Nathan is suffering from amnesia, an illness that prevents the retrieval of new memories. in the context of the organization of the brain, which area of nathan's brain is most likely responsible for the amnesia?

The  area of Nathan's brain that is most likely responsible for the amnesia would be: Hippocampus

Hippocampus in our brain plays an important role in storing information to both our short-term spatial memory and long-term spatial memory.
If this part of our brain is damaged, we wouldn't be able to remember anything that we see/learn, which caused the amnesia.