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10. A 2-year-old girl is brought in with developmental delay, speech impairment, and seizures. She also exhibits a happy demeanor with frequent laughter and hand-flapping movements. Genetic testing reveals a deletion of the maternal allele on chromosome 15, while the paternal allele is unaffected. Which of the following is most likely to be deficient in this patient?
A) UBE3A
B) SOD1
C) HTT
D) FMR1
E) MECP2
Answer: A) UBE3A
Explanation: Angelman syndrome results from a deletion of the maternal UBE3A gene on chromosome 15, leading to deficient UBE3A protein, a key ubiquitin ligase involved in synaptic plasticity and neuronal function. The disease presents with developmental delay, seizures, inappropriate laughter, and motor impairments.
9. A patient presents with fatigue, glossitis, and megaloblastic anemia. Lab tests show elevated homocysteine and methylmalonic acid. Which of the following enzymes is impaired due to the vitamin deficiency?
A) Methionine synthase
B) Thymidylate synthase
C) Homogentisate oxidase
D) Phenylalanine hydroxylase
E) Ornithine transcarbamylase
Answer: A) Methionine synthase
Explanation: Vitamin B12 deficiency leads to accumulation of homocysteine and methylmalonic acid. B12 is required for methionine synthase and methylmalonyl-CoA mutase activity.
8. A patient is found to have a mutation in the gene encoding the LDL receptor. Which lipid profile finding would most likely be seen in this individual?
A) Increased HDL
B) Decreased triglycerides
C) Increased LDL
D) Decreased VLDL
E) Normal lipid profile
Answer: C) Increased LDL
Explanation: Familial hypercholesterolemia (Type IIa) results from defective LDL receptors, leading to elevated LDL levels and early-onset atherosclerosis.
7. A researcher is studying the urea cycle and identifies a compound that accumulates in a patient with a deficiency in ornithine transcarbamylase. What substance is most likely elevated in the blood?
A) Arginine
B) Orotic acid
C) Urea
D) Phenylalanine
E) Ammonia
Answer: B) Orotic acid
Explanation: OTC deficiency is the most common urea cycle disorder. It leads to hyperammonemia and elevated orotic acid, due to excess carbamoyl phosphate entering the pyrimidine synthesis pathway.
6. A 3-year-old boy presents with hepatomegaly, fasting hypoglycemia, and lactic acidosis. Lab results show elevated triglycerides and uric acid. A liver biopsy shows increased glycogen with normal structure. Which enzyme is most likely deficient?
A) Glycogen phosphorylase
B) Glucose-6-phosphatase
C) Debranching enzyme
D) Muscle phosphorylase
E) Fructose-1,6-bisphosphatase
Answer: B) Glucose-6-phosphatase
Explanation: This is Von Gierke disease (Glycogen Storage Disease Type I). The deficiency in glucose-6-phosphatase prevents the final step of gluconeogenesis and glycogenolysis, causing severe fasting hypoglycemia, hepatomegaly, and metabolic derangements.
5. A 1-week-old infant becomes lethargic and develops vomiting after feeding. Physical exam reveals jaundice and hepatomegaly. Reducing substances are present in the urine. What enzyme is most likely deficient?
A) Galactokinase
B) Aldolase B
C) Galactose-1-phosphate uridyltransferase
D) Fructokinase
E) Glucokinase
Answer: C) Galactose-1-phosphate uridyltransferase
Explanation: This is classic galactosemia, caused by a deficiency in GALT. It leads to accumulation of galactose-1-phosphate, which is toxic to the liver, brain, and kidneys. Presents in neonates shortly after milk ingestion.
4. A 21-year-old college athlete reports muscle cramps and fatigue during workouts. After brief rest, his symptoms improve. A muscle biopsy reveals excess glycogen with normal structure. Which of the following enzymes is most likely deficient?
A) Myophosphorylase
B) Acid maltase
C) Debranching enzyme
D) Pyruvate kinase
E) PFK-1
Answer: A) Myophosphorylase
Explanation: This is McArdle disease (GSD type V), due to deficiency in muscle glycogen phosphorylase (myophosphorylase). Patients can’t break down glycogen in muscle, leading to exercise intolerance and “second wind” phenomenon.
3. A 3-month-old infant presents with vomiting, poor feeding, and a peculiar sweet odor in the urine. Laboratory testing reveals elevated levels of leucine, isoleucine, and valine. This condition is most likely due to a deficiency in which of the following enzymes?
A) Homogentisate oxidase
B) Branched-chain α-ketoacid dehydrogenase
C) Phenylalanine hydroxylase
D) Tyrosinase
E) Dihydropteridine reductase
Answer: B) Branched-chain α-ketoacid dehydrogenase
Explanation: The condition is Maple Syrup Urine Disease (MSUD), caused by a defect in the degradation of branched-chain amino acids.
2. A patient is found to have a deficiency of biotin. Which of the following types of metabolic reactions would be most directly affected by this deficiency?
A) Transamination
B) Decarboxylation
C) Carboxylation
D) Deamination
E) Hydroxylation
Answer: C) Carboxylation
Explanation: Biotin is a cofactor for carboxylation enzymes (e.g., pyruvate carboxylase, acetyl-CoA carboxylase).
1. A 2-year-old child with developmental delay, self-mutilation behavior, and elevated uric acid levels is diagnosed with Lesch-Nyhan syndrome. Which enzyme is most likely deficient in this patient?
A) Adenosine deaminase
B) Xanthine oxidase
C) Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)
D) PRPP synthetase
E) Carbamoyl phosphate synthetase II
Answer: C) Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)
Explanation: Lesch-Nyhan syndrome is caused by HGPRT deficiency, leading to defective purine salvage and excess uric acid.
Simon Downes, MD, PhD 