What would be the most appropriate next step when a 64-year-old man presents with a palpable liver that is 4 centimeters below the costal arch?

In the scenario presented, where a 64-year-old man has a palpable liver extending 4 centimeters below the costal arch, determining the liver span by percussion is an essential next step. Percussion provides valuable information regarding the size, consistency, and potential abnormalities of the liver. Assessing the liver span can help distinguish between normal anatomical variations and pathological enlargements, providing initial insights into the underlying cause of hepatomegaly. Knowing the exact size of the liver can inform further diagnostic steps and guide treatment decisions.

While checking an ultrasound of the liver or obtaining a hepatitis panel may be valuable later in the evaluation process, these steps would typically follow an initial hands-on assessment of the liver. An ultrasound can visualize liver structures and detect lesions, while a hepatitis panel can identify any viral infections, but these tests do not replace the immediate physical evaluation of the liver size and contour. Adopting a "watchful waiting" approach may not be appropriate given the palpable liver enlargement in this older patient, as it could allow potential underlying pathology to progress without intervention. Thus, percussion to gauge liver span is the most logical and immediate next step in the clinical assessment of this patient.