**Core Concept**
Whiplash injury is a type of neck injury caused by the sudden, forceful movement of the neck, typically resulting from a rear-end or side-impact motor vehicle collision. This injury is characterized by the rapid extension and flexion of the cervical spine, leading to damage to the ligaments, muscles, and spinal joints.

**Why the Correct Answer is Right**
The correct answer, **B. Acute hyperextension of the spine**, is the most common mechanism of whiplash injury. When a vehicle is struck from behind, the head is thrown forward and then snaps back, causing the neck to extend beyond its normal range of motion. This excessive extension can lead to the stretching or tearing of ligaments and muscles in the neck, resulting in pain, stiffness, and limited mobility. The rapid extension of the spine can also cause the spinal joints to become misaligned, further contributing to the development of whiplash symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** A fall from a height is a different mechanism of injury, typically resulting in a more severe and complex pattern of trauma, including fractures, head injuries, and other systemic injuries. While a fall from a height can cause neck injuries, it is not the primary mechanism of whiplash injury.
**Option C:** A blow to the top of the head is more likely to cause a skull fracture or other head injuries, rather than a whiplash injury. This type of injury would typically result from a direct impact to the head, rather than the rapid extension and flexion of the neck.
**Option D:** Acute hyperflexion of the spine is the opposite movement of hyperextension, and while it can cause neck injuries, it is not the primary mechanism of whiplash injury.

**Clinical Pearl / High-Yield Fact**
It's essential to note that whiplash injuries can occur at speeds as low as 5-10 km/h, and the severity of the injury is not always correlated with the speed of the impact. This highlights the importance of a thorough medical evaluation and documentation of the injury mechanism in cases of whiplash.

**✓ Correct Answer: B. Acute hyperextension of the spine**