A 40-year-old male with a 5-year history of chronic active hepatitis and ascites for 1 year presents with gingival bleeding, epistaxis, and scattered ecchymoses over the past 2 months. Physical exam: chronic liver disease facies, icteric sclera, spleen palpable 5 cm below the costal margin, shifting dullness (+). The most likely cause of his significant bleeding tendency is: