5-Hydroxy-5-methylhexan-3-one, $\mathrm{CH}{}_3\mathrm{CH}{}_2-\mathrm{CO}-\mathrm{CH}{}_2-\mathrm{C}(\mathrm{OH})(\mathrm{CH}{}_3){}_2$, is warmed with dilute aqueous $\mathrm{NaOH}$. The product is

Only the CH2 group at C-4, between the carbonyl and the carbinol carbon, has acidic hydrogens next to a carbon bearing a leaving group. Its enolate expels hydroxide from C-5 (E1cB), giving the conjugated enone $\mathrm{CH}{}_3\mathrm{CH}{}_2-\mathrm{CO}-\mathrm{CH}=\mathrm{C}(\mathrm{CH}{}_3){}_2$, 5-methylhex-4-en-3-one. Hydrogens of the methyl groups on C-5 are not acidic and are not removed.