The 45-year-old female patient had received a diagnosis of seronegative RA 4 years before presentation. She had received several DMARDs including methotrexate (MTX), sulfasalazine (SSZ), and hydroxychloroquine (HCQ). Her current treatment included leflunomide (LEF) and intermittent usage of low-dose prednisolone. Her past medical history was significant for arterial hypertension, which was treated with metoprolol and amlodipine.
The patient presented with acute symptoms, as she had been suffering from increasing pain and swelling of her right hand. Two weeks before presentation, she had already taken 20 mg prednisolone for 10 days with only slight improvement of symptoms. On presentation, there was swelling and tenderness of the third metacarpophalangeal (MCP) joint of the right hand. A flare of her RA was assumed and prednisone increased to 40 mg daily. Another 2 weeks later, she returned for a follow-up visit with new cutaneous findings: Clinical examination at that time revealed a visible and palpable swelling of the back of the right hand and third MCP joint (Fig. 1a). Other joints were not affected. A musculoskeletal ultrasound was performed and showed tenosynovitis of the third extensor tendon with increased power doppler signal (Fig. 2a and b). Laboratory analysis showed a thrombocytosis of 422 x 10^3/mul (normal range, NR: 150-350 x 10^3/mul), a leukocytosis of 15,9 x 10^3/mul (NR: 4-11 x 10^3/mul) and slightly increased CRP of 7,6 mg/l (NR: <= 5 mg/l). The differential blood count showed increased neutrophilic granulocytes with 83,9% (NR: 40-76%). Otherwise, there were no pathological findings.  
Due to the novel skin findings, the patient was urgently referred to our institution's Dermatology Clinic. In addition to the findings on the right hand, a complete examination of the skin revealed a single reddish-purplish erythematous nodule in a linear "sporotrichoid" form on the right proximal forearm (Fig. 1b). No palpable lymph nodes were found in the axillary region. Upon further questioning, the patient reported regular cleaning of fish tanks and recent travel to the seaside. A clinical diagnosis of a so-called "swimmer's granuloma" was made and a biopsy scheduled for microbiological analysis. In light of this information, the nailfolds of the third and fourth right finger seemed to be the most likely site of inoculation. The histopathological analysis revealed granulomatous inflammation with necrosis. Ziehl-Neelsen staining and blood-based testing for Mycobacterium tuberculosis (using Quantiferon TB Gold Plus  test) were negative. After several weeks, infection with MM was confirmed in the tissue culture.
Based on the clinical findings, immediate antibiotic treatment with clarithromycin and rifampicin was initiated. There was gradual improvement of the clinical findings over the following months.