This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General anatomy, Subject name: Anatomy
Which type of glycoprotein is collagen? 
Select and explain. 
A. O-linkage, B. N-linkage, C. GPI linkage, D. none
A: O-linkage
Answer: A: O-linkage 
Explanation: O-glycosidic linkage: Mucin proteoglycans collagens Nuclear and cytosolic proteins N-Glycosidic linkage type: complex High mannose Hybrid GPI anchored: Acetylcholinesterase alkaline phosphatase Decay-accelerating factor 5'-nucleotidase Thy 1 antigen Variable surface glycoprotein Reference: Harpers illustrated biochemistry 31st edition pg549