What does the term "undruggable' primarily refer to in the context of medicinal chemistry? A Targets that lack suitable binding sites for drug development. B Drugs that are ineffective against specific diseases. C Drugs that have multiple targets. D Highly selective drug targets.

The term "undruggable" in the context of medicinal chemistry primarily refers to targets that lack suitable binding sites for drug development. This means that these targets are difficult or impossible to drug, making it challenging to develop effective drugs against them. Therefore, the correct answer is A.