Which device is commonly used to measure a patient's blood oxygen level?

The pulse oximeter is the device commonly used to measure a patient's blood oxygen level. It works by utilizing light to assess how much oxygen is being carried by the red blood cells in the bloodstream. This non-invasive device is typically placed on a patient's fingertip or earlobe, where it emits light wavelengths that can differentiate between oxygenated and deoxygenated blood. The result is quickly displayed as a percentage, indicating the saturation level of oxygen in the blood.

In contrast, the other devices serve different purposes. A sphygmomanometer is used to measure blood pressure, providing readings that reflect the force of blood against artery walls. A stethoscope is primarily utilized for listening to internal sounds of the body, such as heartbeats and breathing, but does not measure oxygen levels. Lastly, a spirometer measures lung capacity and function, assessing how well air is inhaled and exhaled, but again, it does not provide information on blood oxygen saturation. This highlights the specialized function of the pulse oximeter in directly measuring oxygen levels, making it the correct choice for this question.