A 45-year-old woman with a long-standing history of Ulcerative Colitis, complicated by recurrent episodes of primary sclerosing cholangitis, presents to an outpatient gastroenterology clinic. She reports a progressive yellowing of her skin and eyes over the past three months, accompanied by relentless generalized itching, significant unintentional weight loss of 7 kg, and profound fatigue. She notes her stools have become unusually pale, almost clay-colored, and her urine has turned a very dark, tea-like color. Her vital signs include a temperature of 37.10C, blood pressure of 128/78 mmHg, pulse of 98/min, respirations of 14/min, and oxygen saturation of 99% on room air. Physical examination reveals diffuse jaundice, prominent scleral icterus, and excoriations from scratching. A rectal exam confirms pale, acholic stool with no visible blood. There is no significant abdominal tenderness or hepatosplenomegaly. Considering this patient's chronic conditions and current presentation, what is the most likely underlying diagnosis and the critical initial diagnostic step?