After discovering an error that caused serious harm to a patient, the pharmacy staff's best course of action would be to perform a:

Failure mode and effects analysis (FMEA) is a proactive risk assessment tool used to identify potential failure modes in a process or system and their potential effects. While valuable for preventive analysis, FMEA is more suited to identifying and mitigating risks before they occur rather than investigating incidents that have already transpired.

Medication reconciliation involves comparing a patient's current medication regimen against any new prescriptions or changes to ensure accuracy and prevent adverse drug events. While critical for patient safety, medication reconciliation is not the appropriate immediate response to a discovered error that has already led to harm.

Therapeutic substitution refers to the practice of replacing one medication with another that is considered therapeutically equivalent. In the context of an error resulting in harm, the focus should be on understanding the root causes of the error rather than making therapeutic substitutions.