high as 80-90% with neuraxial opioids, making it a significant concern for anesthesiologists.

**Why Each Wrong Option is Incorrect**
**Option A:** Nausea and vomiting are common side effects of systemic opioids, but they are not the most common side effect of neuraxial opioids. The incidence of nausea and vomiting with neuraxial opioids is lower compared to pruritus.
**Option C:** Urinary retention is a potential side effect of neuraxial opioids, but it is not the most common side effect. Urinary retention occurs due to the relaxation of the bladder muscle and is more common with higher doses of opioids.
**Option D:** Sedation is a common side effect of systemic opioids, but it is not typically associated with neuraxial opioids. The doses of opioids used in neuraxial anesthesia are generally lower than those used systemically, and the risk of sedation is correspondingly lower.

**Clinical Pearl / High-Yield Fact**
It is essential for anesthesiologists to be aware of the high incidence of pruritus associated with neuraxial opioids and to have a plan in place to manage this side effect, such as administering antihistamines or reducing the dose of opioids.

**✓ Correct Answer: B. Pruritus**