Final answer:
The answer to the question, which describes QRS complexes changing in polarity and amplitude with a ventricular rate ranging from 150-250/min, is ventricular tachycardia.
Step-by-step explanation:
Given the options: ventricular fibrillation, atrial fibrillation, ventricular tachycardia, and sinus tachycardia, and the description that QRS complexes change in polarity and amplitude with a ventricular rate ranging from 150-250/min and appear to be 'turning on a point,' the correct answer is ventricular tachycardia (option c). This condition is characterized by a series of rapid heartbeats that originate in the ventricles and lead to abnormal QRS complexes. It is distinct from atrial fibrillation which involves an irregular electrical pattern prior to the QRS complexes and an increased frequency between them. Ventricular fibrillation is marked by a complete lack of normal electrical activity and is immediately life-threatening. Sinus tachycardia, although involving a rapid heartbeat, does not typically change the shape or turning of the QRS complexes to the extent seen in ventricular tachycardia.