USMLE Step 1 Quiz
Practice Amino Acid Metabolism And Urea Cycle in USMLE Step 1 with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.
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A 2-year-old boy is brought to the pediatrician for developmental delay. The parents report he has had seizures and has a peculiar 'mousy' body odor. On examination, he has fair skin, blonde hair, and blue eyes, which is much lighter than his parents' complexion. A metabolic disorder is suspected.
This patient's condition is most likely caused by an inability to convert which of the following amino acids into its normal metabolic product?
This quiz focuses on Amino Acid Metabolism And Urea Cycle, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 1.
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A 2-year-old boy is brought to the pediatrician for developmental delay. The parents report he has had seizures and has a peculiar 'mousy' body odor. On examination, he has fair skin, blonde hair, and blue eyes, which is much lighter than his parents' complexion. A metabolic disorder is suspected.
This patient's condition is most likely caused by an inability to convert which of the following amino acids into its normal metabolic product?
Explanation: The clinical picture of intellectual disability, seizures, musty body odor, and hypopigmentation is characteristic of phenylketonuria (PKU). PKU is caused by a deficiency of phenylalanine hydroxylase, the enzyme that converts phenylalanine to tyrosine. The lack of tyrosine, a precursor for melanin, explains the hypopigmentation. The accumulation of phenylalanine leads to the formation of phenylketones, which cause the musty odor and neurotoxicity.
A 5-day-old infant is evaluated for poor feeding, vomiting, and progressive lethargy. The mother notes that the infant's urine has a sweet smell, similar to burnt sugar. On examination, the infant displays alternating periods of hypertonia and hypotonia. Laboratory analysis is expected to show elevated plasma concentrations of specific amino acids.
This disorder is caused by a deficient enzyme complex involved in the metabolism of which of the following groups of amino acids?
Explanation: This is a classic presentation of Maple Syrup Urine Disease (MSUD), an autosomal recessive disorder. It is caused by a deficiency in the branched-chain α-ketoacid dehydrogenase complex, which is required for the degradation of the branched-chain amino acids: leucine, isoleucine, and valine. The accumulation of these amino acids and their corresponding α-ketoacids leads to neurotoxicity and the characteristic sweet-smelling urine.
A 52-year-old man presents with severe, long-standing low back pain and stiffness. Physical examination reveals dark, greyish-blue deposits in the sclerae and on the cartilage of his ears. He mentions that his urine has always turned dark brown or black upon standing. Radiographs of his spine show calcification of intervertebral discs.
This patient's condition is due to a deficiency in which of the following enzymes?
Explanation: The patient's symptoms of dark urine (alkaptonuria), pigment deposition in connective tissue (ochronosis), and debilitating arthritis are characteristic of alkaptonuria. This is an autosomal recessive disorder caused by a deficiency of homogentisate oxidase. This enzyme is involved in the degradation pathway of tyrosine and phenylalanine. Its deficiency leads to the accumulation of homogentisic acid, which polymerizes to form a dark pigment that deposits in tissues.
A 10-year-old boy is evaluated for poor vision. His height is above the 95th percentile, and he has disproportionately long arms and legs. Ophthalmic examination reveals ectopia lentis with the lens displaced inferiorly and medially. His medical history is significant for a previous deep vein thrombosis. Laboratory studies show elevated plasma levels of methionine and homocysteine.
A deficiency of which of the following enzymes is the most common cause of this patient's disorder?
Explanation: The combination of Marfanoid habitus, downward lens dislocation (ectopia lentis), and thromboembolic events is highly suggestive of classic homocystinuria. This autosomal recessive disorder is most commonly caused by a deficiency of cystathionine β-synthase, an enzyme that requires pyridoxine (vitamin B6) as a cofactor. The deficiency prevents the conversion of homocysteine to cystathionine, leading to the accumulation of homocysteine and its precursor, methionine.
A neonate with a severe urea cycle defect presents with hyperammonemia. The physician explains to the medical student that the rate-limiting step of the urea cycle is catalyzed by carbamoyl phosphate synthetase I (CPS I), which is often the deficient enzyme.
The activity of CPS I is critically dependent on allosteric activation by which of the following molecules?
Explanation: N-acetylglutamate (NAG) is the essential allosteric activator for carbamoyl phosphate synthetase I (CPS I), the enzyme that catalyzes the first and rate-limiting step of the urea cycle. The synthesis of NAG itself is stimulated by arginine. Therefore, in a fed state with high levels of amino acids (including arginine), NAG is synthesized, CPS I is activated, and the urea cycle proceeds to dispose of excess nitrogen. Arginine is an upstream activator of the process but not the direct allosteric activator of CPS I.
A 10-year-old girl from a low-income family whose diet consists mainly of corn presents with a photosensitive, pigmented rash on her arms and neck. She has also been experiencing episodes of ataxia and emotional irritability. Her mother reports that she has had persistent diarrhea for the past month. A defect in amino acid transport is suspected.
This patient's pellagra-like symptoms are most likely caused by a genetic defect in the transport of which amino acid?
Explanation: This clinical presentation is characteristic of Hartnup disease, an autosomal recessive disorder caused by defective transport of neutral amino acids in the small intestine and kidneys. The most clinically significant consequence is the impaired absorption of tryptophan. Tryptophan is a precursor for niacin (vitamin B3). Niacin deficiency leads to pellagra, which is characterized by the '3 Ds': Dermatitis, Diarrhea, and Dementia (or ataxia/neurologic symptoms). A corn-based diet is also low in niacin and tryptophan, which can unmask or worsen the condition.
A 60-year-old man with alcoholic cirrhosis develops confusion, lethargy, and asterixis. He is diagnosed with hepatic encephalopathy due to hyperammonemia. In addition to lactulose, he is prescribed a medication that provides an alternative pathway for nitrogen excretion.
Drugs such as sodium benzoate lower ammonia levels by conjugating with an amino acid to form a product that is readily excreted in the urine. Which amino acid does benzoate conjugate with?
Explanation: Sodium benzoate is a nitrogen-scavenging drug used to treat hyperammonemia. It combines with glycine to form hippurate, which is then excreted in the urine. This process effectively removes one molecule of nitrogen (in glycine) from the body for every molecule of benzoate administered. Similarly, sodium phenylacetate conjugates with glutamine to form phenylacetylglutamine, removing two nitrogen atoms. These drugs provide an alternative pathway for nitrogen disposal when the urea cycle is impaired.
A medical student is comparing two different urea cycle disorders that present in the neonatal period with severe hyperammonemia. One is ornithine transcarbamylase (OTC) deficiency, and the other is carbamoyl phosphate synthetase I (CPS I) deficiency.
Which of the following laboratory findings would be most useful in distinguishing CPS I deficiency from OTC deficiency?
Explanation: Urine orotic acid level is the key distinguishing feature. In OTC deficiency, carbamoyl phosphate accumulates in the mitochondria and spills into the cytosol, where it is shunted into the pyrimidine synthesis pathway, leading to high levels of orotic acid. In CPS I deficiency, carbamoyl phosphate cannot be synthesized, so orotic acid levels are normal or low. Both disorders will present with very high plasma ammonia, high plasma glutamine (a nitrogen reservoir), and low BUN, as the urea cycle is blocked in both cases.
A biochemistry lecture is covering the synthesis of urea, the primary molecule for disposing of excess nitrogen in humans. The professor emphasizes the origin of each atom in the final urea molecule, which has the structure (NH2)2CO.
The two nitrogen atoms found in a single molecule of urea are directly contributed by which pair of molecules?
Explanation: The synthesis of one molecule of urea requires two nitrogen atoms. The first nitrogen atom enters the urea cycle in the mitochondria as free ammonia (NH3), which is incorporated into carbamoyl phosphate. The second nitrogen atom enters the cycle in the cytosol and is donated by the amino acid aspartate, which condenses with citrulline to form argininosuccinate. Glutamate and glutamine are important in trapping and transporting nitrogen to the liver, but they are not the direct donors to the urea molecule itself.
A 2-day-old infant develops severe hypotonia, lethargy, and intractable seizures. He also has persistent hiccups. An EEG shows a burst-suppression pattern. Laboratory analysis of his cerebrospinal fluid shows a markedly elevated level of glycine.
This severe neonatal encephalopathy is caused by a defect in the enzyme system responsible for the catabolism of which amino acid?
Explanation: The clinical presentation of severe neonatal encephalopathy with seizures, hypotonia, hiccups, and a burst-suppression EEG, combined with elevated CSF glycine, is pathognomonic for nonketotic hyperglycinemia (NKH). This autosomal recessive disorder is caused by a defect in the mitochondrial glycine cleavage system, which is the major pathway for glycine degradation. The accumulation of glycine in the brain acts as an inhibitory neurotransmitter at some receptors and an excitatory co-agonist at NMDA receptors, leading to severe neurotoxicity.
A patient with acute viral hepatitis has laboratory results showing extremely high levels of serum alanine aminotransferase (ALT). This enzyme catalyzes a reversible reaction that is crucial for the transport of nitrogen from muscle to the liver.
In the direction that occurs in the muscle, the ALT-catalyzed reaction transfers an amino group from glutamate to pyruvate. What are the two products of this reaction?
Explanation: Alanine aminotransferase (ALT) catalyzes the reversible transfer of an amino group between alanine and α-ketoglutarate to form pyruvate and glutamate. The question describes the reaction in reverse (the direction it occurs in muscle as part of the glucose-alanine cycle): an amino group is transferred from glutamate to pyruvate. This reaction yields alanine (which travels to the liver) and α-ketoglutarate. The reaction is a key link between amino acid metabolism and carbohydrate metabolism.
A researcher is studying the role of methylation reactions in epigenetic regulation. These reactions are essential for normal development and gene expression and rely on a universal methyl group donor molecule.
This primary methyl group donor is synthesized directly from which of the following amino acids and ATP?
Explanation: The primary methyl group donor in the body is S-adenosylmethionine (SAM). SAM is synthesized from the amino acid methionine and ATP in a reaction catalyzed by methionine adenosyltransferase. After donating its methyl group, SAM is converted to S-adenosylhomocysteine (SAH), which is then hydrolyzed to homocysteine. Homocysteine can then be remethylated back to methionine, completing the cycle. This pathway is central to one-carbon metabolism and numerous biosynthetic and regulatory processes.
A 5-year-old boy presents with progressive difficulty walking, ataxia, and intellectual disability. His parents note that he has had developmental regression over the past year. Physical examination reveals spastic diplegia, which is more pronounced in the lower extremities. Laboratory tests show markedly elevated arginine levels in the plasma and cerebrospinal fluid, with moderate hyperammonemia.
A deficiency of which of the following enzymes best explains this patient's clinical and laboratory findings?
Explanation: This presentation is characteristic of arginase deficiency. Unlike other urea cycle defects that present with severe hyperammonemia in infancy, arginase deficiency often has a later onset and is characterized by progressive spastic diplegia and neurologic deterioration. The enzymatic block is at the last step of the urea cycle, where arginase cleaves arginine into urea and ornithine. This leads to a massive accumulation of arginine, which is thought to be neurotoxic, and a milder degree of hyperammonemia.
A 4-day-old male infant is brought to the emergency department with lethargy, vomiting, and poor feeding that began 24 hours ago. On examination, he is tachypneic and poorly responsive. Laboratory studies reveal a plasma ammonia level of 950 µmol/L (normal < 50 µmol/L) and respiratory alkalosis. Urinalysis is significant for markedly elevated orotic acid levels. The infant is admitted for management of a suspected urea cycle disorder.
A deficiency of which of the following enzymes is the most likely cause of this patient's presentation?
Explanation: This patient's presentation of severe neonatal hyperammonemia, respiratory alkalosis, and elevated urinary orotic acid is classic for ornithine transcarbamylase (OTC) deficiency, the most common urea cycle disorder. OTC deficiency leads to the accumulation of its substrates, carbamoyl phosphate and ornithine. The excess carbamoyl phosphate in the mitochondria leaks into the cytosol, where it enters the pyrimidine synthesis pathway, leading to increased production and excretion of orotic acid. A deficiency in Carbamoyl phosphate synthetase I or N-acetylglutamate synthase would cause hyperammonemia without orotic aciduria, as carbamoyl phosphate would not be produced. Argininosuccinate synthetase deficiency (Citrullinemia Type I) would cause an elevation in citrulline, not orotic acid.
A 58-year-old woman with a history of a slow-growing neuroendocrine tumor of the small intestine presents with episodic flushing, watery diarrhea, and wheezing. Over the past few months, she has also developed a rough, hyperpigmented rash in sun-exposed areas and has become increasingly confused and irritable.
The dermatologic and neurologic symptoms in this patient are likely caused by a deficiency in niacin, resulting from excessive shunting of which amino acid toward serotonin synthesis?
Explanation: This patient has carcinoid syndrome, where a neuroendocrine tumor produces large quantities of serotonin. The precursor for serotonin is the amino acid tryptophan. Tryptophan is also the precursor for the synthesis of niacin (vitamin B3). In carcinoid syndrome, a significant portion of dietary tryptophan is diverted to produce serotonin, leading to a relative deficiency of tryptophan for niacin synthesis. The resulting niacin deficiency causes pellagra, characterized by dermatitis, diarrhea, and dementia, which explains the patient's rash and confusion.
A 45-year-old woman with a history of seizures treated with isoniazid presents with peripheral neuropathy, stomatitis, and cheilosis. Laboratory studies reveal a microcytic anemia. A deficiency of a specific vitamin is suspected, which is known to impair numerous metabolic reactions, including the synthesis and catabolism of amino acids.
A deficiency of the active form of which vitamin would impair all aminotransferase reactions?
Explanation: Pyridoxal phosphate (PLP), the active form of vitamin B6 (pyridoxine), is an essential cofactor for all aminotransferase (transaminase) enzymes. These enzymes are crucial for moving amino groups between amino acids and ketoacids, a central process in amino acid synthesis and degradation. Isoniazid can cause a functional deficiency of vitamin B6, leading to the observed symptoms of neuropathy and anemia (due to its role in heme synthesis). Thiamine is a cofactor for dehydrogenase enzymes, while folate and cobalamin are involved in one-carbon metabolism.
A 7-month-old infant presents with failure to thrive and progressive liver disease. Physical exam shows jaundice and significant hepatomegaly. Laboratory studies show elevated liver transaminases and evidence of renal tubular dysfunction (Fanconi syndrome). The infant's urine has a characteristic 'boiled cabbage' odor.
This severe disorder of tyrosine metabolism is caused by a deficiency of which enzyme?
Explanation: This presentation is classic for hereditary tyrosinemia type I, the most severe form of tyrosinemia. It is caused by a deficiency of fumarylacetoacetate hydrolase, the final enzyme in the tyrosine degradation pathway. This leads to the accumulation of fumarylacetoacetate and its toxic metabolite, succinylacetone, which causes severe liver and kidney damage. The boiled cabbage odor is due to the accumulation of these metabolites.
A newborn is diagnosed with a urea cycle disorder characterized by severe hyperammonemia. A liver biopsy is performed for enzymatic analysis. The pathologist notes that the urea cycle is a compartmentalized pathway, with some reactions occurring in the mitochondria and others in the cytosol.
Which of the following steps of the urea cycle occurs within the mitochondrial matrix?
Explanation: The first two steps of the urea cycle occur in the mitochondrial matrix. These are: 1) the synthesis of carbamoyl phosphate from ammonia (NH3) and bicarbonate (HCO3-) by carbamoyl phosphate synthetase I (CPS I), and 2) the synthesis of citrulline from ornithine and carbamoyl phosphate by ornithine transcarbamylase (OTC). The remaining steps, including the synthesis of argininosuccinate, cleavage of argininosuccinate, and cleavage of arginine to form urea, all take place in the cytosol.
A 3-year-old child with a rare metabolic disorder that prevents the conversion of pyruvate to acetyl-CoA is placed on a specific diet to prevent life-threatening lactic acidosis. The diet must provide energy substrates that bypass the defective enzymatic step.
Metabolism of which of the following amino acids would generate energy in the form of acetyl-CoA without producing pyruvate, making it a suitable component of this patient's diet?
Explanation: The patient likely has pyruvate dehydrogenase complex deficiency. In this condition, substrates that are metabolized to pyruvate (like carbohydrates and glucogenic amino acids) will worsen lactic acidosis. Therefore, a ketogenic diet is recommended. Lysine and Leucine are the only purely ketogenic amino acids, meaning their carbon skeletons are degraded exclusively to acetyl-CoA or acetoacetate. This provides an energy source for the TCA cycle or ketone body synthesis without forming pyruvate. Alanine, glycine, and cysteine are all glucogenic, meaning their catabolism yields pyruvate or TCA cycle intermediates that can be converted to glucose, and would thus exacerbate the patient's condition.
During periods of catabolism, such as prolonged fasting or after major trauma, amino acids are broken down in peripheral tissues, releasing ammonia. Because free ammonia is toxic to the central nervous system, it must be safely transported in the blood to the liver for detoxification.
Which of the following amino acids is the principal carrier of nitrogen from most peripheral tissues to the liver and can carry two nitrogen atoms per molecule?
Explanation: Glutamine is the major, non-toxic carrier of ammonia in the blood. In peripheral tissues, glutamine synthetase combines free ammonia with glutamate to form glutamine. This reaction fixes a molecule of toxic ammonia. Glutamine carries two nitrogen atoms (one from glutamate's amino group and one in the amide group) to the liver. In the liver, glutaminase releases the amide nitrogen as ammonia, which enters the urea cycle. Alanine is another important carrier, primarily from muscle (glucose-alanine cycle), but it only carries one nitrogen atom. Aspartate and arginine are urea cycle intermediates but not primary circulatory transporters of nitrogen from the periphery.