Brain Abscess Likely Secondary to Infective Endocarditis Mimicking High-Grade Glioma in a Patient with Pyelonephritis.

Brain abscesses are serious central nervous system infections that often present with nonspecific infectious and neurologic symptoms and may mimic intracranial neoplasms on imaging. They can develop rapidly or insidiously, and diagnosis is often delayed when symptoms overlap with concurrent systemic infections. We report the case of a 66-year-old woman admitted for pyelonephritis who initially improved with intravenous antibiotics but later developed persistent fever, worsening occipital headache, and acute neurologic deterioration. Initial imaging demonstrated an irregular ring-enhancing lesion suggestive of high-grade glioma; however, intraoperative findings and histopathology confirmed a brain abscess. Gram stain revealed anaerobic gram-positive cocci in pairs, but cultures showed no growth. Further evaluation identified a mitral valve vegetation highly suggestive of infective endocarditis (IE), representing the most likely source of septic embolization; however, a definitive microbiologic link could not be established because blood cultures remained negative and the causative organism was not identified. This case highlights the diagnostic difficulty of brain abscess when it mimics an intracranial neoplasm and occurs alongside systemic infection. Importantly, it also underscores the need to maintain a broad differential diagnosis when clinical status worsens despite appropriate therapy and to consider IE as an occult source of septic emboli in patients with unexplained neurologic decline and concurrent infection.