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ABMGG Clinical Biochemical Genetics study materials
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ABMGG Clinical Biochemical Genetics practice questions

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Question 1Clinical and laboratory diagnosis of inborn errors of metabolism

A newborn screening dried blood spot shows an elevated phenylalanine level with a normal tyrosine level. Which enzyme is most directly deficient in the most common form of phenylketonuria?

  1. Tyrosine aminotransferase
  2. Phenylalanine hydroxylase Correct answer.
  3. Homogentisate oxidase
  4. Dihydropteridine reductase

Explanation

Classic PKU results from deficient phenylalanine hydroxylase, which converts phenylalanine to tyrosine and requires tetrahydrobiopterin as a cofactor. Low tyrosine with high phenylalanine is the biochemical signature. Defects in cofactor recycling cause rarer variant forms.

Question 2Clinical and laboratory diagnosis of inborn errors of metabolism

A 3-day-old infant presents with poor feeding, vomiting, lethargy, and a urine odor resembling maple syrup. Plasma amino acids show markedly elevated branched-chain amino acids. Which enzyme is deficient?

  1. Methylmalonyl-CoA mutase
  2. Isovaleryl-CoA dehydrogenase
  3. Branched-chain alpha-ketoacid dehydrogenase Correct answer.
  4. Propionyl-CoA carboxylase

Explanation

Maple syrup urine disease is caused by deficiency of the branched-chain alpha-ketoacid dehydrogenase complex, producing accumulation of leucine, isoleucine, valine, and their ketoacids. Leucine elevation drives the encephalopathy.

Question 3Clinical and laboratory diagnosis of inborn errors of metabolism

A child has developmental delay, fair skin, and eczema. Blood shows elevated phenylalanine with a normal-to-low tyrosine. Which additional finding most specifically indicates a defect of tetrahydrobiopterin metabolism rather than phenylalanine hydroxylase deficiency?

  1. Elevated urinary neopterin with low biopterin Correct answer.
  2. Elevated urinary homogentisic acid
  3. Elevated plasma methionine
  4. Elevated urinary orotic acid

Explanation

Pterin profiling distinguishes cofactor defects from PAH deficiency. A pattern of elevated neopterin with low biopterin suggests a defect in biopterin synthesis, while elevated biopterin with low neopterin suggests DHPR deficiency. These patients also often have neurotransmitter deficiency.

Question 4Clinical and laboratory diagnosis of inborn errors of metabolism

A neonate develops severe hyperammonemia, respiratory alkalosis, and no urinary orotic acid excretion. Plasma citrulline is very low. Which enzyme deficiency is most likely?

  1. Arginase
  2. Carbamoyl phosphate synthetase I Correct answer.
  3. Argininosuccinate lyase
  4. Ornithine transcarbamylase

Explanation

Carbamoyl phosphate synthetase I deficiency blocks the first step of the urea cycle, so no carbamoyl phosphate is made and orotic acid is not overproduced. Citrulline is low because downstream intermediates are not formed. OTC deficiency also gives low citrulline but with elevated urinary orotic acid.

Question 5Clinical and laboratory diagnosis of inborn errors of metabolism

A patient with recurrent metabolic acidosis, hyperammonemia, and elevated propionylcarnitine (C3) on acylcarnitine profile is suspected of propionic acidemia. Which secondary laboratory abnormality is most characteristic?

  1. Elevated isovalerylglycine
  2. Elevated succinylacetone
  3. Elevated methylmalonic acid
  4. Elevated 3-hydroxypropionic acid and methylcitrate Correct answer.

Explanation

Propionyl-CoA carboxylase deficiency leads to accumulation of propionyl-CoA, which is converted to 3-hydroxypropionic acid and methylcitrate, and these are characteristic markers. Methylmalonic acid is not elevated in isolated propionic acidemia.

Question 6Clinical and laboratory diagnosis of inborn errors of metabolism

A young child presents with hypoglycemia, hepatomegaly, and lactic acidosis that worsens with fasting. Acylcarnitine and urine organic acids are unremarkable, but liver biopsy shows glycogen accumulation. Which enzyme deficiency best fits?

  1. Acid alpha-glucosidase
  2. Glucose-6-phosphatase Correct answer.
  3. Debranching enzyme
  4. Medium-chain acyl-CoA dehydrogenase

Explanation

Glucose-6-phosphatase deficiency (von Gierke disease) impairs both gluconeogenesis and glycogenolysis, causing fasting hypoglycemia, lactic acidosis, and hepatomegaly. Glycogen accumulates because glucose-6-phosphate cannot be released as glucose.

Question 7Clinical and laboratory diagnosis of inborn errors of metabolism

A newborn with a positive screen for medium-chain acyl-CoA dehydrogenase deficiency has an elevated octanoylcarnitine (C8) level. Which clinical feature is most typical of this disorder during an acute crisis?

  1. Severe lactic acidosis with normal glucose
  2. Hyperketotic hypoglycemia
  3. Persistent hyperglycemia
  4. Hypoketotic hypoglycemia Correct answer.

Explanation

Impaired fatty acid oxidation limits ketone production, so fasting or illness produces hypoglycemia with inappropriately low ketones. This hypoketotic hypoglycemia is the hallmark of MCAD deficiency.

Question 8Clinical and laboratory diagnosis of inborn errors of metabolism

A child has coarse facial features, developmental delay, and vacuolated lymphocytes. Enzyme testing confirms a lysosomal storage disorder. Which finding on urine oligosaccharide analysis is most consistent with alpha-mannosidosis?

  1. Elevated glycosaminoglycans with normal oligosaccharides
  2. Elevated sialic acid only
  3. Elevated mannose-rich oligosaccharides Correct answer.
  4. Elevated keratan sulfate

Explanation

Alpha-mannosidase deficiency impairs degradation of mannose-containing oligosaccharides, producing a characteristic mannose-rich oligosaccharide pattern on urine analysis. This distinguishes it from mucopolysaccharidoses and other storage disorders.

Question 9Clinical and laboratory diagnosis of inborn errors of metabolism

A 6-month-old presents with failure to thrive, developmental regression, cherry-red macula, and hepatosplenomegaly. Enzyme assay shows deficient beta-hexosaminidase A activity. Which disorder is this?

  1. Gaucher disease
  2. Niemann-Pick disease type A
  3. Tay-Sachs disease Correct answer.
  4. Fabry disease

Explanation

Tay-Sachs disease results from beta-hexosaminidase A deficiency with GM2 ganglioside accumulation, producing regression, an exaggerated startle, and a cherry-red macula. Hepatosplenomegaly is generally absent in Tay-Sachs, so its presence should prompt reconsideration, but the enzyme result is diagnostic.

Question 10Clinical and laboratory diagnosis of inborn errors of metabolism

A patient has angiokeratomas, acroparesthesias, hypohidrosis, and progressive renal failure. Which enzyme deficiency is responsible?

  1. Sphingomyelinase
  2. Beta-galactosidase
  3. Alpha-galactosidase A Correct answer.
  4. Glucocerebrosidase

Explanation

Fabry disease is an X-linked disorder caused by alpha-galactosidase A deficiency, leading to globotriaosylceramide accumulation. The classic triad includes angiokeratomas, acroparesthesias, and hypohidrosis, with renal and cardiac involvement.

Question 11Clinical and laboratory diagnosis of inborn errors of metabolism

A newborn screening result shows elevated immunoreactive trypsinogen with a positive sweat chloride test. Which diagnosis is most consistent?

  1. Hereditary fructose intolerance
  2. Phenylketonuria
  3. Galactosemia
  4. Cystic fibrosis Correct answer.

Explanation

Elevated immunoreactive trypsinogen is the newborn screening marker for cystic fibrosis, and an elevated sweat chloride confirms the diagnosis. The combination is specific for CF among these options.

Question 12Clinical and laboratory diagnosis of inborn errors of metabolism

An infant with galactosemia is being managed. Which dietary component must be strictly restricted to prevent acute toxicity?

  1. Methionine
  2. Phenylalanine
  3. Lactose and galactose Correct answer.
  4. Fructose

Explanation

Classic galactosemia due to GALT deficiency requires elimination of galactose and lactose from the diet. Continued exposure causes hepatic injury, cataracts, and other complications.

Question 13Clinical and laboratory diagnosis of inborn errors of metabolism

A child with homocystinuria due to cystathionine beta-synthase deficiency is being treated. Which combination of therapies is most appropriate?

  1. Dietary phenylalanine restriction and tetrahydrobiopterin
  2. Low-galactose diet and calcium supplementation
  3. Carnitine and a high-fat diet
  4. Dietary methionine restriction, pyridoxine, and betaine Correct answer.

Explanation

Cystathionine beta-synthase deficiency is treated by restricting methionine, supplementing pyridoxine in responsive patients, and using betaine to remethylate homocysteine to methionine. This lowers homocysteine and reduces vascular and lens complications.

Question 14Clinical and laboratory diagnosis of inborn errors of metabolism

A patient with tyrosinemia type I is monitored during treatment. Which analyte is the most sensitive marker of residual disease activity and risk of hepatocellular carcinoma?

  1. Orotic acid
  2. Homogentisic acid
  3. Phenylalanine
  4. Succinylacetone Correct answer.

Explanation

Succinylacetone is the pathognomonic and most sensitive marker of tyrosinemia type I, reflecting fumarylacetoacetate hydrolase deficiency. Persistent elevation correlates with ongoing liver injury and hepatocellular carcinoma risk.

Question 15Clinical and laboratory diagnosis of inborn errors of metabolism

A neonate has ambiguous genitalia, severe salt wasting, and hyperkalemia. Which enzyme deficiency is most likely?

  1. 21-hydroxylase Correct answer.
  2. Aldosterone synthase
  3. 11-beta-hydroxylase
  4. 17-alpha-hydroxylase

Explanation

21-hydroxylase deficiency is the most common form of congenital adrenal hyperplasia and causes cortisol and aldosterone deficiency with androgen excess. The salt-wasting form presents with hyponatremia, hyperkalemia, and virilization in females.

Question 16Clinical and laboratory diagnosis of inborn errors of metabolism

A child has episodic ataxia, confusion, and hyperammonemia triggered by illness. Urine shows elevated orotic acid. Which enzyme is deficient?

  1. Ornithine transcarbamylase Correct answer.
  2. N-acetylglutamate synthase
  3. Carbamoyl phosphate synthetase I
  4. Argininosuccinate synthetase

Explanation

Ornithine transcarbamylase deficiency leads to carbamoyl phosphate accumulation, which is shunted into pyrimidine synthesis and appears as orotic aciduria. It is X-linked and can present with episodic hyperammonemia in heterozygous females.

Question 17Clinical and laboratory diagnosis of inborn errors of metabolism

A patient is evaluated for a fatty acid oxidation defect. Which analyte pattern on acylcarnitine profiling is most consistent with very long-chain acyl-CoA dehydrogenase deficiency?

  1. Elevated C8 acylcarnitine
  2. Elevated C5 acylcarnitine
  3. Elevated C14:1 and C14 acylcarnitines Correct answer.
  4. Elevated C3 acylcarnitine

Explanation

VLCAD deficiency produces elevated long-chain acylcarnitines, particularly C14:1 and C14 species. This pattern distinguishes it from medium- and short-chain defects.

Question 18Clinical and laboratory diagnosis of inborn errors of metabolism

A 2-year-old presents with acute liver failure, coagulopathy, and a Fanconi-like renal tubular dysfunction. Urine succinylacetone is elevated. Which enzyme is deficient?

  1. Phenylalanine hydroxylase
  2. Fumarylacetoacetate hydrolase Correct answer.
  3. Tyrosine aminotransferase
  4. Homogentisate oxidase

Explanation

Tyrosinemia type I is caused by fumarylacetoacetate hydrolase deficiency, with accumulation of succinylacetone, which is toxic to liver and kidney. It produces hepatic failure and renal tubular dysfunction.

Question 19Clinical and laboratory diagnosis of inborn errors of metabolism

A child with developmental delay and self-injurious behavior has elevated urinary uric acid and a striking absence of hypoxanthine-guanine phosphoribosyltransferase activity. Which disorder is this?

  1. Lesch-Nyhan syndrome Correct answer.
  2. Adenosine deaminase deficiency
  3. Xanthinuria
  4. Purine nucleoside phosphorylase deficiency

Explanation

Lesch-Nyhan syndrome is an X-linked disorder caused by HPRT deficiency, leading to uric acid overproduction and a neurologic phenotype with self-injury. Hyperuricemia and hyperuricosuria are characteristic.

Question 20Clinical and laboratory diagnosis of inborn errors of metabolism

A newborn is found to have elevated C3 acylcarnitine and elevated methylmalonic acid with low vitamin B12 levels. Which interpretation is most appropriate?

  1. This pattern may reflect nutritional vitamin B12 deficiency rather than an inherited enzyme defect Correct answer.
  2. This pattern indicates propionic acidemia
  3. This pattern is diagnostic of methylmalonic acidemia due to mutase deficiency
  4. This pattern indicates a fatty acid oxidation defect

Explanation

Vitamin B12 is a cofactor for methylmalonyl-CoA mutase, so nutritional deficiency can produce elevated methylmalonic acid and C3. Distinguishing acquired deficiency from inherited methylmalonic acidemia requires assessing B12 status and response to supplementation.

Question 21Clinical and laboratory diagnosis of inborn errors of metabolism

A patient with suspected biotinidase deficiency is being evaluated. Which combination of findings is most characteristic?

  1. Coarse facies and corneal clouding
  2. Angiokeratomas and renal failure
  3. Alopecia, skin rash, seizures, and elevated urinary 3-hydroxyisovaleric acid Correct answer.
  4. Cherry-red macula and hepatosplenomegaly

Explanation

Biotinidase deficiency impairs biotin recycling, causing alopecia, seborrheic dermatitis, seizures, and organic acid accumulation including 3-hydroxyisovaleric acid. It is treatable with biotin supplementation.

Question 22Clinical and laboratory diagnosis of inborn errors of metabolism

A child with a urea cycle disorder is in crisis with hyperammonemia. In addition to stopping protein intake and providing calories, which pharmacologic approach is used to promote nitrogen excretion?

  1. Betaine and pyridoxine
  2. Levodopa and carbidopa
  3. Sodium phenylbutyrate and sodium benzoate Correct answer.
  4. Nitisinone

Explanation

Sodium phenylbutyrate and sodium benzoate provide alternative nitrogen excretion pathways, conjugating waste nitrogen to phenylacetylglutamine and hippurate. They are used acutely and chronically in urea cycle disorders.

Question 23Clinical and laboratory diagnosis of inborn errors of metabolism

A 4-month-old with hypotonia, cardiomyopathy, and elevated creatine kinase has a muscle biopsy showing glycogen accumulation within lysosomes. Which enzyme deficiency is most likely?

  1. Phosphofructokinase
  2. Myophosphorylase
  3. Acid alpha-glucosidase Correct answer.
  4. Carnitine palmitoyltransferase II

Explanation

Pompe disease (glycogen storage disease type II) results from lysosomal acid alpha-glucosidase deficiency, producing glycogen accumulation in muscle and heart. Infantile onset presents with hypotonia and hypertrophic cardiomyopathy.

Question 24Clinical and laboratory diagnosis of inborn errors of metabolism

A patient is suspected of having a disorder of creatine metabolism. Which laboratory finding would most specifically support a creatine transporter defect?

  1. Elevated plasma methionine with low homocysteine
  2. Elevated urine creatine with normal plasma creatine and low brain creatine on spectroscopy Correct answer.
  3. Low plasma and urine creatine with elevated guanidinoacetate
  4. Elevated plasma guanidinoacetate with low creatine

Explanation

Creatine transporter deficiency (SLC6A8) shows increased urinary creatine with relatively normal plasma creatine because synthesis is intact but cellular uptake is impaired. Brain magnetic resonance spectroscopy shows reduced creatine.

Question 25Clinical and laboratory diagnosis of inborn errors of metabolism

A patient with a suspected mitochondrial fatty acid oxidation disorder has a normal acylcarnitine profile while well but becomes symptomatic during fasting. Which additional test is most appropriate to unmask the defect?

  1. A sweat chloride test
  2. A random plasma amino acid panel
  3. Serum ceruloplasmin
  4. A controlled fasting study with serial glucose, ketones, and acylcarnitines Correct answer.

Explanation

Fatty acid oxidation defects may be biochemically silent when the patient is well because flux through the pathway is low. A monitored fasting study can provoke the characteristic hypoketotic hypoglycemia and abnormal acylcarnitine pattern.

Question 26Clinical and laboratory diagnosis of inborn errors of metabolism

A newborn screening dried blood spot shows an elevated phenylalanine level. Which confirmatory test is most appropriate to distinguish classical phenylketonuria from a defect of tetrahydrobiopterin metabolism?

  1. Measurement of plasma homocysteine and methionine
  2. Measurement of the phenylalanine-to-tyrosine ratio in plasma
  3. Assay of urinary organic acids by gas chromatography-mass spectrometry
  4. Analysis of urinary pterins and a dihydropteridine reductase assay Correct answer.

Explanation

Tetrahydrobiopterin defects are distinguished from phenylalanine hydroxylase deficiency by measuring urinary biopterin and neopterin patterns and by assaying dihydropteridine reductase activity. The phenylalanine-to-tyrosine ratio supports the diagnosis of PKU but does not separate the cofactor disorders.

Question 27Clinical and laboratory diagnosis of inborn errors of metabolism

Which laboratory pattern is most characteristic of classic maple syrup urine disease in an acutely ill neonate?

  1. Elevated proline and hydroxyproline with normal branched-chain amino acids
  2. Low citrulline with elevated ammonia and orotic acid
  3. Elevated glycine with a low cerebrospinal fluid-to-plasma glycine ratio
  4. Elevated branched-chain amino acids with elevated alloisoleucine Correct answer.

Explanation

Maple syrup urine disease results from deficient branched-chain ketoacid dehydrogenase, producing accumulation of leucine, isoleucine, and valine, with alloisoleucine being a pathognomonic marker. The other patterns correspond to unrelated disorders of amino acid or urea cycle metabolism.

Question 28Clinical and laboratory diagnosis of inborn errors of metabolism

A child is evaluated for a peroxisomal disorder. Which combination of findings most strongly suggests X-linked adrenoleukodystrophy rather than a Zellweger spectrum disorder?

  1. Elevated very-long-chain fatty acids with normal plasmalogen synthesis and preserved peroxisome number Correct answer.
  2. Elevated very-long-chain fatty acids with deficient plasmalogen synthesis and absent peroxisomes
  3. Deficient plasmalogen synthesis with normal very-long-chain fatty acids
  4. Elevated phytanic acid with normal very-long-chain fatty acids

Explanation

X-linked adrenoleukodystrophy is a single-enzyme defect of very-long-chain fatty acid oxidation with intact peroxisome biogenesis, so plasmalogen synthesis and peroxisome number remain normal. Zellweger spectrum disorders impair peroxisome assembly, affecting multiple pathways.

Question 29Clinical and laboratory diagnosis of inborn errors of metabolism

A 3-day-old infant has profound lactic acidosis, a high anion gap, and a normal glucose. Plasma amino acids show elevated alanine and the urine organic acids show marked lactate and pyruvate elevation with a normal lactate-to-pyruvate ratio. Which diagnosis is most consistent?

  1. Pyruvate dehydrogenase deficiency
  2. Fructose-1,6-bisphosphatase deficiency
  3. Mitochondrial DNA depletion syndrome
  4. Pyruvate carboxylase deficiency Correct answer.

Explanation

Pyruvate carboxylase deficiency causes lactic acidosis with an elevated lactate-to-pyruvate ratio in the direction of pyruvate accumulation, often with elevated alanine and citrulline. A normal lactate-to-pyruvate ratio is more typical of a primary respiratory chain defect.

Question 30Clinical and laboratory diagnosis of inborn errors of metabolism

Which analyte is measured in dried blood spots to screen for classic galactosemia, and which secondary marker is commonly used to reduce false positives?

  1. Phenylalanine with tyrosine
  2. Total galactose with galactose-1-phosphate uridylyltransferase activity Correct answer.
  3. Immunoreactive trypsinogen with sweat chloride
  4. Succinylacetone with delta-aminolevulinic acid

Explanation

Newborn screening for galactosemia measures total galactose and, where available, galactose-1-phosphate uridylyltransferase activity to confirm the enzyme defect. The paired markers in the other options relate to different screening targets.

Question 31Clinical and laboratory diagnosis of inborn errors of metabolism

A child with suspected Lesch-Nyhan syndrome has hyperuricemia. Which enzyme assay would confirm the diagnosis?

  1. Xanthine oxidase activity in plasma
  2. Adenosine deaminase activity in erythrocytes
  3. Purine nucleoside phosphorylase activity in erythrocytes
  4. Hypoxanthine-guanine phosphoribosyltransferase activity in erythrocytes Correct answer.

Explanation

Lesch-Nyhan syndrome is caused by near-complete deficiency of hypoxanthine-guanine phosphoribosyltransferase, which can be measured in erythrocytes or fibroblasts. The other enzymes relate to different purine or immunodeficiency disorders.

Question 32Clinical and laboratory diagnosis of inborn errors of metabolism

A patient with a suspected disorder of fatty acid oxidation has a normal acylcarnitine profile when well. Which additional test is most likely to reveal the defect?

  1. Acylcarnitine profiling after a controlled fasting provocation Correct answer.
  2. Measurement of urinary glycosaminoglycans
  3. Measurement of plasma phytanic acid
  4. Repeat acylcarnitine profiling on a random fed sample

Explanation

Some fatty acid oxidation defects only manifest biochemically during fasting or metabolic stress, so a controlled fasting study can unmask the characteristic acylcarnitine and organic acid abnormalities. Random fed samples may be entirely normal.

Question 33Clinical and laboratory diagnosis of inborn errors of metabolism

A neonate has severe hyperammonemia, respiratory alkalosis, and no urinary orotic acid. Which enzyme deficiency is most likely?

  1. Carbamoyl phosphate synthetase I deficiency Correct answer.
  2. Argininosuccinate lyase deficiency
  3. Ornithine transcarbamylase deficiency
  4. Citrin deficiency

Explanation

Carbamoyl phosphate synthetase I deficiency is proximal to orotate production, so orotic acid remains low despite marked hyperammonemia. Ornithine transcarbamylase deficiency and distal urea cycle defects typically raise urinary orotic acid.

Question 34Clinical and laboratory diagnosis of inborn errors of metabolism

Which finding best supports a diagnosis of a congenital disorder of glycosylation type Ia (PMM2-CDG) on laboratory testing?

  1. Elevated urinary sialic acid with normal transferrin isoforms
  2. Abnormal transferrin isoelectric focusing with a type I pattern Correct answer.
  3. Elevated very-long-chain fatty acids
  4. Low plasma ceruloplasmin with elevated copper

Explanation

PMM2-CDG produces a characteristic type I transferrin isoelectric focusing pattern reflecting underglycosylation of transferrin. This test is the standard biochemical screen for the congenital disorders of glycosylation.

Question 35Clinical and laboratory diagnosis of inborn errors of metabolism

A child with recurrent vomiting, lethargy, and hypoglycemia is found to have elevated urinary ethylmalonic acid and elevated C4 and C5 acylcarnitines. Which diagnosis is most likely?

  1. Short-chain acyl-CoA dehydrogenase deficiency
  2. Glutaric acidemia type I
  3. Isovaleric acidemia
  4. Multiple acyl-CoA dehydrogenase deficiency Correct answer.

Explanation

Multiple acyl-CoA dehydrogenase deficiency produces a broad elevation of short- and medium-chain acylcarnitines with ethylmalonic acid excretion. The combined C4/C5 elevation with ethylmalonic acid is typical of this disorder.

Question 36Clinical and laboratory diagnosis of inborn errors of metabolism

Which test is the most direct way to confirm a diagnosis of cystinuria in a patient with recurrent kidney stones?

  1. 24-hour urine cystine excretion with stone analysis Correct answer.
  2. Plasma amino acid analysis alone
  3. Urinary glycosaminoglycan electrophoresis
  4. Serum cystine concentration

Explanation

Cystinuria is confirmed by demonstrating increased urinary cystine excretion, often with stone analysis showing cystine crystals. Serum levels are typically normal because the defect is in renal tubular reabsorption.

Question 37Clinical and laboratory diagnosis of inborn errors of metabolism

A patient has episodic rhabdomyolysis triggered by exercise and fasting, with elevated long-chain acylcarnitines and a low serum carnitine. Which disorder is most likely?

  1. Medium-chain acyl-CoA dehydrogenase deficiency
  2. Very-long-chain acyl-CoA dehydrogenase deficiency
  3. Carnitine palmitoyltransferase II deficiency Correct answer.
  4. Carnitine palmitoyltransferase I deficiency

Explanation

Adult-onset carnitine palmitoyltransferase II deficiency classically presents with exercise- and fasting-triggered rhabdomyolysis and elevated long-chain acylcarnitines with secondary carnitine depletion. The other disorders have different triggering patterns and acylcarnitine profiles.

Question 38Clinical and laboratory diagnosis of inborn errors of metabolism

Which laboratory finding is most specific for a diagnosis of alkaptonuria?

  1. Elevated plasma tyrosine
  2. Elevated urinary methylmalonic acid
  3. Elevated urinary homogentisic acid Correct answer.
  4. Elevated urinary orotic acid

Explanation

Alkaptonuria results from homogentisate 1,2-dioxygenase deficiency, causing homogentisic acid accumulation and excretion, which darkens urine on standing. The other analytes relate to different metabolic disorders.

Question 39Clinical and laboratory diagnosis of inborn errors of metabolism

A child is suspected of having a mitochondrial respiratory chain disorder. Which combination of tests best supports the diagnosis?

  1. Low ammonia with elevated plasma citrulline
  2. Elevated very-long-chain fatty acids with normal lactate
  3. Normal lactate with elevated urinary glycosaminoglycans
  4. Elevated lactate and pyruvate with an elevated lactate-to-pyruvate ratio and abnormal urine organic acids Correct answer.

Explanation

Respiratory chain defects typically produce lactic acidemia with an elevated lactate-to-pyruvate ratio reflecting impaired oxidative phosphorylation, along with abnormal organic acids such as Krebs cycle intermediates. This combination supports the diagnosis before enzyme or genetic confirmation.

Question 40Clinical and laboratory diagnosis of inborn errors of metabolism

A 5-year-old has developmental regression, vertical supranuclear gaze palsy, and hepatosplenomegaly. Which test is most likely to confirm Niemann-Pick disease type C?

  1. Measurement of plasma very-long-chain fatty acids
  2. Measurement of urinary oligosaccharides
  3. Assay of beta-glucocerebrosidase activity
  4. Cellular cholesterol esterification and filipin staining in cultured fibroblasts Correct answer.

Explanation

Niemann-Pick disease type C is a defect of intracellular cholesterol trafficking, best confirmed by demonstrating impaired cholesterol esterification and abnormal filipin staining in cultured fibroblasts. The other tests assess unrelated storage or peroxisomal disorders.

Question 41Biochemical and molecular testing methodologies (including newborn screening)

In standard newborn screening by tandem mass spectrometry, which analyte is the primary marker for phenylketonuria?

  1. Tyrosine
  2. Methionine
  3. Leucine
  4. Phenylalanine Correct answer.

Explanation

PKU screening relies on detecting elevated phenylalanine in dried blood spots, often evaluated as a phenylalanine-to-tyrosine ratio to improve specificity. Tyrosine is the downstream product and is typically low or normal in PKU.

Question 42Biochemical and molecular testing methodologies (including newborn screening)

Elevated immunoreactive trypsinogen (IRT) on newborn screening is the first-tier marker for which condition?

  1. Cystic fibrosis Correct answer.
  2. Galactosemia
  3. Congenital adrenal hyperplasia
  4. Congenital hypothyroidism

Explanation

IRT is elevated in newborns with cystic fibrosis and serves as the primary screening analyte, usually followed by DNA mutation panel testing or a second IRT. The other conditions use distinct markers such as TSH, 17-hydroxyprogesterone, or galactose metabolites.

Question 43Biochemical and molecular testing methodologies (including newborn screening)

Which laboratory method directly measures enzyme activity by monitoring the reduction of NAD+ to NADH at 340 nm?

  1. Spectrophotometric enzyme assay Correct answer.
  2. Ion-exchange chromatography
  3. Turbidimetric immunoassay
  4. Capillary electrophoresis

Explanation

NADH absorbs light at 340 nm while NAD+ does not, so continuous spectrophotometric assays follow enzyme reactions coupled to NADH production or consumption. The other techniques separate or bind analytes rather than measure this cofactor change.

Question 44Biochemical and molecular testing methodologies (including newborn screening)

A dried blood spot is analyzed by flow injection tandem mass spectrometry using stable isotope-labeled internal standards. What is the primary purpose of these internal standards?

  1. To correct for extraction and ionization variability Correct answer.
  2. To separate isobaric amino acids
  3. To ionize the analytes more efficiently
  4. To prevent carryover between samples

Explanation

Stable isotope-labeled analogs behave nearly identically to native analytes during extraction and ionization, allowing quantitative correction for variability. They do not improve ionization per se, separate compounds, or by themselves prevent carryover.

Question 45Biochemical and molecular testing methodologies (including newborn screening)

In Sanger sequencing, incorporation of a dideoxynucleotide causes chain termination because it lacks which feature?

  1. A 5'-phosphate group
  2. A nitrogenous base
  3. A 3'-hydroxyl group Correct answer.
  4. A phosphodiester backbone

Explanation

Dideoxynucleotides lack the 3'-OH needed for nucleophilic attack on the next incoming nucleotide, so extension stops. The 5'-phosphate and base are present, and the backbone exists up to the point of incorporation.

Question 46Biochemical and molecular testing methodologies (including newborn screening)

A quantitative PCR assay uses a standard curve of known copy numbers to determine the amount of a target gene. Which parameter of the amplification curve is plotted against the log of input quantity?

  1. Melting temperature
  2. Cycle threshold (Ct) Correct answer.
  3. Fluorescence plateau
  4. Amplicon length

Explanation

The cycle threshold, the cycle at which fluorescence crosses a fixed threshold, is inversely linear with the log of starting template. Melting temperature, plateau height, and amplicon length are not used to build the quantification curve.

Question 47Biochemical and molecular testing methodologies (including newborn screening)

Which technique is best suited to detect a large deletion spanning multiple exons of a gene?

  1. Southern blot of a PCR product
  2. Multiplex ligation-dependent probe amplification Correct answer.
  3. Targeted single-nucleotide polymorphism genotyping
  4. Sanger sequencing of a single exon

Explanation

MLPA quantifies copy number across multiple exons simultaneously, making it well suited to detect multi-exon deletions or duplications. Sanger sequencing of one exon and single-SNP genotyping cannot assess copy number across the gene.

Question 48Biochemical and molecular testing methodologies (including newborn screening)

In a urine organic acid analysis by gas chromatography-mass spectrometry, which step is typically required before GC injection?

  1. Alkaline hydrolysis of all esters
  2. Removal of all water by lyophilization only
  3. Addition of ethidium bromide
  4. Derivatization to increase volatility Correct answer.

Explanation

Organic acids are polar and nonvolatile, so they are derivatized (for example, to trimethylsilyl derivatives) to increase volatility for GC. The other steps are not routine prerequisites for GC injection.

Question 49Biochemical and molecular testing methodologies (including newborn screening)

Amino acid analysis using ion-exchange chromatography with post-column ninhydrin detection relies on which property for separation?

  1. Volatility
  2. Molecular size
  3. Charge differences at controlled pH Correct answer.
  4. Hydrophobicity only

Explanation

Ion-exchange chromatography separates amino acids based on their charge, which varies with pH and the resin's charged groups. Ninhydrin then reacts with eluted amino acids to produce a colored signal.

Question 50Biochemical and molecular testing methodologies (including newborn screening)

Enzyme activity in a leukocyte lysate is measured and reported per milligram of protein. Why is protein normalization necessary?

  1. To eliminate the need for a standard curve
  2. To correct for differences in cell number and lysate concentration Correct answer.
  3. To increase the substrate concentration
  4. To convert enzyme units to mass units

Explanation

Normalizing to protein content controls for differing amounts of cells or lysate in each sample, allowing valid comparison between specimens. It does not alter substrate concentration or substitute for calibration.