USMLE-Rx Step 1 Practice Q's

USMLE-Rx Step 1 Qmax Challenge #1539

Check out today’s Step 1 Qmax Question Challenge.

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USMLE-Rx Step 1 Qmax Challenge #1539A 12-year-old boy is brought by his mother to the clinic because of persistent rash and peculiar behavior. She reports that over the past few years the boy has developed a rough, red rash on his face, neck, and hands that gets worse when he is outdoors. She reports that he has been “tottering like an old man” since the beginning of the year and that his hands are consistently shaky. He is not doing well in school. More often than not his stools are loose and watery. The mother reports that he eats a normal, balanced diet. On physical examination, the boy appears well nourished, with an erythematous, scaling rash like that in the image. He has bilateral nystagmus and an intention tremor. He walks with a teetering, wide-based gait. Urinalysis reveals significant aminoaciduria.

Deficiency of which amino acid is likely responsible for this patient’s presentation?

A. Cystine
B. Histidine
C. Isoleucine
D. Tryptophan
E. Tyrosine


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8 replies »

  1. D. Tryptophan. It’s important to recognize the signs and symptoms because many times it’s hard to tell the difference between one health problem or another.

  2. The correct answer is D. This patient is presenting with photosensitive dermatitis, diarrhea, and central nervous system disturbances pointing to the triad of pellagra. Pellagra is the result of niacin deficiency and can occur because of insufficient dietary intake or an inherited error of metabolism. Considering the boy appears well nourished on exam with no history of insufficient dietary intake, the possibility of an inherited error of metabolism is higher on the differential. This possibility is further suggested by the boy’s significant aminoaciduria. Several disorders are caused by deficient renal tubular reabsorption of amino acids that can result in high levels of amino acids lost in the urine. One of these disorders is known as Hartnup disease, which is caused by mutation in the transport of neutral amino acids, including that of tryptophan. Tryptophan is a precursor to niacin and an essential amino acid. Hartnup disease therefore manifests as pellagra due to tryptophan, and consequently niacin, deficiency.

    A is not correct. An excess of cystine (rather than a deficiency) results in cystinuria, the most common genetic error of amino acid transport. It is a disorder caused by impaired reabsorption of cystine and dibasic amino acids (lysine, ornithine, and arginine) in the proximal renal tubule and gastrointestinal tract. Patients present with renal stones resulting in obstructive nephropathy, pyelonephritis, and, rarely, renal failure.

    B is not correct. A relatively common inborn error of metabolism affecting histidine is histidinemia, caused by a deficiency in the enzyme histidase, necessary for the catabolism of histidine. These individuals have elevated levels of histidine in the blood and urine. It is considered to be a benign disorder.

    C is not correct. A metabolic disorder involving isoleucine (as well as other branched-chain amino acids) is maple syrup urine disease. This condition is caused by a deficiency in branched-chain α-keto acid dehydrogenase, leading to elevated levels of valine, leucine, and isoleucine in plasma and urine. Central nervous system problems are common, as are feeding problems, vomiting, dehydration, severe metabolic acidosis, and a characteristic maple syrup odor to the urine. Treatment involves dietary restriction of branched-chain amino acids.

    E is not correct. Deficiency of tyrosine is found in phenylketonuria (PKU), caused by deficiency of the enzyme phenylalanine hydroxylase, which converts phenylalanine to tyrosine. PKU is the most common inborn error of amino acid metabolism. Affected individuals have high phenylalanine levels, intellectual diability, and hypopigmentation (fair hair, light skin color, blue eyes). There is a neonatal screen for PKU. If detected early, these adverse sequelae can be prevented by adhering to a diet low in phenylalanine.

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