Which of these means abnormal, painful, or difficult?

The term that signifies abnormal, painful, or difficult is "dys-." This prefix is frequently used in medical terminology to denote conditions or processes that are not normal or that present challenges. For example, "dyslexia" refers to difficulty with reading, and "dyspepsia" indicates painful or difficult digestion. This prefix helps to communicate various medical issues related to abnormal functioning, sensations, or challenges in the body, making it an essential element in descriptive medical language.

In contrast, "iso-" typically refers to equality or sameness, such as in "isotope." "Anti-" means against or opposed to something, like in "antibiotic." "Post-" denotes something occurring after something else, as seen in terms like "postoperative." Each of these prefixes serves its own distinct purpose in medical vocabulary but does not convey the idea of abnormality or difficulty that "dys-" does.