**Core Concept**
Pulse deficit refers to the difference between the apical pulse rate and the radial pulse rate, often observed in conditions where there is an abnormal heart rhythm or increased cardiac contractility. This phenomenon occurs due to the asynchronous contraction of the heart's ventricles, leading to a discrepancy between the apical and radial pulse rates.

**Why the Correct Answer is Right**
Pulse deficit is classically seen in atrial fibrillation, where the irregular and rapid atrial contractions lead to asynchronous ventricular depolarizations. This results in an irregularly irregular pulse, often with a widened pulse pressure. In atrial fibrillation, the apical pulse rate may be significantly different from the radial pulse rate, causing a noticeable pulse deficit. Additionally, conditions like bundle branch block, ventricular tachycardia, and heart block can also cause pulse deficit due to abnormal ventricular conduction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because pulse deficit is indeed seen in atrial fibrillation, making it the correct answer. Atrial fibrillation is characterized by irregular and rapid atrial contractions, leading to asynchronous ventricular depolarizations and a widened pulse pressure.

**Option B:** This option is incorrect because pulse deficit can be seen in conditions like bundle branch block, where there is abnormal ventricular conduction. Bundle branch block can cause a delay in one of the ventricular conduction pathways, leading to a discrepancy between the apical and radial pulse rates.

**Option C:** This option is incorrect because pulse deficit is not typically seen in sinus tachycardia. Sinus tachycardia is characterized by a regular and rapid heart rate, with a narrow pulse pressure. The apical and radial pulse rates are usually in synchrony in sinus tachycardia.

**Option D:** This option is incorrect because pulse deficit can be seen in ventricular tachycardia, where there is abnormal ventricular conduction and asynchronous ventricular depolarizations. Ventricular tachycardia can cause a widened pulse pressure and a discrepancy between the apical and radial pulse rates.

**Clinical Pearl / High-Yield Fact**
A pulse deficit is often a sign of abnormal heart rhythm or increased cardiac contractility. Remember to check for pulse deficit in patients with irregular heart rhythms or widened pulse pressures.

**Correct Answer: C. Sinus tachycardia.**