A 66 year old woman presents with a 4 month history of fatigue, upper abdominal discomfort and itching. On examination, she has hepatomegaly, jaundice and hyperpigmentation. A subsequent blood test demonstrates the following:
ALT 189 U/L (normal less than 50 U/L)
AST 174 U/L (normal less than 50 U/L)
Alkaline Phosphatase 757 U/L (normal range, 30-130 U/L)
Anti-Smooth Muscle Antibody Negative
Anti-Mitochondrial Antibody Positive
Liver-Kidney Microsomal Antibody Negative
What is the most likely diagnosis?