This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
With regard to the malignant behaviour of leiomyosarcoma, the most important criterion is - 
Select and explain. 
A. Blood vessel penetration by tumor cells, B. Tumor cells in lymphatic channels, C. Lymphocyte infiltration, D. The number of mitoses per high power field
D: The number of mitoses per high power field
Answer: D: The number of mitoses per high power field 
Explanation: The distinction of Leiomyosarcoma from Leiomyomas:
It is based on the combination of: 

The degree of nuclear atypia 
Mitotic index 
Zonal Necrosis

The most important single criteria are the number of nuclear mitosis present

With or without Nuclear atypia (Independent of nuclear atypia) → The presence of ten or more mitoses per ten high power (x400) fields indicates malignancy with or without cellular atypism.
In presence of nuclear atypia or large cells → If the tumor contains nuclear atypia or large epithelioid cells, five mitoses per ten high power fields are sufficient to justify a diagnosis of malignancy.