An infant is brought to a hospital because her wet diapers turn black when they are exposed to air. Physical examination is normal. Urine is positive both for reducing substance and when tested with ferric chloride. This disorder is caused by a deficiency of which of the following?

a. Homogentisic acid oxidase

b. Phenylalanine hydroxylase

c. I-histidine ammonia-lyase

d. Ketoacid decarboxylase

e. Isovaleryl-CoA dehydrogenase

the answer is below…

The United States Medical Licensing Examination, or USMLE for short, is a three-part licensing examination that is required in order to receive a license to practice medicine within the United States.

The USMLE assesses a physician’s ability to apply knowledge, concepts, and principles, and to determine fundamental patient-centered skills that are important in health and disease and that constitute the basis of safe and effective patient care.Examination committees composed of medical educators and clinicians from across the United States and its territories prepare the examination materials each year.

This exam is designed by the Federation of State Medical Boards and the National Board of Medical Examiners to determine whether or not an individual understands and can apply the knowledge necessary to practice medicine safely and intelligently.

The USMLE is actually comprised of three different exams that are referred to as steps, which examine the individual’s knowledge of specific topics related to the field of medicine such as basic science, medical knowledge, medical skills, clinical science, and the application of all of these skills and areas of knowledge in the medical field.

All three steps of the USMLE include a series of computerized multiple-choice questions, but the format of the exam and the information covered in each multiple-choice section is different for each step of the USMLE. The USMLE Step II also has a clinical skills portion that examines an individual’s ability to work with real patients and the USMLE Step III has a computerized patient simulation portion in addition to the multiple-choice section of the exam. In order for an individual to receive a license to practice medicine, the individual must pass all three steps of the USMLE.

Medical doctors with an M.D. degree are required to pass this examination before being permitted to practice medicine in the United States of America

The correct answer is a; Homogentisic acid oxidase.[3]

The infant described in the question has alkaptonuria, an autosomal recessive disorder caused by a deficiency of homogentisic acid oxidase. The diagnosis is made in infants when their urine turns dark brown or black on exposure to air due to the oxidation of homogentisic acid. Affected children are asymptomatic. In adults, ochronosis—the deposition of a bluish pigment in cartilage and fibrous tissue—develops; symptoms of arthritis may appear later. No specific treatment is available for patients who have alkaptonuria, although supplemental ascorbic acid may delay the onset of the disorder and reduce clinical symptoms. The other deficiencies listed in the question are found in phenylketonuria, histidinemia, maple syrup urine disease, and isovaleric acidemia, respectively.