What crucial element should an IND application include according to FDA guidelines?

The correct answer emphasizes the importance of a structured formula for the drug in an Investigational New Drug (IND) application according to FDA guidelines. When submitting an IND, it is essential to provide specific information about the drug substance, which includes its chemical and structural formulation. This is critical for evaluating the drug’s safety and efficacy prior to clinical trials, enabling the FDA to assess both the manufacturing process and the identity of the drug.

A structured formula helps in understanding the molecular composition of the drug, which is integral for ensuring compliance with safety protocols and regulatory requirements. It allows for comprehensive review processes that ensure the drug's intended use is based on solid scientific ground.

In terms of other choices, while a marketing plan and a list of potential side effects are important in the broader context of drug development and commercialization, they are not a mandatory part of the IND application. Similarly, while statistical methods play a significant role in the analysis of clinical trial data, they do not pertain to the structural formulation of the drug itself and thus are not a crucial element of the IND submission.