Which of the following substances may increase TIBC levels?

Total Iron Binding Capacity (TIBC) is a clinical measure that reflects the blood's capacity to bind iron with transferrin, an important protein in iron metabolism. TIBC levels can change due to various physiological and pathological conditions, and certain substances can indeed influence these levels.

Fluoride has been noted to impact TIBC levels because it can alter the metabolism of various minerals in the body, including iron. That modification in iron metabolism can lead to an increase in TIBC as the body responds to perceived iron deficiency by increasing transferrin synthesis, aimed at transporting more iron in the bloodstream.

Corticotropic Hormone (ACTH) and Chloramphenicol do not have a well-documented direct effect on TIBC levels. While ACTH primarily influences cortisol levels and can affect many other hormonal cascades, it does not typically increase TIBC. Similarly, Chloramphenicol is an antibiotic that does not inherently influence TIBC or transferrin synthesis in any significant way.

Thus, while considering the role of fluoride in increasing TIBC levels through its effect on iron metabolism, it is appropriate to identify it as the substance that may lead to such an increase.