**Core Concept**
The Swan-Ganz catheter is a type of pulmonary artery catheter used for hemodynamic monitoring in critically ill patients. It allows for the measurement of various parameters related to cardiac function and blood flow.

**Why the Correct Answer is Right**
The Swan-Ganz catheter is inserted through a large vein and guided into the pulmonary artery, where it can measure pressures in the right atrium, right ventricle, pulmonary artery, and left atrium. It also has a balloon that can be inflated to occlude the pulmonary artery, allowing for the measurement of cardiac output using the thermodilution technique. This involves injecting a cold saline solution into the pulmonary artery and measuring the temperature change in the blood downstream, which is used to calculate cardiac output.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the Swan-Ganz catheter is not primarily used for measuring blood gas levels.
**Option B:** Incorrect because while the Swan-Ganz catheter can provide some information about cardiac output, it is not its primary function.
**Option C:** Incorrect because the Swan-Ganz catheter is not used for measuring urine output.

**Clinical Pearl / High-Yield Fact**
The Swan-Ganz catheter is a high-risk procedure that should only be performed by experienced operators, and its use should be limited to critically ill patients who require close hemodynamic monitoring.

**Correct Answer:** C. Cardiac output.