**Core Concept**
Diabetes mellitus is a metabolic disorder characterized by chronic hyperglycemia due to impaired insulin secretion, insulin resistance, or both. The complications of diabetes mellitus can be broadly classified into microvascular and macrovascular complications. The sixth complication of diabetes mellitus is related to the gastrointestinal system.

**Why the Correct Answer is Right**
The sixth complication of diabetes mellitus is gastroparesis. Gastroparesis is a form of delayed gastric emptying that occurs due to damage to the vagus nerve, which controls gastric motility. The damage to the vagus nerve is often caused by prolonged hyperglycemia, which can lead to inflammation and neuropathy. The resulting gastroparesis can cause symptoms such as nausea, vomiting, abdominal bloating, and weight loss.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not correspond to a recognized complication of diabetes mellitus.
**Option B:** This option is incorrect because it is a type of retinopathy, which is a microvascular complication of diabetes mellitus, not a gastrointestinal complication.
**Option C:** This option is incorrect because it is a type of nephropathy, which is a microvascular complication of diabetes mellitus, not a gastrointestinal complication.

**Clinical Pearl / High-Yield Fact**
Gastroparesis is a common complication of diabetes mellitus, especially in patients with long-standing disease. It is essential to recognize the symptoms of gastroparesis and manage it promptly to prevent malnutrition and dehydration.

**Correct Answer: C. Diabetic gastroparesis**