The definition in option B is the official Uniform Hospital Discharge Data Set (UHDDS) definition used for inpatient coding: the principal diagnosis is the condition determined—after evaluation—to be chiefly responsible for the admission. It is not simply the first condition written, nor necessarily the “worst” or most severe condition; it is the reason for admission once the workup clarifies the clinical picture. CDI practice reinforces this because principal diagnosis selection drives DRG assignment, quality metrics, and reporting, and errors often stem from confusing presenting symptoms with the final established diagnosis. Although outpatient settings use different concepts (e.g., first-listed diagnosis for the encounter), ACDIS education frequently contrasts inpatient “principal diagnosis” with outpatient “first-listed” to prevent documentation and coding misalignment. Clinicians should document the definitive condition when known (and link symptoms to that condition), and clearly describe diagnostic uncertainty when not yet established. This clarity supports compliant coding, accurate benchmarking, and defensible medical necessity across settings.