USMLE® Step 1 style questions USMLE
A 10-year-old girl comes to the emergency department because of a sudden onset rash across her body. The rash appeared after she played outside under the sun for 30 minutes. She also reports double vision. Physical examination shows dry, scaly, well-marginated eruptions on the face, backs of the hands, and lower back. When asked to walk, her gait is wide and unsteady. Which of the following is the most likely diagnosis?
Cystinuria is an inherited metabolic disorder named because of high levels of the amino acid cystine found in urine. The chemically similar amino acids arginine, lysine, and ornithine are also found in high levels.
The kidneys are two bean-shaped organs in the body that filter out waste products and excess water from blood to produce urine. Typically, amino acids are recovered from urine and reabsorbed into the blood by the kidneys. But in cystinuria, impaired reabsorption of cystine, arginine, lysine, and ornithine leads to excessive amounts in urine. At high concentration, cystine clumps together to form crystals and stones, also called calculi. These can be found along the urinary tract, including the kidneys, ureters, and urinary bladder.
While high levels of cystine, arginine, lysine, and ornithine in the urine are characteristics of the disorder, the main signs and symptoms are caused only by the cystine crystals and stones. As they stick together and grow larger, they form lemon-yellow stones. Stones are jagged and can cause sharp pains in the sides and lower back that can radiate to the lower abdomen and groin when they cause obstruction of the ureters. The pain can come in waves and may be present when urinating. Stones can also cause injury to tissues in the urinary tract, which can lead to blood in the urine. Frequent recurrences may ultimately lead to lasting kidney damage. Depending on their size, stones may obstruct the urinary tract and slow or stop the flow of urine. This can lead to additional complications, like urinary tract infections. Some individuals won’t form stones, but others typically begin having these symptoms between 10 and 30 years old.
With a normal pH of urine, high levels of arginine, lysine, and ornithine can be excreted in massive amounts without additional complications. But high levels of cystine are not as soluble and need urine with a higher pH to completely dissolve. Undissolved cystine clumps together to form crystals and stones. And clumping is more likely when urine volume is low as cystine becomes more concentrated.
Cystinuria is an inherited disorder caused by changes, or mutations, in either the SLC3A1 or SLC7A9 gene. These mutations impair the normal reabsorption of cystine, arginine, lysine, and ornithine. Cystinuria is an autosomal recessive disorder, which means an individual needs to inherit a mutated copy of the SLC3A1 or SLC7A9 gene from each parent to be affected. If an individual inherits one mutated and one normal copy, they are considered carriers for the disorder. Carriers usually have minimal to no signs and symptoms of cystinuria. A few people seem to make stones with only one abnormal SLC7A9 gene.