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: Physiology
C-peptide occurs in: 
Select and explain. 
A. Proinsulin, B. Glucagon, C. Parathormone, D. Thyroxine
A: Proinsulin
Answer: A: Proinsulin 
Explanation: Preproinsulin contains a signal peptide of 23­-amino acid. The signal peptide is removed as it enters the endoplasmic reticulum. The remainder of the molecule is pro-insulin. This peptide contains A, B chains  and the connecting peptide (C peptide). Separation of C peptide produces insulin.