This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology, Subject name: Physiology
C-peptide occurs in 
Select and explain. 
A. Proinsulin, B. Glucagon, C. Paratharmone, D. Thyroxine
A: Proinsulin
Answer: A: Proinsulin 
Explanation: C-peptideA biologically inactive residue of insulin formation in the beta cells of the pancreas. When proinsulin is conveed to insulin, an equal amount of C peptide, a chain of amino acids, is also secreted into the bloodstream. Beta cell secretory function can be determined by measuring the C peptide in a blood sample.Proinsulin is a precursor of insulin, having low biologic activity.Glucagon is a polypeptide hormone secreted by the alpha cells of the islets of Langerhans in response to hypoglycemia or to stimulation by growth hormone.Thyroxine is a thyroid hormone that contains iodine and is a derivative of the amino acid tyrosine, occurring naturally as l-thyroxine; its chemical name is tetraiodothyronine.Ref: Ganong&;s review of medical physiology; 24th edition; page no-433