Differential Diagnosis
- Single most likely diagnosis
- Psoriatic arthritis: The patient's chronic rash on elbows and knees, which is likely psoriasis, combined with back pain that worsens in the morning and tender sacroiliac joints, points towards psoriatic arthritis. The absence of rheumatoid factor and ANCA also supports this diagnosis, as psoriatic arthritis is typically seronegative.
- Other Likely diagnoses
- Ankylosing spondylosis: The patient's back pain worsening in the morning and tender sacroiliac joints could also suggest ankylosing spondylosis, a form of spondyloarthritis. However, the presence of a chronic rash on elbows and knees is more indicative of psoriatic arthritis.
- Reactive arthritis: Although reactive arthritis is a possibility, it is less likely given the chronic nature of the rash and the absence of a clear infectious trigger.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Inflammatory bowel disease-associated arthritis: Although not directly suggested by the patient's symptoms, inflammatory bowel disease can cause arthritis and sacroiliitis, and missing this diagnosis could have significant consequences.
- Spinal infection or tumor: It is essential to consider and rule out spinal infection or tumor, as these conditions can cause back pain and sacroiliac joint tenderness, and delaying diagnosis could be catastrophic.
- Rare diagnoses
- Enteropathic arthritis: A rare form of arthritis associated with inflammatory bowel disease, which could be considered if the patient has a history of gastrointestinal symptoms.
- Spondyloarthritis with psoriasis-like rash: Other forms of spondyloarthritis, such as undifferentiated spondyloarthritis, could present with a psoriasis-like rash, although this is less likely given the patient's symptoms.