**Core Concept**
Einthoven's law describes the relationship between the amplitudes of the electrocardiogram (ECG) leads and the distances between the recording electrodes. This law is based on the principle that the amplitude of an ECG lead is inversely proportional to the square of the distance between the recording electrode and the heart. It is a fundamental concept in electrocardiography.

**Why the Correct Answer is Right**
The correct answer is related to the concept of lead placement and the resultant amplitudes. According to Einthoven's law, the amplitude of an ECG lead is inversely proportional to the square of the distance between the recording electrode and the heart. This means that as the distance between the recording electrode and the heart increases, the amplitude of the ECG lead decreases. Conversely, as the distance decreases, the amplitude increases. This law is essential for understanding the principles of lead placement and the interpretation of ECG tracings.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe Einthoven's law. Einthoven's law specifically states that the amplitude of an ECG lead is inversely proportional to the square of the distance between the recording electrode and the heart, not directly proportional.
**Option B:** This option is incorrect because it does not take into account the inverse square law. The correct answer should reflect the inverse relationship between distance and amplitude, not a direct relationship.
**Option C:** This option is incorrect because it does not accurately describe the relationship between lead placement and ECG amplitude. Einthoven's law specifically addresses the inverse square law, not the direct relationship implied by this option.

**Clinical Pearl / High-Yield Fact**
A key application of Einthoven's law is in the interpretation of ECG tracings, particularly in the context of lead placement and the resultant amplitudes. Understanding this law is essential for accurate ECG interpretation and is a fundamental concept in electrocardiography.

**Correct Answer:** D. The law states that the amplitude of an ECG lead is inversely proportional to the square of the distance between the recording electrode and the heart.