Fracture of the hook of the hamate: This is a common injury in sports involving a bat or racket, as the hook of the hamate is vulnerable to direct impact from the handle of the bat. The mechanism of injury from a line drive softball to the wrist makes this a highly plausible diagnosis.

Fracture of the palmar base of the fourth metacarpal: Given the location of the impact, it's possible that the force was transmitted to the fourth metacarpal, causing a fracture. This area is also susceptible to injury from direct blows.
Contusion or bruising of the wrist: The impact from the softball could result in soft tissue injury, including contusions or bruising, without an underlying fracture.
Wrist sprain: The force of the impact could also cause ligamentous injury, leading to a wrist sprain.

Scaphoid fracture: Although less likely given the specific location of the impact, a scaphoid fracture is a critical diagnosis not to miss due to its potential for complications, such as avascular necrosis, if not promptly treated.
Ulnar nerve injury: The ulnar nerve runs close to the surface of the wrist near the hook of the hamate and could be injured in a similar mechanism, leading to neurological symptoms.
Compartment syndrome: This is a rare but serious condition that could occur after a significant injury to the wrist, characterized by increased pressure within the muscle compartments, which can lead to muscle and nerve damage.

Pisiform fracture: The pisiform is a small carpal bone that could potentially be injured in a direct blow to the wrist, but it is less commonly involved compared to other carpal bones or the metacarpals.
Triquetrum fracture: Another carpal bone, the triquetrum, could be injured, but fractures here are less common and might not be the first consideration given the mechanism described.