**Core Concept**
Duodenal blowout, also known as duodenal stump blowout, is a serious complication following gastrectomy, particularly Billroth procedures. It occurs due to leakage from the duodenal stump, which is the area where the duodenum is resected and closed during surgery. Anatomy plays a crucial role in understanding this complication, as the duodenal stump's blood supply, suturing technique, and closure method can all impact its integrity.

**Why the Correct Answer is Right**
Duodenal blowout most commonly occurs on the 4th day post-operatively due to the delayed onset of ischemia in the duodenal stump. This is because the blood supply to the duodenum is initially maintained through the rich vascular network surrounding the area, but as the edema resolves and the inflammatory response subsides, the ischemia becomes apparent, leading to stump leakage. The 4th day is a critical period when the balance between blood supply and demand is disrupted, making it the most susceptible time for duodenal blowout.

**Why Each Wrong Option is Incorrect**
**Option A:** The 2nd day is too early for duodenal blowout to occur, as the initial post-operative period is characterized by edema and inflammation, which can mask the underlying ischemia.
**Option C:** The 6th day is too late, as the ischemia has usually become apparent by this time, and the stump leakage would have been identified earlier.
**Option D:** The 12th day is highly unlikely, as the risk of duodenal blowout decreases significantly after the 4th day, and the stump would have been well-healed by this time.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the timing of duodenal blowout can vary depending on the individual patient's anatomy, surgical technique, and post-operative care. However, in general, the 4th day is considered the most critical period for monitoring and preventing this complication.

**✓ Correct Answer: B. 4th day**