Differential Diagnosis

The patient presents with acute onset of rectal and body pain, a positive CTC of the sacroiliac joint, and various laboratory findings including HLA-B27 negativity, ANA positivity, elevated CRP, and the presence of certain antibodies. Based on these findings, the differential diagnosis can be categorized as follows:

• Single Most Likely Diagnosis
  • Ankylosing Spondylitis (AS): Although the patient is HLA-B27 negative, which is a common marker for AS, the disease can still occur in its absence. The positive CTC of the sacroiliac joint and elevated CRP support this diagnosis, as AS is characterized by inflammation of the sacroiliac joints and spine. The presence of body pain and elevated inflammatory markers further aligns with AS.
• Other Likely Diagnoses
  • Psoriatic Arthritis (PsA): Given the patient's body pain and elevated CRP, PsA is a consideration, especially if there are skin or nail changes indicative of psoriasis. The absence of HLA-B27 does not rule out PsA, as it is associated with different HLA antigens.
  • Reactive Arthritis: This condition can follow a gastrointestinal or genitourinary infection and presents with arthritis, often involving the sacroiliac joints. The rectal pain could be a symptom of the initial infection.
  • Enteropathic Arthritis: Associated with inflammatory bowel diseases like Crohn's disease or ulcerative colitis, this condition could explain the rectal pain and body pain, especially if there's an underlying gastrointestinal condition.
• Do Not Miss Diagnoses
  • Infectious Spondyloarthropathy: Infections such as tuberculosis or brucellosis can cause sacroiliac joint inflammation and must be considered, especially if the patient has been exposed to these pathogens or has risk factors.
  • Malignancy: Although less likely, malignancies can cause widespread pain and elevated inflammatory markers. A thorough evaluation is necessary to rule out cancer.
• Rare Diagnoses
  • Sarcoidosis: This autoimmune disease can cause inflammation in various parts of the body, including the joints, and could potentially explain the patient's symptoms, although it is less common.
  • Relapsing Polychondritis: A rare autoimmune disorder that can cause inflammation in various cartilaginous structures, including joints, and could be considered if other diagnoses are ruled out.

Each of these diagnoses requires careful consideration of the patient's clinical presentation, laboratory findings, and potentially additional diagnostic testing to determine the most accurate diagnosis.