A 3-day-old female infant presents with poor feeding, lethargy, vomiting after feeding, and seizures. Labs revealed ketoacidosis and elevated hydroxypropionic acid levels. Upon administration of parenteral glucose and protein devoid of valine, isoleucine, methionine, and threonine, and carnitine, the infant began to recover. Which of the following enzymes is most likely deficient in this infant?
Q2
A 29-year-old female presents to the family physician concerned over the blue-black discoloration of her sclera and skin. She notes that at the end of her day, there are black stains in her underwear. The incomplete breakdown of which of the following amino acids is responsible for this presentation?
Q3
You are counseling a mother whose newborn has just screened positive for a deficit of phenylalanine hydroxylase enzyme. You inform her that her child will require dietary supplementation of which of the following?
Q4
A 4-day-old boy is brought to the physician by his mother because of vomiting, irritability, and poor feeding. Pregnancy and delivery were uncomplicated. Physical examination shows increased muscle tone in all extremities. He appears lethargic. His diapers emit a caramel-like odor. Urine studies are positive for ketone bodies. Supplementation of which of the following is most likely to improve this patient's condition?
Q5
A 2-day-old newborn boy is brought to the emergency department because of apnea, cyanosis, and seizures. He is severely hypoglycemic and does not improve with glucagon administration. His blood pressure is 100/62 mm Hg and heart rate is 75/min. Blood tests show high lactate levels. Physical examination is notable for hepatomegaly. Which of the following enzymes is most likely to be deficient in this baby?
Q6
A 2-month-old boy is brought to his pediatrician’s office to be evaluated for new onset seizures and poor weight gain. The patient’s father says he is unable to track with his eyes and is unresponsive to verbal stimuli. The patient is hypotonic on physical exam. Further studies show elevated serum lactate levels and elevated levels of alanine and pyruvate. Family history reveals that several distant family members suffered from neurological diseases and died of unknown causes at a young age. Which of the following amino acids should be increased in this patient’s diet?
Q7
An 8-day-old boy is brought to the physician by his mother because of vomiting and poor feeding. The pregnancy was uncomplicated, and he was born at full term. He appears pale and lethargic. Physical examination shows diffusely increased muscle tone. His urine is noted to have a sweet odor. This patient's symptoms are most likely caused by the accumulation of which of the following?
Q8
A 9-month-old infant presents to your office for a check-up. Exam reveals developmental delay, microcephaly, and a mousy odor to his breath. You should be concerned that the infant may have which of the following?
Q9
A 7-year-old boy is brought to the emergency department because of sudden-onset abdominal pain that began 1 hour ago. Three days ago, he was diagnosed with a urinary tract infection and was treated with nitrofurantoin. There is no personal history of serious illness. His parents emigrated from Kenya before he was born. Examination shows diffuse abdominal tenderness, mild splenomegaly, and scleral icterus. Laboratory studies show:
Hemoglobin 9.8 g/dL
Mean corpuscular volume 88 μm3
Reticulocyte count 3.1%
Serum
Bilirubin
Total 3.8 mg/dL
Direct 0.6 mg/dL
Haptoglobin 16 mg/dL (N=41–165 mg/dL)
Lactate dehydrogenase 179 U/L
Which of the following is the most likely underlying cause of this patient's symptoms?
Q10
A 5 year old child was brought to the physician with a history of black urine. There is no history of fever or any other complaints. There is no growth retardation and all the developmental milestones are normal. The child is suspected to have an enzyme defect for metabolism of an aromatic amino acid. What is the enzyme deficient
Branched-chain amino acid metabolism US Medical PG Practice Questions and MCQs
Question 1: A 3-day-old female infant presents with poor feeding, lethargy, vomiting after feeding, and seizures. Labs revealed ketoacidosis and elevated hydroxypropionic acid levels. Upon administration of parenteral glucose and protein devoid of valine, isoleucine, methionine, and threonine, and carnitine, the infant began to recover. Which of the following enzymes is most likely deficient in this infant?
A. Branched-chain ketoacid dehydrogenase
B. Propionyl-CoA carboxylase (Correct Answer)
C. Cystathionine synthase
D. Phenylalanine hydroxylase
E. Homogentisate oxidase
Explanation: ***Propionyl-CoA carboxylase***
- The presence of **ketoacidosis** and elevated **hydroxypropionic acid** levels is characteristic of propionic acidemia, which is caused by a deficiency in **propionyl-CoA carboxylase**.
- The therapeutic benefit from a diet restricted in **valine, methionine, threonine**, and **isoleucine** (precursors of propionyl-CoA) along with carnitine supplementation further supports this diagnosis.
*Branched-chain ketoacid dehydrogenase*
- A deficiency in this enzyme leads to **Maple Syrup Urine Disease**, characterized by elevated **branched-chain ketoacids** and associated with a distinctive sweet odor in urine.
- While it causes neurotoxicity and poor feeding, the specific finding of elevated **hydroxypropionic acid** points away from this diagnosis.
*Cystathionine synthase*
- Deficiency in **cystathionine synthase** causes **homocystinuria**, leading to elevated **homocysteine** levels.
- Symptoms include developmental delay, ectopia lentis, and skeletal abnormalities, but not typically elevated **hydroxypropionic acid** or severe neonatal ketoacidosis in this manner.
*Phenylalanine hydroxylase*
- This enzyme is deficient in **phenylketonuria (PKU)**, resulting in high levels of **phenylalanine** and its metabolites.
- PKU is typically associated with intellectual disability, seizures, and a musty odor, but not ketoacidosis or elevated **hydroxypropionic acid**.
*Homogentisate oxidase*
- A deficiency in this enzyme causes **alkaptonuria**, characterized by the accumulation of **homogentisic acid**.
- This condition is usually benign in infancy, primarily manifesting as dark urine upon standing and later developing into ochronosis and arthritis, without acute neonatal ketoacidosis or elevated **hydroxypropionic acid**.
Question 2: A 29-year-old female presents to the family physician concerned over the blue-black discoloration of her sclera and skin. She notes that at the end of her day, there are black stains in her underwear. The incomplete breakdown of which of the following amino acids is responsible for this presentation?
A. Leucine
B. Valine
C. Tyrosine (Correct Answer)
D. Methionine
E. Isoleucine
Explanation: ***Tyrosine***
- The symptoms described (blue-black **scleral and skin discoloration**, and dark urine staining underwear) are classic for **alkaptonuria**.
- Alkaptonuria results from a deficiency of **homogentisate oxidase**, an enzyme in the catabolic pathway of **tyrosine**, which leads to the accumulation of **homogentisic acid**.
*Leucine*
- Leucine is a **branched-chain amino acid**; disorders in its metabolism are typically associated with conditions like **Maple Syrup Urine Disease**.
- **Maple Syrup Urine Disease** presents with neurological symptoms, poor feeding, and a characteristic odor, not blue-black discoloration.
*Valine*
- Valine is another **branched-chain amino acid**, and its metabolic defects are also associated with conditions like **Maple Syrup Urine Disease**.
- Its accumulation does not lead to the specific pigmentary changes seen in alkaptonuria.
*Methionine*
- Methionine metabolism disorders include conditions such as **homocystinuria**.
- **Homocystinuria** is characterized by skeletal abnormalities, ocular problems, and intellectual disability, not blue-black discoloration.
*Isoleucine*
- Isoleucine is the third **branched-chain amino acid** and is also implicated in **Maple Syrup Urine Disease** when its metabolism is impaired.
- Clinical features of isoleucine metabolism defects do not include the blue-black pigmentation described.
Question 3: You are counseling a mother whose newborn has just screened positive for a deficit of phenylalanine hydroxylase enzyme. You inform her that her child will require dietary supplementation of which of the following?
A. Aspartame
B. Niacin
C. Homogentisic Acid
D. Tyrosine (Correct Answer)
E. Leucine
Explanation: ***Tyrosine***
- A deficit of **phenylalanine hydroxylase** prevents the conversion of phenylalanine to tyrosine, making **tyrosine** an essential amino acid that must be supplemented.
- Dietary restriction of **phenylalanine** is also crucial to prevent the accumulation of toxic byproducts that can cause severe neurological damage.
*Aspartame*
- **Aspartame** is an artificial sweetener that contains **phenylalanine**, which would be harmful for a child with phenylalanine hydroxylase deficiency.
- Consumption of aspartame would increase the body's phenylalanine load, exacerbating the metabolic disorder.
*Niacin*
- **Niacin** (vitamin B3) is a vitamin and its supplementation is not related to the phenylalanine hydroxylase pathway or its deficiency.
- Deficiency of niacin is associated with **pellagra**, characterized by dermatitis, diarrhea, and dementia.
*Homogentisic Acid*
- **Homogentisic acid** is an intermediate in the metabolism of tyrosine, and its accumulation is characteristic of **alkaptonuria**, a different metabolic disorder.
- It is not a therapeutic supplement for phenylalanine hydroxylase deficiency.
*Leucine*
- **Leucine** is a branched-chain amino acid, and its metabolism is unrelated to phenylalanine hydroxylase deficiency.
- Supplemental leucine is not required in this condition and would not address the metabolic defect.
Question 4: A 4-day-old boy is brought to the physician by his mother because of vomiting, irritability, and poor feeding. Pregnancy and delivery were uncomplicated. Physical examination shows increased muscle tone in all extremities. He appears lethargic. His diapers emit a caramel-like odor. Urine studies are positive for ketone bodies. Supplementation of which of the following is most likely to improve this patient's condition?
A. Tyrosine
B. Cysteine
C. Tetrahydrobiopterin
D. Thiamine (Correct Answer)
E. Leucine
Explanation: ***Thiamine***
- This clinical scenario describes an infant with **vomiting, irritability, poor feeding, increased muscle tone, lethargy, and a caramel-like odor in the urine**, along with **ketone bodies in the urine**. These are classic signs of **Maple Syrup Urine Disease (MSUD)**.
- MSUD is caused by a deficiency in the **branched-chain alpha-keto acid dehydrogenase complex (BCKDC)**, an enzyme that requires **thiamine** as a cofactor. In some cases, high-dose thiamine supplementation can improve enzyme activity and alleviate symptoms.
*Tyrosine*
- **Tyrosine** is an amino acid. Disorders of tyrosine metabolism, such as tyrosinemia, present with liver dysfunction, renal tubulopathy, and neurological crises, but typically do not involve a caramel-like odor or respond to thiamine.
- Tyrosine supplementation is not indicated for MSUD and would not improve the patient's condition.
*Cysteine*
- **Cysteine** is a sulfur-containing amino acid. Disorders involving cysteine metabolism, like homocystinuria, are characterized by ocular, skeletal, vascular, and neurological abnormalities.
- There is no direct role for cysteine supplementation in treating MSUD, and it would not address the underlying enzymatic defect.
*Tetrahydrobiopterin*
- **Tetrahydrobiopterin (BH4)** is a cofactor for several enzymes involved in amino acid metabolism, particularly in the synthesis of neurotransmitters and the metabolism of phenylalanine.
- BH4 deficiency causes phenylketonuria-like symptoms or other neurological issues, but not MSUD, and it would not address the BCKDC deficiency.
*Leucine*
- **Leucine** is one of the branched-chain amino acids (BCAAs) that accumulate in MSUD due to the defective BCKDC enzyme.
- Supplementing leucine would worsen the patient's condition by increasing the toxic BCAA levels, as the body cannot properly metabolize it.
Question 5: A 2-day-old newborn boy is brought to the emergency department because of apnea, cyanosis, and seizures. He is severely hypoglycemic and does not improve with glucagon administration. His blood pressure is 100/62 mm Hg and heart rate is 75/min. Blood tests show high lactate levels. Physical examination is notable for hepatomegaly. Which of the following enzymes is most likely to be deficient in this baby?
A. α-ketoacid dehydrogenase
B. Phenylalanine hydroxylase
C. Glucose-6-phosphatase (Correct Answer)
D. Glucocerebrosidase
E. Sphingomyelinase
Explanation: ***Correct: Glucose-6-phosphatase***
- The presentation of severe **hypoglycemia** not responsive to glucagon, coupled with **hepatomegaly** and **lactic acidosis** in a neonate, is highly suggestive of **Type I glycogen storage disease (von Gierke disease)**.
- Deficiency of **glucose-6-phosphatase** prevents the liver from releasing glucose into the bloodstream (the final step of both gluconeogenesis and glycogenolysis), leading to profound hypoglycemia.
- **Key diagnostic clue**: Lack of response to glucagon occurs because glucagon stimulates glycogenolysis, but without functional glucose-6-phosphatase, glucose-6-phosphate cannot be converted to free glucose for release.
- Accumulated glucose-6-phosphate shunts to glycolysis, producing **lactate** (lactic acidosis), and to glycogen synthesis, causing **hepatomegaly**.
*Incorrect: α-ketoacid dehydrogenase*
- Deficiency of **branched-chain α-ketoacid dehydrogenase** causes **maple syrup urine disease (MSUD)**, which presents with poor feeding, vomiting, lethargy, and a characteristic maple syrup odor in urine.
- While MSUD can cause neurological symptoms and seizures, **severe hypoglycemia unresponsive to glucagon** and **hepatomegaly** as primary features are not typical.
*Incorrect: Phenylalanine hydroxylase*
- Deficiency in **phenylalanine hydroxylase** causes **phenylketonuria (PKU)**, which is primarily characterized by intellectual disability, seizures (if untreated), and a musty odor, usually manifesting later in infancy.
- PKU does not present with acute neonatal hypoglycemia, lactic acidosis, or hepatomegaly.
*Incorrect: Glucocerebrosidase*
- Deficiency in **glucocerebrosidase** leads to **Gaucher disease**, a lysosomal storage disorder characterized by hepatosplenomegaly, bone crises, and neurological symptoms in severe infantile forms.
- While hepatomegaly may be present, Gaucher disease does not cause acute, severe neonatal hypoglycemia, lactic acidosis, or lack of response to glucagon.
*Incorrect: Sphingomyelinase*
- Deficiency in **sphingomyelinase** causes **Niemann-Pick disease**, another lysosomal storage disorder, which typically presents with hepatosplenomegaly, neurological deterioration, and "cherry-red spots" in the retina.
- This condition does not cause acute neonatal hypoglycemia, lactic acidosis, or glucagon unresponsiveness.
Question 6: A 2-month-old boy is brought to his pediatrician’s office to be evaluated for new onset seizures and poor weight gain. The patient’s father says he is unable to track with his eyes and is unresponsive to verbal stimuli. The patient is hypotonic on physical exam. Further studies show elevated serum lactate levels and elevated levels of alanine and pyruvate. Family history reveals that several distant family members suffered from neurological diseases and died of unknown causes at a young age. Which of the following amino acids should be increased in this patient’s diet?
A. Isoleucine
B. Valine
C. Arginine (Correct Answer)
D. Leucine
E. Methionine
Explanation: ***Arginine***
- The symptoms of **seizures, developmental delay, hypotonia, and elevated lactate/pyruvate** levels, along with a family history of early deaths, are highly suggestive of a **mitochondrial disorder**, specifically **pyruvate dehydrogenase complex (PDC) deficiency**.
- **Arginine** supplementation is used in certain **mitochondrial disorders** to enhance **nitric oxide (NO) production**, which improves **endothelial function and tissue perfusion**. While the primary treatment for PDC deficiency is a **ketogenic diet** and cofactor supplementation (thiamine, lipoic acid), among the amino acid options listed, arginine has supportive evidence in mitochondrial cytopathies.
- Arginine may help counteract **endothelial dysfunction** and improve oxygen delivery to tissues affected by mitochondrial impairment.
*Isoleucine*
- **Isoleucine** is a **branched-chain amino acid (BCAA)** that is catabolized to produce acetyl-CoA and succinyl-CoA for energy.
- In **PDC deficiency**, the problem is the conversion of **pyruvate to acetyl-CoA**, not BCAA metabolism. Increasing BCAAs is not a therapeutic strategy for this condition.
- Excessive BCAA intake could potentially worsen metabolic imbalances without addressing the underlying enzymatic defect.
*Valine*
- **Valine** is another **branched-chain amino acid (BCAA)** with no specific role in managing PDC deficiency.
- Like isoleucine, valine supplementation does not address the **pyruvate dehydrogenase complex defect** and is not part of standard dietary management.
- BCAAs require careful monitoring in metabolic disorders and are not indicated for mitochondrial disorders like PDC deficiency.
*Leucine*
- **Leucine** is a **branched-chain amino acid (BCAA)** and potent activator of mTOR signaling for protein synthesis.
- Direct leucine supplementation is **not indicated for PDC deficiency** and does not address the metabolic block at the pyruvate dehydrogenase step.
- In disorders like **maple syrup urine disease (MSUD)**, BCAAs including leucine must be **restricted**, not supplemented.
*Methionine*
- **Methionine** is a sulfur-containing amino acid important for **methylation reactions** through S-adenosylmethionine (SAM) synthesis.
- There is **no established role** for methionine supplementation in **pyruvate dehydrogenase complex deficiency** or mitochondrial disorders presenting with lactic acidosis.
- Methionine metabolism is not directly related to the pathophysiology of PDC deficiency.
Question 7: An 8-day-old boy is brought to the physician by his mother because of vomiting and poor feeding. The pregnancy was uncomplicated, and he was born at full term. He appears pale and lethargic. Physical examination shows diffusely increased muscle tone. His urine is noted to have a sweet odor. This patient's symptoms are most likely caused by the accumulation of which of the following?
A. Phenylalanine
B. Homogentisic acid
C. Isoleucine (Correct Answer)
D. Homocysteine
E. Phytanic acid
Explanation: ***Isoleucine***
- The combination of **vomiting**, poor feeding, lethargy, **increased muscle tone**, and a **sweet-smelling urine** (often described as maple syrup odor) in a neonate strongly points to **Maple Syrup Urine Disease (MSUD)**.
- MSUD is caused by a defect in the **branched-chain alpha-keto acid dehydrogenase complex**, leading to the accumulation of branched-chain amino acids (leucine, isoleucine, and valine) and their corresponding alpha-keto acids.
*Phenylalanine*
- Accumulation of **phenylalanine** is characteristic of **Phenylketonuria (PKU)**, which typically presents with intellectual disability, seizures, and an eczematous rash if untreated, but not a maple syrup odor in urine or acute neonatal crisis.
- The urine odor in PKU is often described as **mousy** or musty, distinctly different from a sweet or maple syrup odor.
*Homogentisic acid*
- Accumulation of **homogentisic acid** is seen in **Alkaptonuria**, an inborn error of metabolism that primarily causes dark urine upon standing (due to oxidation of homogentisic acid), ochronosis (blue-black pigmentation of cartilage and connective tissue later in life), and arthritis.
- It does not present with acute neonatal symptoms such as vomiting, lethargy, or a sweet urine odor.
*Homocysteine*
- Elevated levels of **homocysteine** are found in **Homocystinuria**, which can lead to intellectual disability, developmental delay, dislocation of the ocular lens (ectopia lentis), Marfanoid habitus, and thromboembolic events.
- It does not typically present in the neonatal period with a sweet urine odor or acute neurological symptoms like increased muscle tone.
*Phytanic acid*
- Accumulation of **phytanic acid** is characteristic of **Refsum disease**, a rare peroxisomal disorder that causes progressive neurological symptoms such as retinitis pigmentosa, peripheral neuropathy, ataxia, and deafness in childhood or adulthood.
- It does not present in the neonatal period with the described acute symptoms or unique urine odor.
Question 8: A 9-month-old infant presents to your office for a check-up. Exam reveals developmental delay, microcephaly, and a mousy odor to his breath. You should be concerned that the infant may have which of the following?
A. Excess tetrahydrobiopterin cofactor
B. Deficit of porphobilinogen deaminase activity
C. Deficit of tyrosine hydroxylase activity
D. Excess phenylalanine hydroxylase activity
E. Deficit of phenylalanine hydroxylase activity (Correct Answer)
Explanation: ***Deficit of phenylalanine hydroxylase activity***
- The combination of **developmental delay**, **microcephaly**, and a **mousy odor** is characteristic of **phenylketonuria (PKU)**.
- PKU is caused by a deficient **phenylalanine hydroxylase** enzyme, leading to a buildup of phenylalanine and its metabolites, which are toxic to the developing brain.
*Excess tetrahydrobiopterin cofactor*
- This condition (**BH4 excess**) is rare and does not typically present with the classic signs of PKU; rather, it often involves neurological symptoms due to other metabolic imbalances.
- An excess of the BH4 cofactor would theoretically enhance rather than inhibit phenylalanine hydroxylase activity, if the enzyme itself were functional.
*Deficit of porphobilinogen deaminase activity*
- A deficit in **porphobilinogen deaminase** is associated with **Acute Intermittent Porphyria (AIP)**, which presents with acute neurovisceral attacks.
- Symptoms of AIP include severe abdominal pain, psychiatric disturbances, and neurological deficits, but not developmental delay or a mousy odor.
*Deficit of tyrosine hydroxylase activity*
- A deficiency in **tyrosine hydroxylase** affects the synthesis of **dopamine** and other catecholamines, leading to neurological disorders, including **dystonia** and **Parkinsonian symptoms**.
- While it can cause developmental delay, it does not typically present with a mousy odor or microcephaly, and its primary symptoms relate to motor control.
*Excess phenylalanine hydroxylase activity*
- An **excess** of phenylalanine hydroxylase activity would lead to increased breakdown of phenylalanine, preventing its buildup.
- This would not cause the symptoms described; instead, it would likely result in lower-than-normal phenylalanine levels, which is generally not problematic.
Question 9: A 7-year-old boy is brought to the emergency department because of sudden-onset abdominal pain that began 1 hour ago. Three days ago, he was diagnosed with a urinary tract infection and was treated with nitrofurantoin. There is no personal history of serious illness. His parents emigrated from Kenya before he was born. Examination shows diffuse abdominal tenderness, mild splenomegaly, and scleral icterus. Laboratory studies show:
Hemoglobin 9.8 g/dL
Mean corpuscular volume 88 μm3
Reticulocyte count 3.1%
Serum
Bilirubin
Total 3.8 mg/dL
Direct 0.6 mg/dL
Haptoglobin 16 mg/dL (N=41–165 mg/dL)
Lactate dehydrogenase 179 U/L
Which of the following is the most likely underlying cause of this patient's symptoms?
A. Defective red blood cell membrane proteins
B. Lead poisoning
C. Defect in orotic acid metabolism
D. Absent hemoglobin beta chain
E. Enzyme deficiency in red blood cells (Correct Answer)
Explanation: ***Enzyme deficiency in red blood cells***
- The patient's symptoms (abdominal pain, scleral icterus, mild splenomegaly, anemia, elevated reticulocyte count, increased unconjugated bilirubin, low haptoglobin, and elevated LDH) are consistent with **hemolytic anemia**. The recent use of **nitrofurantoin**, an oxidative stressor, in a patient of African descent, strongly suggests a diagnosis of **Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency**.
- G6PD deficiency is an **X-linked recessive** inherited enzyme defect causing red blood cells to be susceptible to oxidative damage, leading to hemolysis when exposed to certain drugs (like nitrofurantoin) or infections.
*Defective red blood cell membrane proteins*
- This describes conditions like **hereditary spherocytosis** or **hereditary elliptocytosis**. While these cause hemolytic anemia, the acute onset triggered by a drug (nitrofurantoin) is less typical.
- Hereditary spherocytosis is characterized by **microspherocytes** on a peripheral smear and is usually diagnosed earlier in life or has a chronic course, often without an acute precipitating drug.
*Lead poisoning*
- Lead poisoning typically causes **microcytic anemia** with **basophilic stippling** and neurological symptoms, not the type of hemolytic anemia and jaundice described.
- It does not present as an acute hemolytic crisis triggered by an oxidative drug.
*Defect in orotic acid metabolism*
- This can lead to conditions like **hereditary orotic aciduria**, which presents with **megaloblastic anemia** (without B12 or folate deficiency) and developmental delay.
- It is not associated with acute hemolytic episodes triggered by oxidative drugs or the specific lab findings seen here.
*Absent hemoglobin beta chain*
- This refers to **beta-thalassemia major**, which causes **microcytic hypochromic anemia** that is typically chronic and presents early in childhood with severe anemia requiring regular transfusions.
- Beta-thalassemia does not present as an acute hemolytic crisis triggered by nitrofurantoin, and the MCV in this patient is normal (88 μm³), not microcytic.
Question 10: A 5 year old child was brought to the physician with a history of black urine. There is no history of fever or any other complaints. There is no growth retardation and all the developmental milestones are normal. The child is suspected to have an enzyme defect for metabolism of an aromatic amino acid. What is the enzyme deficient
A. Homogentisate dehydrogenase
B. Homogentistae oxidase (Correct Answer)
C. Tyrosine Transaminase
D. Tryptophan Hydroxylase
E. Phenylalanine Hydroxylase
Explanation: ***Homogentistae oxidase***
- The presentation of a child with **black urine** (alkaptonuria) in the absence of other symptoms is characteristic of a deficiency in **homogentisate oxidase**.
- This enzyme is crucial in the catabolism of **tyrosine**, and its deficiency leads to the accumulation of **homogentisic acid**, which oxidizes upon exposure to air, turning urine black.
*Homogentisate dehydrogenase*
- This enzyme is not a recognized component of the **tyrosine degradation pathway** in humans.
- The correct enzyme involved in the breakdown of **homogentisate** is an oxidase, not a dehydrogenase, in this context.
*Tyrosine Transaminase*
- A deficiency in **tyrosine transaminase** (tyrosinemia type II) would lead to elevated tyrosine levels and typically presents with symptoms affecting the eyes, skin, and intellectual disability, not primarily black urine.
- This condition is characterized by **ocular findings** (corneal ulcers), **skin lesions**, and **neurological symptoms**.
*Tryptophan Hydroxylase*
- This enzyme is involved in the synthesis of **serotonin** and **melatonin** from tryptophan, a different amino acid pathway.
- A deficiency or abnormality in **tryptophan hydroxylase** would not cause black urine but could lead to neurological or mood disorders.
*Phenylalanine Hydroxylase*
- A deficiency in **phenylalanine hydroxylase** causes **phenylketonuria (PKU)**, which affects phenylalanine metabolism, not tyrosine metabolism directly.
- PKU typically presents with **intellectual disability**, **musty odor**, **fair skin**, and **seizures** if untreated, not black urine.
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