**Core Concept**
Delta waves in ECG represent a high-amplitude, slow-frequency wave pattern typically seen in infants and young children. This phenomenon is attributed to the immaturity of the sinoatrial node and the presence of an accessory electrical conduction pathway, known as the Wolff-Parkinson-White (WPW) syndrome.

**Why the Correct Answer is Right**
Delta waves are a result of the simultaneous activation of the atria and ventricles through the accessory conduction pathway, bypassing the normal atrioventricular (AV) node conduction. This leads to a shorter PR interval and an increased amplitude of the QRS complex. In infants, delta waves are often seen due to the incomplete development of the AV node, allowing for an accessory pathway to conduct electricity directly from the atria to the ventricles.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because delta waves are not characteristic of atrial fibrillation, which presents with a rapid, irregularly irregular ventricular response and a lack of distinct P waves.
**Option B:** This choice is incorrect because ventricular tachycardia typically presents with a narrow QRS complex and a rapid ventricular rate, not delta waves.
**Option C:** This choice is incorrect because Wolff-Parkinson-White syndrome, which can present with delta waves, is a separate entity from the normal physiology of infancy, although it can be present in infants.

**Clinical Pearl / High-Yield Fact**
Delta waves can be a normal finding in infants, but in older children and adults, they are often indicative of an underlying cardiac pathology, such as WPW syndrome. It is essential to differentiate between delta waves in infants and those seen in adults to avoid misdiagnosis and inappropriate treatment.

**Correct Answer: D.**