A 67-yeаr-оld pаtient with primаry hypertensiоn returns fоr follow-up after 6 weeks of treatment with losartan. Blood pressure has improved from 164/96 mm Hg to 148/88 mm Hg but remains above goal. The patient reports good adherence and no adverse effects. Renal function and serum potassium are normal. Which medication change is most appropriate?