An apparently healthy 6-year-old boy is enrolled in a research study designed to assess the amino acid absorptive capacity of the intestine. As part of the investigation, he is administered an oral solution containing free amino acids. Blood samples are then obtained at 15 minute intervals for the next 2 hours. The boy is found to have significantly decreased intestinal absorption of lysine, arginine, ornithine, and cysteine as compared to the other study participants. If his condition is left untreated, which of the following complications is this patient at greatest risk of developing?

Question

An apparently healthy 6-year-old boy is enrolled in a research study designed to assess the amino acid absorptive capacity of the intestine.  As part of the investigation, he is administered an oral solution containing free amino acids.  Blood samples are then obtained at 15 minute intervals for the next 2 hours.  The boy is found to have significantly decreased intestinal absorption of lysine, arginine, ornithine, and cysteine as compared to the other study participants.  If his condition is left untreated, which of the following complications is this patient at greatest risk of developing?

Quizplus · Accepted Answer

11ec160b_3b52_6692_9249_6102353df93d_MD0001_00 The dibasic amino acids cysteine, ornithine, lysine, and arginine (cola) share a common transporter in the intestinal lumen and kidneys. In patients with cystinuria, this transporter is defective, resulting in impaired renal and intestinal absorption of these amino acids. In the intestine, this causes absent (or diminished) intestinal absorption of these free amino acids. However, patients do not develop amino acid deficiencies, as these amino acids are absorbed in sufficient quantities as oligopeptides. In the kidneys, impaired tubular reabsorption of these amino acids leads to a high urinary cystine concentration, resulting in the formation of cystine kidney stones. The other amino acids (eg, ornithine, lysine, and arginine) are relatively soluble in urine and do not result in the formation of kidney stones. Risk factors for cysteine precipitation include low urine pH (pH <7), the presence of a preexisting crystal nidus, and urine supersaturation. (Choice A) Aortic dissection is a well-known complication of Marfan syndrome and other connective tissue disorders. (Choice B) Emphysema is a common complication of α1-antitripsin deficiency. Liver involvement is also common in this disorder. (Choice C) Fat malabsorption is typically seen in conditions that cause exocrine pancreas dysfunction, such as cystic fibrosis. (Choice D) Intellectual disability occurs in several inborn errors of amino acid metabolism, including phenylketonuria, homocystinuria, and in some patients with maple syrup urine disease (branched-chain ketoaciduria). However, intellectual disability is not seen in cystinuria. (Choice F) Rickets is a failure of osteoid calcification (secondary to vitamin D deficiency) that occurs in children. This disorder is more likely to occur in infants who are exclusively breastfed, who receive no oral vitamin D supplementation, and who have darkly pigmented skin. Cystinuria does not affect bone calcification. Educational objective: Cystinuria is an autosomal recessive disorder caused by defective transportation of cystine, ornithine, arginine, and lysine across the intestinal and renal tubular epithelium. Recurrent nephrolithiasis is the only clinical manifestation. Urinalysis shows pathognomonic hexagonal cystine crystals.

An Apparently Healthy 6-Year-Old Boy Is Enrolled in a Research