Blockage of trafficking vesicles from the endoplasmic reticulum to the Golgi leads to what change in the endoplasmic reticulum?

• A. Decreased RER
• B. No change
• C. Mitochondrial proliferation
• D. Increased RER because products back up

Answer: (D)

Blockage of vesicle transport from the endoplasmic reticulum to the Golgi causes proteins to back up in the rough endoplasmic reticulum, since they cannot progress along the secretory pathway. This backlog leads to dilation and expansion of the ER membrane, particularly the rough ER rich in ribosomes, making it appear enlarged. The increased load from accumulated proteins prompts the ER to swell rather than shrink, so the correct outcome is an increased RER due to products backing up. The other options don’t fit because there’s no reason for the ER to decrease, for there to be no change, or for mitochondrial proliferation to be a direct consequence of this trafficking block.