Final answer:
The gastrointestinal manifestation of stomatitis in patients with CKD is primarily caused by metabolic acidosis and poor tubular reabsorption of sodium.
Step-by-step explanation:
The gastrointestinal (GI) manifestation of stomatitis in patients with chronic kidney disease (CKD) is primarily caused by metabolic acidosis due to the accumulation of sulfates, phosphates, uric acid, and other substances. This leads to altered enzyme activity and increased excitability of cardiac and neuronal membranes. Additionally, poor tubular reabsorption of sodium in CKD with acidosis can contribute to the development of gastrointestinal symptoms.