(See pages xx for the Answer to the Photo Quiz.) A 48-year-old man with living with well-controlled human immunodeficiency virus (HIV) and treated plasmablastic lymphoma presented with 3 weeks of left shin pain and 8 days of right shin and left forearm pain. There was no history of trauma. The pain was described as “sharp” and “prickling” and was exacerbated by movement and weight-bearing. There had been intermittent swelling present over the left shin, but no muscle weakness or associated skin changes. He reported unintentional weight loss of 6 kg over the preceding 6 months; he had no fevers or night sweats. He was receiving antiretroviral treatment with tenofovir disoproxil, emtricitabine, and dolutegravir. Regular testing demonstrated undetectable HIV viral loads for the previous 2 years. Plasmablastic lymphoma was treated in 2016 with R-CHOP-B chemotherapy, and follow-up positron emission tomography–computed tomography (CT) imaging confirmed remission. During the lymphoma treatment, he was diagnosed with primary syphilis via direct fluorescent antibody staining from a penile ulcer, which showed multiple fluorescing treponemes. He was treated with 2 weeks of doxycycline due to a history of developing a rash following prior treatment with penicillin. Following this treatment, the rapid plasma reagin (RPR) was stable at 1:2. Physical examination revealed a trace of edema in the left lower leg and tenderness over the left anterior tibia. Plain X-rays of the left leg showed no fracture or focal osseous lesion. He had normocytic anemia (hemoglobin 97 g/L) with normal neutrophil and lymphocyte counts; 6 months prior, his full blood count was normal (hemoglobin 132 g/L). C-reactive protein was elevated at 23 mg/L; HIV viral load was undetectable and CD4 count was 666 (34%) cells/mm3; lactic acid dehydrogenase was normal. Magnetic resonance imaging and CT (Figures 1, 2) showed a left tibial lesion. There was an area of permeative change within the cortex of the mid-shaft of the tibia with a rim of surrounding enhancing soft tissue extending from the proximal diametaphyseal region to approximately 4.5 cm proximal to the ankle joint (Figures 1, 2). He underwent a radiologically guided biopsy of the tibial lesion. The immunohistochemical stain is shown in Figure 3. What is your diagnosis?
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