A number of sports nutrition companies add $\beta$-Alanine to their supplements as a way to increase the overall concentration of carnosine in the muscles to reduce fatigue during strenuous exercise.
Carnosine has a number of biological benefits as well, but it suffers a short half-life in the body.
The schematic below showcases the synthesis of carnosine.
Based on the schematic, structure A must be
, which would make structure B
The incorporation of structure A would result in structure C being