Read the passage from "Flowers for Algernon." As I review the records and data on Algernon, I see that although he is still in his physical infancy, he has regressed mentally. Motor activity is impaired; there is a general reduction of glandular activity; there is an accelerated loss of coordination.

Which paraphrase best expresses the meaning of the word although?

Algernon is going backward mentally, similar to other young mice, which was not expected.
First Algernon was young, then he went backward mentally, which was not expected.
As a result of being a young mouse, Algernon is going backward mentally, as one would expect.
Algernon is still a young mouse, but he is going backward mentally, in contrast to what one would expect.