**Core Concept**
Ventricular tachycardia (VT) and supraventricular tachycardia (SVT) are two distinct types of arrhythmias with different underlying mechanisms. VT originates from the ventricles, often due to re-entry or abnormal automaticity, whereas SVT originates above the ventricles, typically from the atria or AV node.

**Why the Correct Answer is Right**
The correct answer is the feature that does not reliably differentiate between VT and SVT. VT is often characterized by a narrow QRS complex in the presence of a wide QRS complex, as the abnormal electrical activity originates from the ventricles. The presence of a wide QRS complex (>120 ms) is a key feature that suggests VT, as it indicates that the electrical impulse is being generated within the ventricles rather than above them. Other features that can differentiate between VT and SVT include the presence of a P wave, the direction of the initial QRS vector, and the presence of a capture beat or fusion beat.

**Why Each Wrong Option is Incorrect**
**Option A:** Presence of a P wave. This feature can actually differentiate between VT and SVT, as a P wave is typically absent in VT and present in SVT.
**Option B:** Direction of the initial QRS vector. The direction of the initial QRS vector can be helpful in differentiating between VT and SVT, as VT often has a leftward or downward directed initial QRS vector.
**Option C:** Presence of a capture beat or fusion beat. The presence of a capture beat or fusion beat can be a key feature that suggests VT, as it indicates that the normal sinus rhythm is being interrupted by the abnormal ventricular rhythm.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the presence of a wide QRS complex (>120 ms) is a key feature that suggests VT, as it indicates that the electrical impulse is being generated within the ventricles rather than above them.

**Correct Answer:** D