Which of the following signifies "difficult" or "painful"?

The prefix "Dys-" signifies "difficult" or "painful." This prefix is commonly used in medical terminology to describe conditions that involve dysfunction or abnormalities, which can lead to difficulty or discomfort. For example, "dysfunction" refers to impaired or abnormal functioning of an organ or system, while "dysphagia" denotes difficulty swallowing.

The other prefixes do not carry the same meaning. "Endo-" refers to something that is inside or within, commonly used in terms like "endocardial," describing something related to the inner layer of the heart. "Brady-" indicates slowness, as in "bradycardia," which refers to a slower than normal heart rate. "Ante-" means before or in front of, used in terms like "antecedent," which refers to something that precedes another in time or order. Thus, the use of "Dys-" specifically correlates with difficulty or pain, making it the correct choice.