**Question:** All of the following statements about WPW syndrome are true, Except:

A. **A.** WPW syndrome is caused by the presence of an accessory pathway that allows for antegrade or retrograde conduction during atrial fibrillation.
B. **B.** The accessory pathway causes a decrease in the QRS width during sinus rhythm.
C. **C.** The accessory pathway is found in the atrioventricular (AV) node.
D. **D.** The accessory pathway is permanent in WPW syndrome.

**Correct Answer:** D. **D.** The accessory pathway is permanent in WPW syndrome.

**Core Concept:** WPW syndrome, or Wolff-Parkinson-White syndrome, is a congenital disorder characterized by the presence of an accessory pathway between the atria and ventricles. This accessory pathway allows for antegrade or retrograde conduction during atrial fibrillation, leading to a widened QRS complex and pre-excitation syndrome.

**Why the Correct Answer is Right:** The correct answer, D, highlights an important distinction between WPW syndrome and other conditions with pre-excitation syndromes, such as atrioventricular (AV) node re-entry tachycardia. In WPW syndrome, the accessory pathway is not necessarily permanent, but rather variable, depending on the presence of atrial fibrillation or other triggers that facilitate conduction through the accessory pathway. In contrast, in AV node re-entry tachycardia, the accessory pathway is constitutively present and causes pre-excitation in all heart rates.

**Why Each Wrong Option is Incorrect:**

A. **A.** The statement is incorrect because WPW syndrome is not solely caused by the presence of an accessory pathway during atrial fibrillation. The condition has a genetic basis and is present at birth.

B. **B.** This statement is incorrect because the QRS complex narrows during sinus rhythm due to the presence of a functional block in the accessory pathway, not because of a decrease in the QRS width.

C. **C.** The accessory pathway in WPW syndrome is not located in the AV node but in the atrioventricular (AV) septum or the right atrium-right ventricle junction. The AV node is involved in AV nodal re-entry tachycardia.

**Why the Correct Answer is Right:**

D. **D.** The accessory pathway in WPW syndrome is not always present but rather can be triggered by various factors. This is an essential distinction from conditions like AV node re-entry tachycardia where the accessory pathway is constitutive.

**Clinical Pearls:** Understanding the distinction between WPW syndrome and AV node re-entry tachycardia is crucial for accurate diagnosis and appropriate management strategies. WPW syndrome is often diagnosed during exercise stress tests, tilt table tests, or triggered by medications like beta-blockers or digitalis. The treatment involves medications like adenosine, beta-blockers, or cardioversion, depending on the specific clinical presentation and the type of arrhythmia.