**Core Concept**
Kussmaul's sign is a clinical finding that describes an increase in jugular venous pressure (JVP) with inspiration, which is often indicative of right-sided heart failure or constrictive pericarditis.

**Why the Correct Answer is Right**
This phenomenon occurs due to the impaired filling of the right ventricle during inspiration, leading to a decrease in right ventricular volume. As a result, the increase in negative intrathoracic pressure during inspiration causes a greater increase in JVP, which can be palpable and is known as Kussmaul's sign. This sign is often associated with conditions that restrict the diastolic filling of the right ventricle, such as constrictive pericarditis or right ventricular failure.

**Why Each Wrong Option is Incorrect**
**Option A:** Kussmaul's sign is not typically associated with left-sided heart failure, as the left ventricle is not directly involved in the inspiratory increase in JVP.
**Option B:** While high-output states can cause an increase in JVP, they do not characteristically produce Kussmaul's sign.
**Option C:** Pneumonia can cause jugular venous distension, but it is not a classic cause of Kussmaul's sign.

**Clinical Pearl / High-Yield Fact**
Kussmaul's sign is a valuable clinical finding that can help differentiate between cardiac and non-cardiac causes of jugular venous distension, and should be remembered as a key feature of constrictive pericarditis.

**Correct Answer:** C. Constrictive pericarditis