**Core Concept**
In electrocardiography (ECG), the P wave represents the depolarization of the atria. The orientation of the P wave in different ECG leads can vary due to the unique perspective and orientation of each lead. The normal P wave is typically upright in most leads but can be inverted in specific leads.

**Why the Correct Answer is Right**
The P wave is inverted in lead aVR because of the unique orientation of this lead. Lead aVR is placed on the right arm and is directed away from the heart. As a result, it views the atrial depolarization from a direction that is opposite to the other leads. In lead aVR, the P wave is inverted because the electrical forces of atrial depolarization are directed away from the electrode, resulting in a negative deflection.

**Why Each Wrong Option is Incorrect**
**Option A:** Lead I is a bipolar lead that views the heart from the front and is not typically associated with an inverted P wave.
**Option B:** Lead II is a standard lead that views the heart from the left leg and is usually associated with an upright P wave.
**Option C:** Lead aVF is a standard lead that views the heart from the left leg and is usually associated with an upright P wave.

**Clinical Pearl / High-Yield Fact**
Remember that the P wave orientation can provide valuable information about atrial enlargement or abnormal depolarization patterns. An inverted P wave in lead aVR can be a sign of left atrial enlargement or other conditions affecting atrial depolarization.

**✓ Correct Answer: D. aVR**