Which of the following substances must be included in child-resistant packaging according to OTC regulations?

The correct choice is based on the requirements established by the Poison Prevention Packaging Act (PPPA). According to OTC regulations, certain substances are classified as requiring child-resistant packaging due to their potential risk if ingested by children. Lidocaine, being a topical anesthetic, is considered a drug that could pose a significant risk to children if consumed orally, which is why it must be packaged in child-resistant containers.

In contrast, the other substances listed have different risk profiles. Amoxicillin, while a prescription antibiotic, does not fall under the OTC regulations for child-resistant packaging specifically because it is generally not an over-the-counter product. Calamine lotion is a topical product used primarily for soothing skin irritations and is typically not classified as requiring child-resistant packaging. Vitamin D, while an important dietary supplement, does not meet the criteria for mandatory child-resistant packaging as it is often available in forms and doses that are considered less hazardous.

Thus, the importance of child-resistant packaging lies in its role in preventing accidental poisonings in children, making Lidocaine's inclusion essential in this context.