Urine that darkens after air exposure, along with scleral pigmentation in a patient with back pain and difficulty walking, should raise suspicion for alkaptonuria. This autosomal recessive disorder of tyrosine metabolism causes homogentisic acid (HGA) accumulation. Over time, HGA oxidizes and polymerizes into a dark pigment that deposits in connective tissues, a process known as ochronosis. These deposits lead to ochronotic arthropathy, seen in this patient’s imaging. Elevated urinary HGA levels confirm the diagnosis. Darkening urine may be the first sign of alkaptonuria in infancy, so remember: a routine diaper change could be a diagnostic game changer.
Comment
Urine that darkens after air exposure, along with scleral pigmentation in a patient with back pain and difficulty walking, should raise suspicion for alkaptonuria. This autosomal recessive disorder of tyrosine metabolism causes homogentisic acid (HGA) accumulation. Over time, HGA oxidizes and polymerizes into a dark pigment that deposits in connective tissues, a process known as ochronosis. These deposits lead to ochronotic arthropathy, seen in this patient’s imaging. Elevated urinary HGA levels confirm the diagnosis. Darkening urine may be the first sign of alkaptonuria in infancy, so remember: a routine diaper change could be a diagnostic game changer.