According to Texas Racing Commission rules, which of the following substances is prohibited in a horse's body?

Prohibited drugs are substances that are not allowed in a horse's body during racing or competition scenarios as governed by the Texas Racing Commission's regulations. These substances can enhance performance, mask pain, or have other effects that compromise the integrity of the race. The rules set forth by the Commission are intended to ensure fair play and the well-being of the horses involved in racing.

Furosemide, also known as Lasix, is commonly used in racing to prevent exercise-induced pulmonary hemorrhage (EIPH) and is permitted under specific circumstances. Electrolytes and vitamins are also generally allowed to support the horse's health and hydration, provided they are not administered in excess or combined with prohibited substances. Thus, the option that correctly identifies substances that are not permitted in the horse's system is the category of prohibited drugs, reflecting the regulatory framework designed to maintain the integrity of horse racing.