**Core Concept**
Cerebrovascular accidents (CVAs) can lead to various complications, including late neurological deterioration. This occurs due to secondary mechanisms that exacerbate brain injury beyond the initial ischemic event.

**Why the Correct Answer is Right**
Vasospasm is a significant cause of late neurological deterioration in CVAs, particularly after subarachnoid hemorrhage (SAH). It occurs due to the contraction of smooth muscle cells in the blood vessel walls, leading to reduced blood flow and increased vascular resistance. This can result in cerebral ischemia, further brain injury, and worsening of neurological symptoms. The pathophysiology of vasospasm involves the release of vasoactive substances, such as serotonin and endothelin, which stimulate smooth muscle contraction. The most common site of vasospasm is the middle cerebral artery, followed by the anterior cerebral artery and posterior cerebral artery.

**Why Each Wrong Option is Incorrect**
**Option A:** Rebleeding is a potential complication of aneurysmal SAH, but it is not the most common cause of late neurological deterioration. Rebleeding usually occurs within the first 24 hours after the initial hemorrhage.
**Option C:** Embolism can occur during the acute phase of CVA, but it is not typically associated with late neurological deterioration. Embolism usually presents as an acute stroke, whereas late deterioration is a complication of the initial event.
**Option D:** Hydrocephalus can occur after SAH, but it is not the most common cause of late neurological deterioration. Hydrocephalus usually presents with symptoms such as headache, nausea, and vomiting, but it is not typically associated with worsening neurological deficits.

**Clinical Pearl / High-Yield Fact**
Be aware that vasospasm can occur up to 14 days after the initial CVA and may be delayed in onset. It is essential to monitor for symptoms such as worsening headache, nausea, and vomiting, as these may indicate the onset of vasospasm.

**✓ Correct Answer: B. Vasospasm**