**Question:** Thromboangitis obliterans involves which vessels -

A. Arteries
B. Veins
C. Capillaries
D. Venules

**Core Concept:**
Thromboangitis obliterans, also known as Buerger's disease, is a vasculitis affecting small and medium-sized arteries and veins in the limbs, typically in young, tobacco-using individuals. It is a rare, idiopathic (unknown cause) disease characterized by inflammation, thrombosis (formation of blood clots), and occlusion (closure) of small and medium-sized vessels.

**Why the Correct Answer is Right:**
Thromboangitis obliterans primarily affects small and medium-sized arteries and veins, making option A (arteries) and option D (venules) correct answers. In this disease, there is a combination of inflammation, thrombosis, and occlusion involving arteries and veins, leading to the characteristic symptoms and complications associated with the disease.

**Why Each Wrong Option is Incorrect:**
Option B (veins) and option C (capillaries) are incorrect as they represent larger vessels that are less commonly affected by thromboangitis obliterans. Capillaries are even smaller vessels than veins and are not involved in this disease process.

**Clinical Pearl:**
Thromboangitis obliterans primarily affects tobacco users, particularly those who use Indian tobacco (such as bidi or khaini) due to the presence of alkaloids. It is a medical emergency since it can lead to critical limb ischemia, gangrene, and amputation if not promptly diagnosed and treated with anticoagulation and antiplatelet therapy.

**Correct Answer: A and D**