Final answer:
The nurse practitioner should order a CBC and fecal occult blood test (FOBT) to evaluate the patient's epigastric pain and darker stools, as this may indicate a possible ulcer or gastrointestinal bleeding.
Step-by-step explanation:
The primary concern in a patient presenting with epigastric pain and darker stools is to rule out conditions such as ulcers or gastrointestinal bleeding. While antacids can help relieve the symptoms of ulcers, which include a burning pain, a change in stool color suggests the possibility of bleeding (which can be a sign of a more serious condition like a bleeding ulcer or potentially something like gastric cancer). Therefore, the nurse practitioner should order a CBC (Complete Blood Count) and fecal occult blood test (FOBT) to assess for anemia or the presence of blood in the stool, which are immediate next steps in evaluation. Further referral to a specialist or additional endoscopic procedures such as a colonoscopy might be considered based on the results of these tests.