**Core Concept**
The patient's triple vessel disease and longstanding diabetes make him a high-risk candidate for general anesthesia due to potential cardiovascular instability and impaired glucose regulation. Regional anesthesia is often preferred in such cases to minimize complications.

**Why the Correct Answer is Right**
The best mode of anesthesia for this patient would be spinal anesthesia, which provides effective analgesia for lower limb procedures while minimizing the risks associated with general anesthesia. Spinal anesthesia involves injecting a local anesthetic into the cerebrospinal fluid, which numbs the lower limb and provides pain relief. This approach allows for better hemodynamic stability and reduced risk of respiratory complications.

**Why Each Wrong Option is Incorrect**
**Option A:** General anesthesia is generally contraindicated in this patient due to his high risk of cardiovascular instability and impaired glucose regulation.
**Option B:** Epidural anesthesia may be an option, but it requires more time to establish and may not provide as rapid onset as spinal anesthesia.
**Option C:** Peripheral nerve blocks may not be sufficient to provide adequate pain relief for a major procedure like gangrene debridement.

**Clinical Pearl / High-Yield Fact**
In high-risk patients, regional anesthesia is often preferred over general anesthesia to minimize complications. This is because regional anesthesia tends to have fewer cardiovascular and respiratory side effects.

**Correct Answer:** C.