Angiotensin converting enzyme (ACE) inhibitors are contraindicated in patients with renal artery stenosis because the hemodynamic changes that have occurred due to the RAS have made renal perfusion in these patients dependent on angiotensin II. An anaphylactic reaction would have presented with acute inflammatory signs. ACE inhibitors have no known hypercoagulability reactions, so thrombosis is unlikely. Acute tubular necrosis (ATN) can result in acute renal failure and is most commonly a result of drugs or toxins. However, ACE inhibitors are not known to cause ATN, so this would be an unlikely cause.