Which condition refers to weakness that affects only one leg?

Monoparesis refers to weakness affecting a single limb, which can be a leg or an arm. This condition is characterized by a partial loss of strength or motor power in that specific limb, distinguishing it from other types of paresis that affect multiple limbs or regions of the body. In the context of the question, since it specifies weakness that affects only one leg, monoparesis is the most appropriate term to describe this condition.

Paraperesis typically denotes weakness in both legs, which does not apply to a situation where only one leg is affected. Quadriplegia involves paralysis of all four limbs and is clearly unrelated to the specific weakness of a single leg. Diplegia generally refers to weakness in similar parts on both sides of the body, such as both legs, which again does not fit the description provided in the question. Therefore, given the need to identify weakness restricted to one leg, monoparesis is the correct and most fitting choice.