What prefix indicates a decrease or low condition?

The prefix that indicates a decrease or low condition is "hypo." This prefix is commonly used in medical terminology to denote a lower-than-normal level of a substance or function. For example, "hypoglycemia" refers to abnormally low levels of glucose in the blood.

In contrast, the other prefixes carry different meanings: "hyper" signifies an above-normal level or an increase, "brady" refers specifically to a slower than normal heart rate, and "tachy" indicates a rapid heart rate. Each of these prefixes is important in conveying specific medical information, but "hypo" distinctly indicates a decrease or deficiency, making it the correct choice for this question.