The most likely diagnosis is polyarteritis nodosa (PAN), the most common medium-sized vasculitis that affects the mesenteric and renal arteries. Patients usually present with nonspecific inflammatory symptoms as well as abdominal symptoms (chronic or intermittent ischemic pain), neurologic involvement (mononeuritis multiplex), and skin findings (livedo reticularis, purpura, and painful subcutaneous nodules). Kidney disease is based on decreased renal artery blood flow rather than glomerulonephritis. The presence of hepatitis B virus (HBV) infection is a strong risk factor for the development of PAN, although in areas where HBV vaccination is common, the incidence of HBV-associated PAN has diminished greatly. This patient with HBV infection (positive hepatitis B surface antigen) presents with involvement of the abdomen, kidneys, nerves, joints, and skin, accompanied by hypertension but sparing the upper and lower respiratory tracts. He also has mononeuropathy, arthralgia, fever, and livedo reticularis (shown in the figure) but lacks the active urine sediment characteristic of glomerulonephritis. Together these findings suggest a systemic vasculitis of which PAN is the most likely. Testicular involvement is also common, as in this case. Diagnosis of PAN is best established by demonstrating necrotizing arteritis in biopsy specimens, or characteristic medium-sized artery aneurysms and stenoses on imaging studies of the mesenteric or renal arteries using either angiography or CT angiography.

Goodpasture syndrome affects the kidneys and often the lungs. The lack of active urine sediment and the presence of medium arterial involvement are inconsistent with Goodpasture syndrome, as are the abdominal symptoms.

Granulomatosis with polyangiitis (GPA; formerly known as Wegener granulomatosis) can involve the kidneys, and neuropathy and purpura can occur; however, this disease is associated with glomerulonephritis, which is absent in this patient. Furthermore, medium-sized artery involvement does not occur in GPA, and the absence of ANCA makes GPA much less likely.

Henoch-Schönlein purpura (HSP) can affect the kidneys, nerves, and skin as well as cause abdominal pain. However, HSP is an immune complex disease affecting small vessels; active urine sediment is common, complements (particularly C4) are typically low, and IgA levels may be elevated, none of which is the case in this patient.