Differential Diagnosis for 74-year-old Male with Mouth and Nose Sores and Positive Atypical p-ANCA

• Single most likely diagnosis:
  • Microscopic polyangiitis (MPA): This condition is characterized by the presence of ANCA, particularly p-ANCA, and can present with mucosal ulcers and sores. The absence of MPO and PR3 antibodies, which are more commonly associated with other forms of vasculitis like granulomatosis with polyangiitis (GPA) and eosinophilic granulomatosis with polyangiitis (EGPA), makes MPA a strong consideration given the atypical p-ANCA positivity.
• Other Likely diagnoses:
  • Eosinophilic granulomatosis with polyangiitis (EGPA): Although less likely due to the absence of PR3 and MPO antibodies, EGPA can present with a wide range of symptoms including mucosal involvement. The diagnosis might be considered if there are other systemic symptoms or eosinophilia, even though they are not mentioned.
  • Granulomatosis with polyangiitis (GPA): Typically associated with c-ANCA (PR3), GPA can occasionally present with p-ANCA. The presence of mouth and nose sores could fit within the spectrum of GPA, especially if there's any granulomatous involvement.
• Do Not Miss diagnoses:
  • Wegener's granulomatosis (a form of GPA): Given the potential for severe and life-threatening complications, it's crucial not to miss this diagnosis. Even though the typical ANCA pattern for GPA involves PR3, atypical presentations can occur.
  • Infections (e.g., syphilis, tuberculosis): These can mimic autoimmune diseases and present with mucosal lesions. Missing an infectious cause could lead to inappropriate treatment and significant morbidity.
• Rare diagnoses:
  • Relapsing polychondritis: This condition involves inflammation of cartilaginous structures and can present with mucosal lesions, though it's less commonly associated with ANCA positivity.
  • Behçet's disease: Characterized by recurrent oral ulcers, genital ulcers, and uveitis, Behçet's can occasionally present with positive ANCA, though it's not a primary diagnostic criterion.

Each of these diagnoses requires careful consideration of the patient's full clinical picture, including any additional symptoms, laboratory findings, and potentially, biopsy results to confirm the diagnosis.