Brain MRS, also called proton magnetic resonance spectroscopy, is an MRI-based test that looks at chemical signals inside selected brain regions. Unlike standard MRI, which mainly shows brain structure, MRS can help detect metabolic peaks such as lactate, NAA, creatine, choline, Glx, GABA, glycine, and glutathione under specific conditions. In this video, we explain how brain MRS may help doctors think about inborn errors of metabolism, mitochondrial disease, Canavan disease, cerebral creatine deficiency syndromes, glycine encephalopathy, GLUT1 deficiency, neurotransmitter disorders, and other rare neurometabolic conditions. We also discuss why MRS cannot diagnose most conditions by itself, why CSF testing and genetic/metabolic workups may still be needed, and why red flags such as developmental regression, difficult-to-treat seizures, abnormal movements, white matter changes, basal ganglia abnormalities, or worsening after infection or fasting should not be ignored. This content is for medical education only and does not replace diagnosis or treatment by a physician.