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Introduction Infective endocarditis (IE) in prosthetic valves is a challenging condition associated with high morbidity. This case highlights a patient with multivalvular IE complicated by a rapidly enlarging ascending aortic pseudoaneurysm and culture-negative findings, emphasizing the role of multimodal imaging and multidisciplinary management.Methods A 77-year-old male with a history of aortic valve replacement (AVR) and ascending aortic root replacement presented with chest and upper abdominal pain. Transthoracic (TTE) and transesophageal echocardiography (TOE) identified vegetations on the prosthetic aortic valve, tricuspid valve, and pacemaker lead, along with a large pseudoaneurysm. Despite extensive microbiological testing, including prolonged culture incubation and fungal assays, no causative organism was identified. CT angiogram (CTA) on 14/02/25 demonstrated significant pseudoaneurysm enlargement with mass effect on the right pulmonary artery, superior vena cava, and right superior pulmonary vein. Imaging demonstrated marked pseudoaneurysm enlargement (figure 1A-B) and thrombus formation within it (figure 2).Results The patient was managed conservatively with IV vancomycin, gentamicin, and rifampicin. He remained hemodynamically stable, but given the progressive pseudoaneurysm enlargement, surgical intervention was discussed. However, due to the high surgical risk, a multidisciplinary team recommended close surveillance and supportive management. Vegetations identified by TOE were notable on multiple cardiac structures (figure 3).Conclusion This case underscores the complexity of managing culture-negative prosthetic valve endocarditis with pseudoaneurysm progression. A multimodal imaging approach, early microbiological assessment, and multidisciplinary input are crucial in guiding treatment decisions, particularly in high-risk patients where surgical intervention is not immediately feasible.Full Case Report Case Presentation Patient Information: A 77-year-old male presented on December 27, 2024, with chest pain and upper abdominal pain. His medical history included:Cardiac history: AVR, aortic root replacement, coronary artery re-implantation (April 2024), dual-chamber pacemaker.Hematologic: Polycythemia rubra vera (JAK2 mutation positive) on hydroxycarbamide.Ex-smoker.Investigations CTA Aorta (30/12/24): Large pseudoaneurysm of the ascending aorta.TTE (30/12/24): Suspected prosthetic AVR vegetation, mildly dilated LV (EF ~38%), RV basal dilation.TOE (30/12/24): Confirmed vegetation on prosthetic AVR, tricuspid valve, and pacemaker lead with a large thrombus in the pseudoaneurysm.CTA Aorta (14/02/25):Pseudoaneurysm progression: Now measuring 95 x 76 x 80 mm, previously 39 x 43 x 52 mm.Mass effect on right pulmonary artery, superior vena cava, and right superior pulmonary vein.Self-limiting dissection flap arising from the aortic root.Blood cultures: Negative (5 samples, including prolonged incubation).Fungal tests: Beta-glucan and galactomannan negative.Inflammatory markers: CRP 54 mg/L, WCC 8 x 10^9/L.Management Antibiotics: IV vancomycin, gentamicin, rifampicin.Blood pressure control: Labetalol infusion to maintain SBP 100–120 mmHg.Surgical discussion: Deemed unsuitable for surgery or percutaneous closure.Multidisciplinary approach: Conservative management due to high surgical risk.Discussion Diagnostic Challenges Culture-negative IE requires a high index of suspicion and multimodal imaging.TOE played a key role in identifying multivalvular vegetations and pseudoaneurysm thrombus.Therapeutic Dilemmas Empiric therapy targeted staphylococci, streptococci, and enterococci.High surgical risk prevented intervention despite pseudoaneurysm progression.Key Learning Points Culture-negative prosthetic valve IE requires early multimodal imaging.Pseudoaneurysm progression demands frequent imaging surveillance.Multidisciplinary team input is essential for complex cardiac infections.Abstract 1-023 Figure 1Significant pseudoaneurysm progression (sagittal view)Abstract 1-023 Figure 2Significant pseudoaneurysm progression (axial view)Abstract 1-023 Figure 3Thrombus in pseudoaneurysmAbstract 1-023 Figure 4TOE – Vegetations on tricuspid valve, prosthetic AVR, pacemaker leadReferences Habib G, et al. 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