The most appropriate next step in management is colonoscopy. Primary sclerosing cholangitis (PSC) is diagnosed by measuring liver enzymes and performing cholangiography. Serum alkaline phosphatase values are 3 to 10 times the upper limit of normal, and serum alanine aminotransferase and aspartate aminotransferase levels are two to three times the upper limit of normal. Serum total bilirubin levels may be normal in 60% of patients. Serum antinuclear and anti-smooth muscle antibodies are present in 20% to 50% of patients, but antimitochondrial antibodies are rarely found in PSC. The gold standard for diagnosis of PSC is cholangiography. Diagnostic findings consist of segmental bile duct fibrosis with saccular dilatation of normal intervening areas, resulting in the characteristic “beads on a string” appearance. Magnetic resonance cholangiopancreatography (MRCP) has been increasingly used and has an overall diagnostic accuracy rate of 90%. This patient's clinical presentation and cholangiographic findings are consistent with PSC. Eighty percent of patients with PSC have ulcerative colitis. Given this patient's 3-year history of diarrhea, colonoscopy should be performed to evaluate for ulcerative colitis. Patients with PSC and ulcerative colitis are at increased risk for colon cancer and should receive surveillance. There is no effective medical therapy for PSC. Endoscopic dilatation of biliary strictures and removal of stones may be necessary in patients with progressive cholestasis or symptoms of cholangitis. PSC is generally a progressive disease that often requires liver transplantation.

MRCP has already been performed; endoscopic retrograde cholangiopancreatography (ERCP) will not add any additional useful information to the clinical picture. ERCP will be useful in managing choledocholithiasis, occurring in 10% to 25% of symptomatic patients, and dilatation of dominant strictures, occurring in 5% to 10% of patients.

Liver biopsy is usually not necessary for the diagnosis of PSC. Liver biopsy is required for making a diagnosis of small-duct PSC when cholangiography is normal. Periductal fibrosis with inflammation, bile duct proliferation, and ductopenia are the main histologic findings.

IgG4-associated cholangitis may mimic PSC, but patients will usually have abnormalities in the pancreas on cross-sectional imaging. This patient did not have these findings, so serum IgG4 measurement is not required.