I have a question regarding insulin secretion. In multiple answer explanations on the Uworld Step I Qbank, it is mentioned that Proinsulin is cleaved in the endoplasmic reticulum by endopeptidases, THEN the C-peptide fragment and Insulin is packaged into secretory granules in the Golgi.

However, numerous sources I have researched (First Aid, UpToDate etc.) indicated that PREproinsulin is cleaved in the ER to Proinsulin, Proinsulin is transported into the Golgi apparatus and subsequently packaged into secretory granules, it is in the secretory granules that endopeptidases cleave Proinsulin into C-Peptide + insulin.

Can someone confirm or deny this process and which one is correct?

As per my knowledge I would say that PREproinsulin is cleaved in the ER to Proinsulin, still for more information visit this website.