Aldol condensation is a very important reaction type for biochemistry. For example, in energy metabolism, the cleavage of the 1,6 phosphate diester of fructose in glycolysis is a retro-aldol reaction (aldol condensation in reverse). As another example from energy metabolism, the addition of oxaloacetate to acetyl CoA, which begins the citric acid cycle, is an aldol condensation. As an example from the wider biochemical context, the intramolecular covalent cross-links stabilizing collagen fibers are formed by an aldol condensation between aldehyde derivatives of lysine side chains. As you can imagine, aldol condensation is an MCAT favorite.