Differential Diagnosis for the Presented Case

The patient's symptoms and test results suggest a range of possible diagnoses. Here's a categorized differential diagnosis:

• Single Most Likely Diagnosis

  • Anal fistula and fissure complications: The presence of an anal fistula, anal fissure, and bright blood in the feces, particularly associated with the consumption of vegetables leading to mashy stools, points towards complications related to these conditions as the primary cause of symptoms. The normal inflammatory markers and the absence of severe abdominal pain further support this as the most likely diagnosis.

• Other Likely Diagnoses

  • Irritable Bowel Syndrome (IBS): Given the patient's description of mashy poop, especially after eating vegetables, and the occasional normal poop, IBS could be considered. However, the presence of blood and the anal fistula/fissure might distinguish this from a typical IBS presentation.
  • Diverticulitis: Although less likely given the patient's age and lack of severe pain, diverticulitis could potentially cause similar symptoms, including changes in bowel habits and blood in the stool. However, the normal CRP and lack of left lower quadrant pain make this less likely.

• Do Not Miss Diagnoses

  • Inflammatory Bowel Disease (IBD): Despite the patient's assertion that they want to rule out IBD and the lack of family history, normal inflammatory markers, and absence of severe abdominal pain, IBD (including Crohn's disease and ulcerative colitis) must be considered due to its potential for serious complications if left untreated. The presence of an anal fistula can be a feature of Crohn's disease, making it crucial not to miss this diagnosis.
  • Colorectal Cancer: Although rare in a 28-year-old, colorectal cancer can present with changes in bowel habits and blood in the stool. Given the potential severity of this condition, it should not be overlooked, especially if other symptoms or signs suggestive of cancer are present.

• Rare Diagnoses

  • Infectious Colitis: Certain infections can cause colitis, leading to symptoms like those described. However, this would typically be accompanied by other signs of infection, such as fever or elevated inflammatory markers, which are not present in this case.
  • Microscopic Colitis: This condition can cause chronic diarrhea and could potentially lead to changes in stool consistency. However, it is less likely to cause bright blood in the stool and is more commonly diagnosed in older adults.