A 23-year-old apparently healthy man who recently immigrated to the United States comes to an outpatient clinic to establish care. When asked about his past medical history, he says that he has no significant medical problems. However, his mother told him that he was born with "a problem metabolizing sugar." The patient maintains no dietary restrictions and regularly eats vegetables, fruits, meats, and processed foods. Urine samples show a repeatedly positive copper reduction test, but glucose oxidase dipstick testing is negative. Which of the following enzymes is most likely to be deficient in this patient?

Question

A 23-year-old apparently healthy man who recently immigrated to the United States comes to an outpatient clinic to establish care.  When asked about his past medical history, he says that he has no significant medical problems.  However, his mother told him that he was born with "a problem metabolizing sugar."  The patient maintains no dietary restrictions and regularly eats vegetables, fruits, meats, and processed foods.  Urine samples show a repeatedly positive copper reduction test, but glucose oxidase dipstick testing is negative.  Which of the following enzymes is most likely to be deficient in this patient?

Quizplus · Accepted Answer

11ec160b_369b_6e9a_9249_fd3a1b27f868_MD0001_00 This asymptomatic patient with history of an inborn error of sugar metabolism most likely has essential fructosuria. This benign, autosomal recessive disorder causes some of the dietary fructose load to be secreted in the urine unchanged due to defective metabolism by fructokinase. Fructose, similar to glucose and galactose, is a reducing sugar and can be detected by a copper reduction test, which nonspecifically detects the presence of reducing sugar. A urine dipstick, however, uses glucose oxidase to ascertain the presence of urinary glucose and will not test positive in the presence of fructose or galactose. (Choice A) Acid α-glucosidase (or acid maltase) deficiency causes glycogen storage disease type II (Pompe disease). Affected infants have cardiomyopathy, muscle weakness, and hypotonia. (Choice B) Aldolase B deficiency is a life-threatening disorder caused by the inability to metabolize fructose-1-phosphate (a toxic intermediate that accumulates in cells and depletes intracellular phosphate). Patients become acutely symptomatic after ingesting fructose-containing foods and eventually develop liver failure. Treatment includes elimination of dietary fructose. (Choice D) Galactosemia is an autosomal recessive disorder caused by galactose-1-phosphate uridyl transferase deficiency. It is characterized by neonatal jaundice, vomiting, cataract formation, hepatomegaly, and failure to thrive. Treatment includes elimination of all milk products from the diet and feeding with soy-based infant formula. (Choice E) Lactase is a mucosal enzyme responsible for the digestion of lactose. Acquired lactase deficiency is the most common cause of selective carbohydrate malabsorption. Patients with lactase deficiency experience gastrointestinal symptoms (eg, bloating, diarrhea) following the ingestion of dairy products. Educational objective: Unlike hereditary fructose intolerance (aldolase B deficiency) and classic galactosemia (galactose-1-phosphate uridyl transferase deficiency), essential fructosuria (fructokinase deficiency) is a benign disorder. Although affected patients are asymptomatic, their urine will test positive for a reducing sugar due to the presence of unmetabolized fructose.

A 23-Year-Old Apparently Healthy Man Who Recently Immigrated to the United