Inflammatory Bowel Disease (IBD): Given the patient's symptoms of persistent hematochezia, RLQ abdominal pain, and a borderline fecal calprotectin level, IBD (such as Crohn's disease or ulcerative colitis) is a strong consideration. The moderate to large colonic stool burden, particularly in the right colon, could be indicative of inflammation or altered bowel habits associated with IBD.

Diverticulitis: Although the CT scan did not show wall thickening or masses, diverticulitis could still be a possibility, especially if the patient has a history of diverticula. The symptoms of hematochezia and RLQ pain could be consistent with diverticulitis, particularly if the inflammation is mild or localized.
Colonic Polyps or Angiodysplasia: These conditions could cause hematochezia, and while the CT did not show masses, smaller polyps or angiodysplastic lesions might not be visible on this imaging modality. Further evaluation with colonoscopy would be necessary to rule out these conditions.
Infectious Colitis: Given the patient's symptoms and the presence of a borderline fecal calprotectin, an infectious cause of colitis should be considered, especially if the patient has recently traveled, has been exposed to someone with a similar illness, or has a history of antibiotic use.

Colorectal Cancer: Although less likely in a 39-year-old, colorectal cancer must be considered, especially with symptoms of hematochezia. A thorough evaluation, including colonoscopy, is crucial to rule out this potentially life-threatening condition.
Intussusception: This condition, although rare in adults, can cause abdominal pain and hematochezia. It is essential to consider intussusception, especially if there are signs of bowel obstruction or if the patient's symptoms worsen.
Ischemic Colitis: This condition can present with abdominal pain and hematochezia and is a critical diagnosis not to miss due to its potential for significant morbidity and mortality.

Meckel's Diverticulum: A congenital anomaly that can cause gastrointestinal bleeding, Meckel's diverticulum is a rare but possible cause of the patient's symptoms, especially if other more common causes are ruled out.
Hemangiomas or Other Vascular Malformations: These are rare causes of gastrointestinal bleeding and would typically require specific diagnostic tests (like angiography) for diagnosis.
Ehlers-Danlos Syndrome or Other Connective Tissue Disorders: These conditions can lead to gastrointestinal manifestations, including bleeding, due to vascular fragility or other mechanisms, but they are rare and would require a broader diagnostic approach.