The most appropriate next action is to notify senior management immediately because an incident involving exposed personally identifiable information requires rapid organizational escalation for governance, legal, operational, and communications decisions. Once the security officer initiates the investigation, executive leadership must be engaged right away to activate the incident response structure, allocate resources, approve containment actions that may affect clinical operations (e.g., taking systems offline), and ensure required stakeholders are involved (legal counsel, privacy officer, compliance, risk management, public relations, and clinical leadership). Early senior leadership notification supports timely decision-making and preserves evidence, while ensuring consistent internal and external messaging.

Waiting until the investigation is completed (option B) risks delays in containment, reporting decisions, and organizational coordination. Options C and D focus on notifying affected individuals within a specific timeframe; however, individual notification requirements vary by jurisdiction and circumstance, and generally depend on confirming the scope, impacted individuals, and whether the incident meets the definition of a reportable breach. Those steps come after leadership is engaged and the response process is coordinated. Therefore, immediate senior management notification is the best next step to manage risk, compliance, and patient trust effectively.