Final answer:
Sam's dad is paralyzed from the neck down due to a spinal cord injury from a broken neck, causing disruption of the brain's communication with the body below the injury site, which often results in quadriplegia or tetraplegia.
Step-by-step explanation:
The reason why Sam's dad is paralyzed from the neck down after the accident is because of a spinal cord injury resulting from the broken neck.
When the spinal cord is damaged, it inhibits the brain's ability to communicate with the body parts below the site of injury.
Since the neck is high on the spinal cord, damage at this level typically results in quadriplegia or tetraplegia, which is paralysis of all limbs and often some parts of the body's trunk, as the spinal cord at this level controls signals to a large portion of the body.