Question

Blackening of urine occurs when exposed to air, is a metabolic disorder in human beings. This is due to
A. Phenylalanine
B. Tyrosine
C. Valine replacing glutamine
D. Homogentisic acid
E. Glumine replacing valine

Answer 1

Hint: Urine turning to black in color when exposed to air is an inherited condition called alkaptonuria. People with alkaptonuria usually develop arthritis, especially in the spine and large joints, which begins in early adulthood. Other manifestations of this condition include heart problems, kidney stones, and also prostate stones.

Complete answer:
Ochronosis which is a buildup of dark pigment in connective tissues such as cartilage and skin, is also characteristic of alkaptonuria. This blue-black pigmentation usually appears after the age of thirty. Defect in the HGD gene is responsible for causing alkaptonuria.
Phenylalanine is an essential amino acid which means that it cannot be made in the body and should be ingested through diet. Due to the defect in the HGD gene the body is unable to break down phenylalanine hence it does not cause blackening of urine.
Tyrosine is a nonessential amino acid and is formed by the hydroxylation of phenylalanine in the liver when the intake of tyrosine in the diet is less. Due to the defect in the HGD gene the body does not break down tyrosine. Therefore, tyrosine is not responsible for blackening urine.
When valine replaces glutamic acid, it causes the hemoglobin to become less soluble. This condition is called sickle cell hemoglobin which does not cause urine to be black in color when exposed to air.
The HGD gene defect makes the body unable to break down certain amino acids like tyrosine and phenylalanine. Therefore, a substance called homogentisic acid builds up in the skin and also in other body tissues. The acid leaves the body in the form of urine. This urine turns into brownish-black in color when it is mixed with air.
Sixth amino acid which is glutamic acid is replaced by valine. A hydrophobic site is present on the outside of the HbS beta chain. Thereby creating a HbS polymer that deforms the red blood cell and causes a vaso-occlusive crisis in the capillary venous pole. Hence, it does not cause blackening of urine.

Therefore, the correct option is D.

Note:
Alkaptonuria is inherited in an autosomal recessive pattern, which means that both the copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carries one copy of the mutated gene, but they usually do not show any signs and symptoms of this condition.