**Question:** A 40 year old male presented with sudden onset of severe chest pain radiating to the left upper arm. ECG was suggestive of myocardial infarction. Investigations revealed poor blood supply to the anterior 2/3rd of the interventricular septum. Which of the following blood vessels is most likely to be blocked

A. Left anterior descending artery (LAD)
B. Left circumflex artery (LCx)
C. Right coronary artery (RCA)
D. Posterior descending artery (PDA)

**Correct Answer:** **D. Posterior descending artery (PDA)**

**Core Concept:** Coronary artery occlusion and myocardial infarction (MI)

**Why the Correct Answer is Right:**

The patient is suffering from a myocardial infarction (heart attack) due to poor blood supply to the anterior 2/3rd of the interventricular septum. In a typical coronary circulation, the anterior wall of the heart receives blood supply from multiple arteries. However, the posterior wall gets its blood supply from a single artery, the posterior descending artery (PDA). When the PDA is occluded, it results in a specific pattern of myocardial infarction involving the posterior wall of the heart, which matches the described clinical scenario.

**Why Each Wrong Option is Incorrect:**

A. **Left anterior descending artery (LAD):** Although LAD supplies the anterior wall, occlusion of LAD would lead to a more global myocardial infarction affecting the entire anterior wall, not just the 2/3rd portion.

B. **Left circumflex artery (LCx):** Similar to LAD, LCx supplies the entire left side of the heart and occlusion would result in a more widespread myocardial infarction.

C. **Right coronary artery (RCA):** RCA supplies the posterior wall, but occlusion of RCA would result in a myocardial infarction affecting the entire posterior wall, not just the 2/3rd portion.

D. **Posterior descending artery (PDA):** As mentioned earlier, occlusion of PDA results in a specific pattern of myocardial infarction involving the posterior wall, which aligns with the described clinical scenario.

**Clinical Pearl:**

The occlusion of the PDA is a rare but important cause of myocardial infarction, particularly in patients with a history of chest pain in the left arm and posterior wall of the heart, as described in the question. This clinical scenario is called left main stem occlusion or right coronary artery occlusion with distal embolization, which can occur due to various reasons like embolism, atherosclerosis, or spasm. Early recognition and treatment of such cases can potentially save lives and prevent severe complications.