Final answer:
Manganese must be carefully monitored in long-term parenteral nutrition patients to prevent toxicity, as excessive accumulation can lead to adverse neurological effects.
Step-by-step explanation:
Failure to monitor the micronutrient manganese in long-term parenteral nutrition (PN) patients is most likely to result in toxicity. Manganese is a trace element that, when present in excess, can lead to neurological symptoms such as Parkinsonism. Its intake must be carefully controlled in clinical settings like parenteral nutrition, where patients receive nutrients intravenously because they cannot ingest them by mouth. PN formulations include essential micronutrients, but excessive accumulation, particularly of manganese, can occur if monitoring is inadequate.