The answer is C) Ibuprofen. Lithium is renally cleared, and anything that can adversely affect the functioning of the kidneys could potentiate a lithium toxicity. In this case, taking higher doses of an NSAID like Ibuprofen after an ankle injury may have decreased his renal clearance, which could have precipitated higher lithium levels in the blood, leading to an acute toxicity. While Li toxicity can occur at any level, it usually occurs with levels higher than the therapeutic window (>1.5). In mild lithium toxicity, symptoms include weakness, worsening tremor, mild ataxia, poor concentration and diarrhea. With worsening toxicity, vomiting, the development of a gross tremor, slurred speech, confusion and lethargy may occur. Cerebellar signs may be present with worsening toxicity and may include ataxia and poor coordination, and are potentially irreversible. Treatment depends on severity and includes discontinuing lithium, normal saline infusion, and possibly hemodialysis if levels are high enough or symptoms severe enough. The other answer choices are not as likely to affect renal clearance nor are associated with mechanisms that would increase levels of lithium in the blood.

Publication reference : Gitlin M. Lithium side effects and toxicity: prevalence and management strategies. Int J Bipolar Disord. 2016 Dec;4(1):27. Review. PubMed PMID: 27900734; PubMed Central PMCID: PMC5164879.
Reference URL: https://www-ncbi-nlm-nih-gov/articles/PMC5164879/