The racemic hydroxy aldehyde shown is treated with HCN (cyanide, then acid) and the product is then hydrolysed with hot aqueous acid to a hydroxy acid. How many stereoisomers of the final hydroxy acid are formed in total (configurational isomers of the constitution)?

2-Hydroxypropanal gives a cyanohydrin with a new stereocentre, and hot acid converts CN to COOH, giving 2,3-dihydroxybutanoic acid. It has two unrelated stereocentres, so $2^2=4$ stereoisomers (two diastereomeric pairs) are possible and all four form from the racemic aldehyde. Enumeration gives 4.