Question

A 42-year-old woman is brought to the emergency department by her husband due to progressive confusion, unsteadiness, and hand tremors over the past 3 days. She has a history of bipolar disorder and has been stable on a fixed dose of lithium carbonate for the last three years. Two weeks ago, she was diagnosed with osteoarthritis of the knee and began taking a newly prescribed daily oral medication for joint pain. Her vital signs are temperature $37.0^\circ\text{C}$ ($98.6^\circ\text{F}$), blood pressure $138/85\text{ mmHg}$, pulse $88/\text{min}$, and respiratory rate $14/\text{min}$. Physical examination reveals a coarse resting tremor and marked ataxia. Laboratory studies show a serum lithium concentration of $2.1\text{ mEq/L}$ (therapeutic range: $0.6-1.2\text{ mEq/L}$) and a serum creatinine of $1.4\text{ mg/dL}$ (baseline: $0.8\text{ mg/dL}$). The newly prescribed medication is identified as the likely cause of her current condition. Which of the following pharmacokinetic mechanisms best explains this patient's acute toxicity?