McDonnell Gambit, La Bourdonnais-Denker Gambit

After 3...Bxb4, White continues with 4. c3, gaining a tempo on the bishop and preparing d4 to build a broad center, in the manner of the Evans Gambit but with the g1-knight undeveloped and the f-pawn free to advance. White trails by a pawn and has an isolated a-pawn. Black usually retreats with 4...Bc5, less often 4...Ba5 or 4...Be7. The line is rare and the engine considers it roughly balanced.