The correct answer is A, "Date of last terminal clean of the infected patient rooms," as this is the most critical information the infection preventionist (IP) needs from the Environmental Services director to begin the investigation of a cluster of Aspergillus fumigatus infections in the transplant unit. According to the Certification Board of Infection Control and Epidemiology (CBIC) guidelines, Aspergillus fumigatus is an environmental fungus that thrives in areas with poor ventilation, construction dust, or inadequate cleaning, posing a significant risk to immunocompromised patients, such as those in transplant units. A terminal clean—thorough disinfection and cleaning of a patient room after discharge or transfer—is a key infection control measure to eliminate fungal spores and other pathogens (CBIC Practice Analysis, 2022, Domain III: Infection Prevention and Control, Competency 3.4 - Implement environmental cleaning and disinfection protocols). Determining the date of the last terminal clean helps the IP assess whether lapses in cleaning schedules or procedures could have contributed to the cluster, guiding further environmental sampling or process improvements.

Option B (hospital grade disinfectant used on the transplant unit) is relevant to the investigation but is secondary; the IP would need to know the cleaning schedule first to contextualize the disinfectant’s effectiveness. Option C (use of dust mitigating strategies during floor care) is important, as Aspergillus spores can be aerosolized during floor maintenance, but this is a specific procedural detail that follows the initial focus on cleaning history. Option D (date of the last cleaning of the fish tank in the waiting room) is unlikely to be a priority unless evidence suggests a direct link to the transplant unit, which is not indicated here; Aspergillus is more commonly associated with air quality and room cleaning rather than fish tanks.

The focus on the date of the last terminal clean aligns with CBIC’s emphasis on investigating environmental factors in healthcare-associated infection (HAI) clusters, enabling the IP to collaborate with Environmental Services to pinpoint potential sources and implement corrective actions (CBIC Practice Analysis, 2022, Domain II: Surveillance and Epidemiologic Investigation, Competency 2.2 - Analyze surveillance data). This step is foundational to controlling the outbreak and protecting vulnerable patients.