**Core Concept**
The **PR interval** is a measure of the time from the onset of the **P wave** to the start of the **QRS complex** on an electrocardiogram (ECG), reflecting the time the electrical impulse takes to travel from the **atria** to the **ventricles**.

**Why the Correct Answer is Right**
The correct answer corresponds to the period from the beginning of the **P wave** to the start of the **QRS complex**, which is the definition of the **PR interval**. This interval encompasses the time for the electrical impulse to move through the **atrioventricular (AV) node** and down to the **ventricles**.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not accurately represent the definition of the **PR interval**.
**Option B:** Incorrect as it does not correspond to the standard ECG measurement for the **PR interval**.
**Option C:** Incorrect because it represents a different ECG interval.
**Option D:** Incorrect as it is not related to the **PR interval** measurement.

**Clinical Pearl / High-Yield Fact**
The **PR interval** is crucial for assessing **AV node** function and can be abnormal in conditions such as **first-degree AV block**, where the **PR interval** is prolonged.

**Correct Answer:** D. The period from the beginning of the P wave to the start of the QRS complex.