C

Spironolactone should not be started when serum potassium is >5.0 mEq/L or creatinine is >2.0–2.5 mg/dL because it markedly increases the risk of life-threatening hyper-kalaemia in heart-failure patients [Prescrire Int, 2000, PMID 11603418; Shah et al., 2005, PMID 16139135].  
Given this patient’s K⁺ 5.5 mEq/L and creatinine 2.5 mg/dL, spironolactone is the drug most likely to precipitate dangerous hyper-kalaemia and is therefore best avoided.