**Core Concept**
Microvascular disease and neuropathy are the primary mechanisms behind diabetic gangrene. Diabetic neuropathy leads to loss of sensation in the affected limb, making it difficult for the patient to recognize and respond to injuries, infections, or other insults. Microvascular disease, on the other hand, results from damage to the small blood vessels (capillaries) that supply the limb, leading to reduced blood flow and impaired wound healing.

**Why the Correct Answer is Right**
The combination of microvascular disease and neuropathy in diabetes mellitus leads to a heightened susceptibility to gangrene. Microvascular disease is characterized by thickening of the basement membrane, endothelial dysfunction, and reduced capillary density, resulting in inadequate blood supply to the tissues. Neuropathy, specifically sensory neuropathy, impairs the patient's ability to detect and respond to pain, temperature, and pressure changes, making it more likely for injuries to go unnoticed and progress to gangrene. The autonomic neuropathy component further exacerbates the problem by causing vasospasm and reducing blood flow to the affected limb.

**Why Each Wrong Option is Incorrect**
**Option A:** While peripheral artery disease is a significant cause of gangrene in non-diabetic patients, it is not the primary mechanism behind diabetic gangrene.
**Option B:** Infection is a common complication of diabetic gangrene, but it is not the primary cause. In fact, infection often arises as a secondary consequence of the underlying microvascular disease and neuropathy.
**Option C:** While smoking is a significant risk factor for peripheral artery disease and gangrene, it is not the primary mechanism behind diabetic gangrene.

**Clinical Pearl / High-Yield Fact**
Diabetic patients with a history of neuropathy, foot ulcers, or previous episodes of gangrene are at high risk of developing further episodes. Regular foot exams and prompt treatment of injuries or infections are crucial in preventing the progression to gangrene.

**Correct Answer: D.**