Acetaldehyde, $\mathrm{CH}{}_3-\mathrm{CHO}$, is treated with $\mathrm{HCN}$ in the presence of a trace of base. The product is

Cyanide ion adds to the carbonyl carbon and the alkoxide is protonated by $\mathrm{HCN}$. This gives the cyanohydrin $\mathrm{CH}{}_3-\mathrm{CH}(\mathrm{OH})-\mathrm{CN}$, i.e. 2-hydroxypropanenitrile. Hydrolysis to the acid would need a separate aqueous acid step.