<prompt> Answer this question truthfullyWhat is the significance of free light chain excretion in the urine, and which medical condition is commonly associated with it, known as Bence Jones proteinuria?  <response>: <reason> Free light chain excretion in the urine is significant as it is commonly associated with Bence Jones proteinuria, a medical condition characterized by the presence of free light chain excretion in the urine. This condition is often associated with multiple myeloma, which is a type of cancer affecting plasma cells in the bone marrow. 