**Core Concept**
Atrial fibrillation is a type of supraventricular tachyarrhythmia characterized by rapid and irregular atrial contractions, leading to a loss of coordinated atrial contractions and a decrease in cardiac output. The underlying pathophysiology involves abnormal electrical conduction through the atria, which disrupts the normal sinus rhythm.

**Why the Correct Answer is Right**
During atrial fibrillation, the atria contract in a rapid and irregular manner due to the presence of multiple ectopic foci. This leads to a decrease in the effective atrial contraction and an increase in the risk of thromboembolic events. The irregular rhythm is due to the chaotic electrical activity in the atria, which prevents the normal coordinated contraction of the atrial muscle.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because atrial fibrillation is not typically associated with a decrease in heart rate, as the ventricles continue to contract at a rapid rate despite the irregular atrial contractions.
* **Option B:** This option is incorrect because atrial fibrillation is not typically associated with a decrease in cardiac output, as the ventricles continue to contract and pump blood despite the irregular atrial contractions.
* **Option C:** This option is incorrect because atrial fibrillation is not typically associated with a decrease in the risk of thromboembolic events, as the irregular rhythm and decreased effective atrial contraction increase the risk of clot formation and embolization.

**Clinical Pearl / High-Yield Fact**
Atrial fibrillation is associated with an increased risk of thromboembolic events, particularly stroke, and patients should be anticoagulated to prevent these events.

**Correct Answer: D. Decreased effective atrial contraction and increased risk of thromboembolic events.**