In alkaptonuria there is increased pigmentation in all of the following locations, EXCEPT: -

Alkaptonuria is an autosomal recessively inherited deficiency of homogentisic acid oxidase enzyme which is involved in the metabolism of phenylalanine and tyrosine. Due to this defect, an oxidation product gets deposited in the cailage throughout the body. In these patients examination of the skin shows a slight darkish blue color below the skin in areas overlying cailage such as in the ears, sclera, conjunctiva and cornea. Metabolites also get deposited in the hea valves leading aoic or mitral stenosis. Patients often develop back pain due to spondylitis. It can be differentiated from ankylosing spondylitis by the absence of fusion of sacroiliac joint. Diagnosis can be made when urine turns black spontaneously when exposed to air due to the presence of homogentisic acid. REf: CURRENT Medical Diagnosis & Treatment 2014 chapter 40.