A perpetual inventory means the pharmacy:

A perpetual inventory system is designed to maintain an ongoing, real-time record of inventory levels for medications, particularly controlled substances. This means that every time a medication is dispensed or a new stock is received, the inventory count is updated immediately, allowing the pharmacy to have an accurate understanding of how much of each medication is currently available.

This system is crucial for controlled substances because it helps ensure compliance with legal regulations surrounding their storage and dispensing. It enables pharmacies to monitor usage patterns and keep track of any discrepancies that might indicate theft or losses, thereby enhancing overall security and accountability.

Other approaches like recording all prescriptions filled, conducting monthly inventory counts, or estimating inventory using a standard formula apply different methodologies and do not provide the continuous accuracy that a perpetual inventory system offers. Therefore, the option highlighting the ongoing, exact count of a controlled substance medication aligns perfectly with the definition and purpose of a perpetual inventory system.