When aldehydes are treated with sodium bisulfite ($NaHSO_3$), a water soluble adduct is formed. This adduct decomposes when treated with mild base (e.g. aqueous $NaHCO_3$). This method is often used for extractive purification of aldehydes since usually only aldehydes undergo this reaction (ketones are too unreactive).
What is the structure corresponding to the question mark in the reaction equation below?