Essential Thrombocythemia (MPL W515)

A 62-year-old woman presents with a platelet count of 980,000/μL, palpable splenomegaly, and mild anemia (Hb 11.2 g/dL). Bone marrow biopsy shows striking megakaryocyte proliferation with mild-to-moderate reticulin fibrosis and focal megakaryocyte atypia. BCR-ABL1 is negative. Targeted sequencing is negative for JAK2 V617F and CALR exon 9 mutations but identifies the mutation marked **A** in the diagram—an MPL W515L gain-of-function point mutation in exon 10 at 38% variant allele frequency. Which of the following best describes the pathophysiologic consequence of the mutation marked **A**?