Lessons of the month 3: Mosaic Klinefelter syndrome unveiled by acute vertebral fracture in a middle-aged man.

Klinefelter syndrome (KS) is the most common sex chromosome disorder in males. It is the result of two or more X chromosomes in a phenotypic male. In addition to primary hypogonadism affecting male sexual development, it is associated with a series of comorbidities such as osteoporosis, psychiatric and cognitive disorders, metabolic syndromes, and autoimmune diseases. A broad spectrum of phenotypes has been described and many cases remain undiagnosed throughout their lifespan. In this case report, we describe a case of mosaic KS unmasked by acute vertebral fracture. © Royal College of Physicians 2021. All rights reserved.