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: Biochemistry
In eukaryotes, which of the following is covalently linked to the protein that is targeted for destruction? 
Select and explain. 
A. Clathrin, B. Pepsin, C. Laminin, D. Ubiquitin
D: Ubiquitin
Answer: D: Ubiquitin 
Explanation: The selective degradation of many sho-lived proteins in eukaryotic cells is carried out by the ubiquitin-mediated proteolytic system. In this pathway, proteins are targeted for degradation by covalent ligation to ubiquitin. Ref: The Ubiquitin-Proteasome Proteolytic System, By Aaron J. Ciechanover, Maria G. Masucci, 2002 ; Progress In Virus Research By Sabine Thebault, Page 79, Structural And Functional Organization Of The Synapse By Johannes W. Hell, Michael D. Ehlers