Chess Opening Theory/1. d4/1...Nf6/2. c4/2...g6/3. Nc3/3...d5/4. cxd5/4...Nxd5/5. e4/5...Nxc3

5...Nxc3
This is forced, as Nb6 and Nf6 lost time, and Nb4 lost to Qa4+ Nc6 d5. Now, Black appears to have improved White's center, but weakened the long diagonal when Black plays Bg7. Black will play ...Bg7 and ...c5 soon after.