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A narrow QRS complex on a cardiac monitor typically indicates that the rhythm involves a conduction pathway through the atria, which is generally characteristic of supraventricular tachycardia (SVT). In SVT, the heart rate is elevated, often exceeding 100 beats per minute, while the narrow complex signifies that the electrical impulses are being conducted normally through the His-Purkinje system without significant delay.

In contrast, bradycardia is defined by a slower than normal heart rate and often features a normal or wide QRS complex depending on the underlying cause. Atrial fibrillation, while it may present with a narrow QRS complex, typically is irregularly irregular and involves chaotic atrial activity rather than a consistent rapid rhythm. Ventricular tachycardia, on the other hand, features a wide QRS complex (greater than 0.12 seconds) and arises from an abnormal focus in the ventricles, which is fundamentally different from the narrow complex seen in supraventricular tachycardia.

Thus, identifying the rhythm as narrow leads to the conclusion of supraventricular tachycardia, which is the correct choice in this scenario.