This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Structure and function of protein, Subject name: Biochemistry
Protein purification and separation is done by all Except 
Select and explain. 
A. Chromatography, B. Precipitation, C. Electrophoresis, D. Densitometry
D: Densitometry
Answer: D: Densitometry 
Explanation: Highly purified protein is essential for the detailed examination of its physical and functional propeies. It may require the successive application of multiple purification techniques. Selective precipitation exploits differences in relative solubility of individual proteins as a function of pH (isoelectric precipitation), polarity (precipitation with ethanol or acetone), or salt concentration (salting out with ammonium sulfate). Chromatographic techniques separate one protein from another based upon the difference in their size (size exclusion chromatography), charge (ion-exchange chromatography), hydrophobicity (hydrophobic interaction chromatography), or ability to bind a specific ligand (affinity chromatography).The most widely used method for determining the purity of a protein is SDS-PAGE--polyacrylamide gel electrophoresis (PAGE) in the presence of the anionic detergent sodium dodecyl sulfate (SDS).Densitometry is a method to quantify the proteinsRef: Harper; 30th edition; Pg: 26