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
Which one of the listed statements best describes the process called chemotaxis? 
Select and explain. 
A. Abnormal fusion of phagosomes to primary lysosomes, B. Attachment of chemicals to extracellular material to increase phagocytosis, C. Dilation of blood vessels by chemotherapeutic drugs, D. Movement of cells toward a certain site or source
D: Movement of cells toward a certain site or source
Answer: D: Movement of cells toward a certain site or source 
Explanation: Chemotaxis is the unidirectional migration of cells toward a certain site or source.