Final answer:
Adam's inability to form new memories while retaining older ones is indicative of anterograde amnesia, a memory disorder often resulting from hippocampal damage.
Step-by-step explanation:
The memory difficulty experienced by Adam after suffering a brain injury in a motorcycle accident most clearly illustrates anterograde amnesia. This condition is characterized by an inability to form new episodic or semantic memories, but often allows the retention of procedural memories and life experiences before the injury. Anterograde amnesia usually involves damage to the hippocampus, leading to difficulties in transferring information from short-term to long-term memory, thereby hindering memory consolidation.