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4…Nf6 5.dxc5 Qa5+: 6.Qd2, not 6.c3
The dossier's second recurring move, in the same line: against 5…Qa5+, you answer 6.c3, and you let slip the advantage the check has just handed you.
Recall: after 7.Kf2 Qb6, which move?