What is the primary assessment when Mr. Duncan complains of dizziness upon standing?

When a patient like Mr. Duncan complains of dizziness upon standing, it is crucial to perform a full assessment that includes measuring vital signs while the patient is lying down and again after they stand up. This process is essential to determine if he is experiencing orthostatic hypotension, a condition where blood pressure drops significantly when a person stands up, leading to symptoms such as dizziness or lightheadedness.

By conducting this complete assessment of lying and standing vitals, healthcare providers can monitor changes in blood pressure and heart rate that may indicate whether Mr. Duncan's dizziness is related to inadequate blood flow to the brain upon positioning changes. This assessment will guide further management and interventions if required.

While checking input/output, documenting findings, or consulting dietary services may be relevant in a broader context, they do not directly address the immediate concern of his dizziness upon standing. Hence, a full assessment focusing on vital signs is the most pertinent action in this scenario.