For reporting Evaluation and Management (E/M) services, the longstanding “key components” framework recognizeshistory, examination, and medical decision making (MDM)as the core elements used to determine the appropriate E/M level when the service is not reported based on time. In outpatient CDI education aligned with ACDIS concepts, MDM is emphasized because it reflects the clinician’s cognitive work and risk-based thinking: the complexity of problems addressed, the amount/complexity of data reviewed and analyzed (labs, imaging, external notes, independent interpretation), and the risk of complications and/or morbidity from additional testing or treatment. “Review of systems” is a sub-element of the history component, not a separate key component. “Nature of presenting problem” and “coordination of care” can be clinically relevant and may support medical necessity or time-based billing (when documented appropriately), but they are not one of the three key components that define E/M reporting structure. Therefore, the best answer is Medical decision making.