**Core Concept**
Thromboangitis obliterans, also known as Buerger's disease, is a non-atherosclerotic, segmental inflammatory disorder that affects the small and medium-sized arteries and veins of the limbs. It is characterized by inflammation, thrombosis, and progressive occlusion of blood vessels, leading to ischemia and tissue damage.

**Why the Correct Answer is Right**
The commonest site of thromboangitis obliterans is the distal portions of the lower limbs, particularly the small and medium-sized arteries and veins of the feet. This is due to the high metabolic rate and temperature of the feet, which may contribute to the development of the disease. The inflammation and thrombosis in Buerger's disease are thought to be triggered by nicotine and other vasoactive substances, which can cause vasospasm and increased blood viscosity.

**Why Each Wrong Option is Incorrect**
**Option A:** Upper limbs are less commonly affected by thromboangitis obliterans, making this option incorrect.
**Option B:** Although the disease can affect the hands, it is more commonly associated with the lower limbs, making this option incorrect.
**Option C:** The disease is not typically associated with the gastrointestinal tract or other non-limb regions, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
Buerger's disease is strongly associated with smoking and nicotine use, and smoking cessation is a critical component of treatment. Patients with Buerger's disease are at high risk of developing gangrene and amputation if left untreated.

**Correct Answer:** C.