Final answer:
An epidural hematoma is the most likely diagnosis for the observed symptoms and CT findings following a fall. This condition occurs when a blow to the head leads to arterial bleeding between the skull and the brain, resulting in increased pressure on the brain.
Step-by-step explanation:
The most likely diagnosis for a 68-year-old man presenting to the emergency department after a fall down twelve stairs, noted with a large parietal scalp hematoma and with a CT scan indicating a head injury, would be an epidural hematoma. This condition is typically associated with a blow to the lateral side of the head, such as, at the pterion where a major artery lies beneath.
If this artery is damaged, it can lead to bleeding that forms a hematoma between the skull and the brain, causing pressure that, if untreated, could lead to death. In contrast, a subdural hematoma often involves venous bleeding and is more diffusely spread across the surface of the brain, while a subarachnoid hemorrhage would typically present with bleeding into the cerebrospinal fluid, and a basilar skull fracture is a specific type of fracture at the base of the skull.