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A heel prick is a method primarily used to collect capillary blood samples, particularly in infants or young children. This method involves puncturing the outer layer of the skin on the heel to obtain a small volume of blood from the capillaries, which are tiny blood vessels where oxygen and nutrients are exchanged with tissues.

Capillary samples are generally used for various tests, including metabolic screening and blood glucose monitoring, because they provide a quick and easy way to obtain blood without requiring more invasive procedures, such as venipuncture. This is particularly beneficial with infants, where venous access can be challenging and carry greater risk.

In contrast, venous samples are drawn from veins and are often used for more extensive laboratory testing requiring larger volumes of blood. Arterial samples, on the other hand, are collected from arteries and are specifically utilized for blood gas analysis, which provides critical information about respiratory function and gas exchange. A mixed sample would imply blood drawn from different sources, which is not the case with a heel prick. Thus, the method is specifically aligned with obtaining capillary samples, confirming that the correct answer is indeed that a heel prick yields a capillary sample.