What is the most likely diagnosis if a urine sample collected for routine testing darkens on standing?
Phenylketonuria
Alkaptonuria
Maple syrup disease
Tyrosinemia
Urine turns dark on standing in alkaptonuria due to the formation of black coloured alkaptone bodies. Ref: Textbook of Biochemistry By DM Vasudevan, 3rd Edition, Page 184 ; Harper's Biochemistry, 25th Edition, Page 332