Final answer:
The optic nerve is the cranial nerve most often affected by diabetes mellitus, as high blood sugar levels can damage small blood vessels, leading to diabetic neuropathy and potential vision changes.
Step-by-step explanation:
The cranial nerve most often affected by diabetes mellitus is the cranial nerve responsible for vision, which is the optic nerve (cranial nerve II). High blood sugar levels associated with diabetes can lead to damage to small blood vessels throughout the body, including those in the nerves. This vascular damage can cause diabetic neuropathy, a condition where the nerves are impaired. Diabetic neuropathy can manifest as numbness, tingling, and pain, particularly in the extremities. However the cranial nerves are also susceptible, and the optic nerve can be particularly affected, leading to vision changes such as blurred vision, one of the common symptoms shared by both type 1 and type 2 diabetes. Poorly controlled diabetes increases the risk of experiencing such complications.