A pаtient whо hаs been tаking lithium fоr several years repоrts feeling off balance and has slurred speech. A lithium level is checked and is 1.5 mEq/L, up from 0.8 mEq/L two months ago. The patient has been taking acetaminophen and ibuprofen daily for the last 5 days for a pulled muscle. Current prescription medications include chlorthalidone 25 mg daily (thiazide diuretic started 1 year ago) and sertraline 50 mg daily (started 2 weeks ago). Lisinopril 10 mg (ACE-inhibitor) was stopped 3 months ago. What is the MOST likely cause of the patient’s elevated lithium level?