**Core Concept:** Einthoven's triangle is a geometric representation of the spatial orientation of the three standard leads (I, II, III) of an electrocardiogram (ECG). Each lead is positioned at a specific angle to the sternum and chest wall, reflecting the electrical activity of the heart.

**Why the Correct Answer is Right:** In Einthoven's triangle, lead III is positioned at a 60-degree angle to lead I, which is placed along the midclavicular line between the 5th and 6th ribs. Lead II is positioned at a 60-degree angle to lead I, between the midclavicular and costal lines.

When lead I is 2 mV and lead II is 1 mV, we can calculate lead III as follows:

Lead III = (Lead I x cos(60°)) + (Lead II x cos(60°))
Lead III = (2 mV x cos(60°)) + (1 mV x cos(60°))
Lead III = (2/2) mV + (1/2) mV
Lead III = 3/2 mV

Lead III is effectively 3/2 mV or 1.5 mV.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because it states that lead III is 1 mV, which is not correct based on the calculation provided above.
B. This option is incorrect because it states that lead III is 2 mV, which is not correct based on the calculation provided above.
C. This option is incorrect because it states that lead III is 3 mV, which is incorrect based on the calculation provided above.
D. This option is incorrect because it states that lead III is 4 mV, which is not relevant to the calculation provided above.

**Clinical Pearls:**

1. Understanding Einthoven's triangle and the spatial orientation of ECG leads is essential for accurately interpreting ECG readings.
2. A normal ECG leads I, II, and III should have a similar amplitude (voltage) range for a normal sinus rhythm.
3. The ECG leads are a valuable tool for diagnosing cardiac arrhythmias, myocardial infarction, and other cardiac disorders.
4. Correctly interpreting ECG leads helps diagnose cardiac dysrhythmias, which are essential for accurate clinical decision-making in cardiac care.