Final answer:
The most useful test in the diagnosis of this patient's disease would be Abdominal CT, which can provide detailed images of the abdominal organs to identify any abnormalities or causes of the symptoms.
Step-by-step explanation:
The most useful test in the diagnosis of this patient's disease would be Abdominal CT. Abdominal CT scans are an imaging technique that can provide detailed images of the abdominal organs, including the intestines, to help identify any abnormalities or causes of the symptoms.
In this case, the patient's symptoms, such as left lower quadrant abdominal pain, heme-positive stool, and an elevated WBC count with a left shift, suggest the possibility of diverticulitis, which can be confirmed or ruled out by an abdominal CT scan.