This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General Microbiology Pa 1 (History, Microscopy, Stains and Structure and Physiology of Bacteria), Subject name: Microbiology
Stain used in electron microscopy 
Select and explain. 
A. 2.5% FAM, B. Phosphotungstic acid, C. Saffranin, D. Coomassie blue
B: Phosphotungstic acid
Answer: B: Phosphotungstic acid 
Explanation: Phosphotungstic acid is electron dense, opaque for electrons. It is a common negative stain for viruses, nerves, polysaccharides and other biological tissue materials for imaging by an Electron microscope.