If a wound to the neck is left uncovered, ________ may be sucked into the blood vessels causing an obstruction called an "air embolism."

In a wound to the neck, if left uncovered, the most critical concern is the potential for air to enter the bloodstream. This phenomenon occurs when atmospheric air is drawn into the blood vessels, particularly through a vein or a larger hole caused by the injury. The entry of air creates an obstruction in the circulatory system known as an air embolism.

An air embolism can lead to serious complications, including impaired blood flow to vital organs, stroke, and cardiovascular collapse. This risk is especially pronounced in neck wounds due to the proximity of large veins, such as the jugular veins, which can easily allow air to be sucked into the bloodstream when the pressure changes, for example, during inhalation.

The other choices—blood, water, and foreign matter—while they may also pose risks in different contexts, do not specifically relate to the condition of air embolism in the same way that air does. Blood would usually remain within the body unless externally applied, water isn't typically a factor in venous access in this context, and while foreign matter can cause infection or other complications, it does not create the same immediate life-threatening obstruction as air does.