**Core Concept**
Ventricular tachycardia (VT) and supraventricular tachycardia (SVT) with aberrancy are two common causes of broad-complex tachycardia. The key to differentiating between these two conditions lies in the presence of specific features that suggest VT.

**Why the Correct Answer is Right**
A very broad QRS complex (> 140 ms) is more characteristic of SVT with aberrancy rather than VT. This is because VT originates from the ventricles and has a more complex path of activation, resulting in a narrower QRS complex. In contrast, SVT with aberrancy has a faster conduction through the His-Purkinje system, leading to a broader QRS complex.

**Why Each Wrong Option is Incorrect**
**Option A:** Fusion beats are a feature of VT, where the ventricular contraction is influenced by both the VT and the normal sinus rhythm. This results in a beat that is intermediate in morphology between the two rhythms.
**Option B:** Extreme left axis deviation is more commonly seen in VT, as it often originates from the left ventricle. This feature is not specific to SVT with aberrancy.
**Option D:** Non-response to carotid sinus massage is a feature of VT, as it often does not respond to vagal maneuvers. This is because VT is often due to a re-entrant circuit that is not influenced by the vagus nerve.

**Clinical Pearl / High-Yield Fact**
In broad-complex tachycardia, the presence of a clear P wave or a change in QRS morphology with a change in rate suggests SVT with aberrancy. In contrast, a regular QRS complex with a rate greater than 150 beats per minute and a lack of P wave suggests VT.

✓ Correct Answer: C. Very broad QRS complexes (> 140 ms)