**Core Concept**
Pulse deficit is a measure of the difference between the apical pulse rate and the radial pulse rate. It is an indicator of atrioventricular (AV) block or other conditions affecting the conduction of impulses from the atria to the ventricles.

**Why the Correct Answer is Right**
A pulse deficit of more than 10 per minute is typically seen in third-degree AV block (also known as complete heart block). In this condition, the atria and ventricles beat independently, resulting in a significant discrepancy between the atrial and ventricular rates. The atria may beat at a rate of 60-100 beats per minute, while the ventricles beat at a slower rate, often in the range of 20-40 beats per minute. This leads to a pulse deficit, which can be calculated by subtracting the radial pulse rate from the apical pulse rate.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a pulse deficit of more than 10 per minute is not typically seen in first-degree AV block, which is characterized by a prolonged PR interval on the ECG but no significant difference between the atrial and ventricular rates.

**Option B:** This option is incorrect because a pulse deficit of more than 10 per minute is not typically seen in second-degree AV block, which is characterized by a periodic failure of atrial impulses to reach the ventricles, resulting in a dropped beat. While second-degree AV block can cause a pulse deficit, it is usually less than 10 per minute.

**Option C:** This option is incorrect because a pulse deficit of more than 10 per minute is not typically seen in sinus tachycardia, which is characterized by a rapid heart rate due to increased sympathetic tone. In sinus tachycardia, the atrial and ventricular rates are usually closely coupled, resulting in a minimal pulse deficit.

**Clinical Pearl / High-Yield Fact**
A pulse deficit of more than 10 per minute is a critical finding that suggests third-degree AV block, which requires immediate medical attention to prevent complications such as heart failure and cardiac arrest.

**Correct Answer: C. Third-degree AV block.**