Final answer:
The symptom 'a veil of blackness coming across my vision' is a crucial symptom to follow-up in a patient with diabetes mellitus, as it suggests the possibility of diabetic retinopathy, which can lead to vision loss or blindness and requires immediate medical intervention.
Step-by-step explanation:
Among the symptoms reported by a patient with diabetes mellitus, the symptom described in option C is most important to follow-up: "There is something like a veil of blackness coming across my vision." This could indicate a serious eye condition related to diabetes, such as diabetic retinopathy, which can lead to vision loss and even blindness. Diabetic retinopathy is caused by damage to the blood vessels in the retina due to high blood glucose levels. Immediate medical attention is necessary to prevent further damage. Options A and B may also warrant attention, as vision changes could be symptoms of diabetes affecting the eyes, but the symptom in C suggests an acute and possibly severe condition. Yellow discharge from one eye (option D) may not be directly related to diabetes but could be an infection or another issue and also requires medical attention.