**Core Concept**
The question requires the identification of a specific type of cardiac arrhythmia based on an ECG tracing. A 2:1 block is a type of heart block where every second P wave is blocked, resulting in a ventricular rate that is half the atrial rate.

**Why the Correct Answer is Right**
In a 2:1 block, every other P wave is blocked at the AV node, resulting in a ventricular rate that is half the atrial rate. This is often seen in conditions that cause AV nodal dysfunction, such as digitalis toxicity, ischemia, or degenerative diseases. The ECG tracing would show a regular rhythm at a rate that is half the normal atrial rate, with every other P wave being blocked.

**Why Each Wrong Option is Incorrect**
**Option A:** PSVT (Paroxysmal Supraventricular Tachycardia) is a type of arrhythmia characterized by a rapid heart rate, typically above 100 beats per minute, due to re-entry circuits in the atria or AV node. However, it does not typically result in a 2:1 block.

**Option B:** Atrial flutter is a type of arrhythmia characterized by a rapid and regular atrial rate, typically between 300-400 beats per minute. While it can be associated with a block, the block is not typically 2:1.

**Option C:** Ventricular bigeminy is a type of arrhythmia characterized by an alternating pattern of normal and premature ventricular contractions. It is not typically associated with a 2:1 block.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize the difference between a 2:1 block and other types of heart blocks, such as Wenckebach block or complete heart block, as each has distinct clinical implications and treatments.

**Correct Answer:**
✓ Correct Answer: D. 2 :1 block