Mutations and Inborn Errors of Metabolism (1C) - Bio & Biochem

1. Two siblings develop episodic muscle weakness and dark urine after short bursts of exercise (e.g., sprinting). Between episodes, neurologic exam is normal. During an episode, labs show elevated creatine kinase and normal blood glucose. A muscle biopsy shows increased glycogen content. Enzyme assay reveals near-absent muscle glycogen phosphorylase activity due to a nonsense mutation in the PYGM gene. The biochemical concept is an inborn error of glycogen metabolism limiting rapid ATP generation from glycogenolysis. Based on the information provided, which outcome is most likely during high-intensity exercise?
   Answer: Decreased ability to generate glucose-6-phosphate from muscle glycogen, reducing glycolytic flux and predisposing to rhabdomyolysis This question tests understanding of inborn errors of glycogen metabolism limiting rapid ATP generation from glycogenolysis. Muscle glycogen phosphorylase catalyzes the rate-limiting step in glycogenolysis, breaking down glycogen to glucose-1-phosphate, which is then converted to glucose-6-phosphate for glycolysis. In McArdle disease (glycogen storage disease type V), the absence of muscle phosphorylase prevents glycogen breakdown during high-intensity exercise when rapid ATP generation is needed. The correct answer (B) accurately describes how this defect reduces glycolytic flux from glycogen-derived glucose-6-phosphate, leading to ATP depletion and muscle damage (rhabdomyolysis) during anaerobic exercise. Choice A incorrectly suggests increased lactate production, when actually lactate production is decreased due to reduced glycolytic substrate availability. When analyzing metabolic defects, remember that enzyme deficiencies reduce flux through pathways, and consider the tissue-specific consequences based on the metabolic demands of that tissue.
2. In a metabolic study, fibroblasts from a patient show normal glucose uptake but reduced lactate production during anaerobic conditions. Enzyme assay demonstrates that phosphofructokinase-1 (PFK-1) has markedly decreased affinity for fructose-6-phosphate (increased $K_m$) with unchanged $V_{max}$. The central concept is how altered enzyme kinetics changes pathway flux. Which of the following is most consistent with this mutation during intense exercise in skeletal muscle?
   Answer: Decreased glycolytic flux leading to reduced ATP generation from substrate-level phosphorylation and early fatigue This question tests understanding of glycolytic enzyme deficiencies and how altered enzyme kinetics affect metabolic flux. Phosphofructokinase-1 (PFK-1) catalyzes the committed step of glycolysis, converting fructose-6-phosphate to fructose-1,6-bisphosphate, and an increased Km means the enzyme requires higher substrate concentrations to achieve half-maximal velocity. With markedly increased Km for fructose-6-phosphate, PFK-1 activity is reduced at physiological substrate concentrations, decreasing glycolytic flux and limiting ATP production from substrate-level phosphorylation, which is critical during anaerobic exercise when oxidative phosphorylation cannot meet ATP demands. The correct answer B accurately describes decreased glycolytic flux and reduced ATP generation leading to early muscle fatigue. Answer A is incorrect because it suggests increased glycolytic flux, when actually the increased Km reduces enzyme activity at normal substrate concentrations, decreasing flux through the pathway. When analyzing enzyme kinetics mutations, increased Km reduces enzyme efficiency at physiological substrate levels, while decreased Vmax reduces maximal catalytic capacity. Remember that PFK-1 deficiency (Tarui disease) specifically impairs anaerobic ATP generation in muscle.
3. A 2-year-old presents with recurrent kidney stones and anemia. Peripheral smear shows signs consistent with hemolysis. Urine studies reveal elevated homocystine. Genetic testing identifies a mutation in cystathionine β-synthase (CBS) that decreases conversion of homocysteine to cystathionine. The central concept is an metabolic block causing upstream accumulation and downstream product deficiency. Which additional biochemical pattern is most consistent with this defect?
   Answer: Increased homocysteine with decreased cysteine availability, potentially reducing glutathione synthesis This question tests understanding of metabolic blocks causing upstream accumulation and downstream product deficiency. Cystathionine β-synthase (CBS) catalyzes the condensation of homocysteine and serine to form cystathionine, the first step in the transsulfuration pathway that ultimately produces cysteine. When CBS is deficient, homocysteine accumulates and forms disulfide bonds creating homocystine (detected in urine), while cysteine production is reduced, potentially limiting glutathione synthesis. The correct answer (B) accurately describes this pattern of increased homocysteine with decreased cysteine availability. Choice A incorrectly suggests decreased methionine, but methionine levels are typically normal or elevated in CBS deficiency as the remethylation pathway remains intact. To analyze amino acid metabolism defects, trace both the fate of accumulating substrates and the consequences of reduced product formation, considering how these changes affect related pathways.
4. A child with developmental regression has elevated very-long-chain fatty acids (VLCFAs) in plasma. A mutation is identified in a peroxisomal membrane transporter required to import activated VLCFAs for peroxisomal $β$-oxidation. Using the concept of compartment-specific fatty acid metabolism, which outcome is most consistent?
   Answer: Accumulation of VLCFAs due to impaired peroxisomal oxidation, with secondary effects on myelin stability This question examines peroxisomal disorders from transporter mutations. The principle is that compartment-specific defects prevent substrate entry, leading to accumulation and secondary tissue damage. The peroxisomal transporter mutation blocks VLCFA import for beta-oxidation, causing their plasma accumulation and myelin instability. This fits the developmental regression in X-linked adrenoleukodystrophy. Choice C is incorrect as it suggests increased oxidation without transporters, ignoring import necessity. For similar questions, emphasize organelle localization. Validate by noting elevated VLCFAs as diagnostic.
5. A researcher engineers a point mutation in a mitochondrial matrix enzyme that converts methylmalonyl-CoA to succinyl-CoA. In cultured hepatocytes expressing the mutant enzyme, basal oxygen consumption is unchanged, but cells show reduced growth when propionate is the primary carbon source. Metabolite profiling shows increased methylmalonic acid and decreased succinyl-CoA-derived labeling of TCA intermediates. The central concept is an enzyme defect that reduces anaplerosis into the TCA cycle. Which of the following best explains the metabolic consequence of the mutation described?
   Answer: Reduced conversion of methylmalonyl-CoA to succinyl-CoA limits replenishment of TCA intermediates from propionyl-CoA metabolism This question tests understanding of enzyme defects that reduce anaplerosis into the TCA cycle. Methylmalonyl-CoA mutase converts methylmalonyl-CoA to succinyl-CoA, providing an anaplerotic entry point for propionyl-CoA metabolism into the TCA cycle. When this enzyme is defective, propionyl-CoA (from propionate or odd-chain fatty acid metabolism) cannot efficiently replenish TCA cycle intermediates through succinyl-CoA formation. The correct answer (C) accurately describes this reduced anaplerotic flux, explaining why cells show impaired growth specifically when propionate is the primary carbon source. Choice B incorrectly reverses the reaction direction, as the enzyme catalyzes methylmalonyl-CoA to succinyl-CoA conversion, not the reverse. To solve similar problems, identify the specific metabolic role of the defective enzyme and consider how substrate-specific growth defects reveal the importance of particular anaplerotic pathways.
6. A 19-year-old college student reports episodic muscle cramps and dark urine after short bursts of intense exercise (e.g., sprinting). During an ischemic forearm exercise test, lactate fails to rise while ammonia rises normally. A homozygous nonsense mutation is found in the gene encoding skeletal muscle glycogen phosphorylase. The biochemical concept is a mutation that blocks glycogen breakdown, limiting glycolytic substrate during anaerobic activity. What physiological effect would be expected from the described genetic mutation?
   Answer: Exercise intolerance with reduced ATP generation from glycolysis during high-intensity activity and risk of myoglobinuria This question tests understanding of McArdle disease (glycogen storage disease type V), caused by deficiency of muscle glycogen phosphorylase. Glycogen phosphorylase catalyzes the rate-limiting step in glycogenolysis, releasing glucose-1-phosphate from glycogen for entry into glycolysis during muscle contraction. The nonsense mutation eliminates enzyme function, preventing glycogen breakdown during anaerobic exercise when muscle relies heavily on glycolysis for rapid ATP generation. The correct answer B accurately describes exercise intolerance during high-intensity activity due to insufficient glycolytic substrate, with risk of rhabdomyolysis (myoglobinuria) from muscle damage. The failed lactate rise during ischemic exercise reflects inability to mobilize glycogen for glycolysis, while normal ammonia rise indicates intact purine nucleotide cycle. Answer C incorrectly attributes symptoms to liver involvement, but the mutation affects only muscle phosphorylase. When evaluating tissue-specific enzyme defects, consider which metabolic demands are unmet in that specific tissue.
7. A patient presents with photosensitivity and blistering skin lesions after sun exposure. Urine darkens on standing. Testing shows elevated uroporphyrinogen and reduced uroporphyrinogen decarboxylase activity. Considering the biochemical concept of porphyrin pathway enzyme defects causing intermediate buildup, which finding is most consistent?
   Answer: Accumulation of porphyrin intermediates that generate reactive species upon light exposure, contributing to cutaneous photosensitivity This question assesses porphyrias from decarboxylase mutations. The principle is that heme synthesis defects cause porphyrin intermediate buildup, leading to photosensitivity from reactive species generation. The uroporphyrinogen decarboxylase deficiency accumulates uroporphyrinogen, oxidizing to porphyrins that cause skin lesions upon light exposure. This explains the photosensitivity and urine darkening. Choice B is misleading because it claims decreased porphyrins with increased heme, reversing the block effect. In related problems, identify the porphyrin pathway step. Check for light-dependent symptoms to confirm.
8. A 4-year-old presents with developmental delay and episodes of dystonia. MRI suggests basal ganglia injury. Plasma shows elevated methylmalonic acid; homocysteine is also elevated. Genetic testing reveals a mutation that disrupts intracellular processing of cobalamin into its active cofactor forms. The central concept is a mutation that impairs a vitamin-derived cofactor, affecting multiple enzymes. Based on the findings, which outcome is most consistent with the underlying biochemical defect?
   Answer: Decreased activity of methylmalonyl-CoA mutase and methionine synthase, causing elevated methylmalonic acid and homocysteine This question tests understanding of combined methylmalonic acidemia and homocystinuria (cblC defect). Mutations affecting intracellular cobalamin processing prevent formation of both methylcobalamin (cofactor for methionine synthase) and adenosylcobalamin (cofactor for methylmalonyl-CoA mutase), causing a functional deficiency of both vitamin B12-dependent enzymes. This dual enzyme deficiency leads to accumulation of both methylmalonic acid (from impaired propionate metabolism) and homocysteine (from impaired methionine synthesis), explaining the combined biochemical abnormalities and neurological symptoms from basal ganglia toxicity. The correct answer D accurately describes decreased activity of both affected enzymes and their respective substrate accumulations. Answer B incorrectly suggests increased enzyme activity, while C focuses only on folate without explaining methylmalonic acid elevation. Recognition of cofactor processing defects requires understanding that a single mutation can affect multiple enzymes sharing the same cofactor.
9. A 6-month-old presents with developmental delay and recurrent episodes of vomiting after high-protein feeds. Plasma shows elevated leucine, isoleucine, and valine; urine has a characteristic sweet odor. Sequencing reveals a loss-of-function mutation in the E1 subunit of branched-chain $\alpha$-ketoacid dehydrogenase (BCKDH). The biochemical concept is impaired oxidative decarboxylation leading to upstream metabolite accumulation. Based on this enzyme defect, which outcome is most likely?
   Answer: Accumulation of branched-chain $\alpha$-ketoacids due to impaired conversion to acyl-CoA derivatives This question tests understanding of maple syrup urine disease and the consequences of blocked branched-chain amino acid catabolism. Branched-chain α-ketoacid dehydrogenase (BCKDH) catalyzes the oxidative decarboxylation of α-ketoacids derived from leucine, isoleucine, and valine to their corresponding acyl-CoA derivatives. When BCKDH is deficient, the α-ketoacids accumulate because they cannot be converted to acyl-CoA products that would normally enter the TCA cycle, causing the characteristic sweet odor in urine and neurological symptoms. The correct answer B accurately describes this accumulation of branched-chain α-ketoacids upstream of the enzymatic block. Answer C is incorrect because it describes phenylketonuria (PKU), which involves a different amino acid (phenylalanine) and enzyme (phenylalanine hydroxylase), not the branched-chain amino acids affected in this case. To distinguish amino acid disorders, remember that maple syrup urine disease affects all three branched-chain amino acids and their α-ketoacids, while PKU specifically affects phenylalanine metabolism. The sweet odor is from α-ketoacid accumulation, not the amino acids themselves.
10. A patient experiences recurrent episodes of fasting hypoglycemia with low ketone bodies. During a supervised fast, plasma free fatty acids are elevated, but acylcarnitine profiling shows increased medium-chain acylcarnitines. Sequencing reveals a mutation that reduces activity of medium-chain acyl-CoA dehydrogenase (MCAD). The biochemical concept is an fatty acid β-oxidation defect limiting acetyl-CoA and NADH production during fasting. Based on the findings, which outcome is most likely?
    Answer: Decreased hepatic gluconeogenesis due to reduced ATP/NADH supply from β-oxidation, contributing to hypoketotic hypoglycemia This question tests understanding of fatty acid β-oxidation defects limiting acetyl-CoA and NADH production during fasting. Medium-chain acyl-CoA dehydrogenase (MCAD) catalyzes the first step of β-oxidation for medium-chain fatty acids, generating FADH2 and creating a double bond in the acyl-CoA. When MCAD is deficient, medium-chain fatty acids cannot be efficiently oxidized, reducing acetyl-CoA production for ketogenesis and NADH/FADH2 generation for gluconeogenesis. The correct answer (B) accurately describes how reduced ATP and reducing equivalent supply from impaired β-oxidation limits hepatic gluconeogenesis, contributing to the characteristic hypoketotic hypoglycemia. Choice A incorrectly suggests increased ketone production, when actually ketogenesis is impaired due to reduced acetyl-CoA availability from blocked β-oxidation. To identify correct answers in metabolic defect questions, trace how the enzyme deficiency affects both substrate fate and energy/cofactor production needed for other pathways.
11. In a screening program, a child is found to have elevated methylmalonic acid and low succinyl-CoA formation from propionate. A mutation reduces methylmalonyl-CoA mutase activity. Using the concept of metabolic intermediate confusion in propionate metabolism, which outcome is most consistent?
    Answer: Accumulation of methylmalonyl-CoA-derived metabolites with reduced entry of propionyl units into the TCA cycle as succinyl-CoA This question tests propionate metabolism disorders from mutase mutations. The principle is that blocks prevent TCA anaplerosis, accumulating upstream acids like methylmalonic. The methylmalonyl-CoA mutase deficiency reduces succinyl-CoA formation, building up methylmalonyl metabolites. This fits the elevated methylmalonic acid. Choice B is incorrect as it suggests reaction reversal, which is not thermodynamically favored. For similar problems, trace anaplerotic paths. Check metabolite patterns for confirmation.
12. In a newborn screening study, infants with poor feeding, vomiting, and lethargy were found to have elevated plasma leucine and its corresponding $α$-ketoacid. Urine organic acids showed increased branched-chain ketoacids. Sequencing revealed a missense mutation in the E1 subunit of branched-chain $α$-ketoacid dehydrogenase (BCKDH) that reduces catalytic activity but does not affect protein abundance. Based on the biochemical concept of an enzyme-catalyzed metabolic block, which outcome is most consistent with this mutation?
    Answer: Accumulation of branched-chain amino acids and their $α$-ketoacids upstream of the BCKDH step This question tests the understanding of inborn errors of metabolism due to mutations in catabolic enzymes, specifically branched-chain amino acid degradation. The relevant biochemical principle is that an enzyme deficiency creates a metabolic block, leading to accumulation of substrates immediately upstream and potential diversion into alternative pathways. In this case, the missense mutation in the E1 subunit of BCKDH reduces its activity, blocking the decarboxylation of branched-chain alpha-ketoacids derived from leucine, isoleucine, and valine. Therefore, branched-chain amino acids and their alpha-ketoacids accumulate upstream, as seen in the elevated plasma leucine and ketoacids, consistent with maple syrup urine disease. Choice A is incorrect because it suggests decreased branched-chain amino acids, which misconstrues the block as increasing flux rather than causing buildup. To approach similar questions, first identify the exact step blocked by the mutation. Then, map out the upstream metabolites that would accumulate and any toxic byproducts formed.
13. In a metabolic study, fibroblasts from a patient show normal levels of lysosomal enzyme protein by Western blot, but markedly reduced enzymatic activity. The substrate of the enzyme accumulates in lysosomes, causing progressive neurodegeneration. Sequencing reveals a point mutation that changes a catalytic residue but does not alter trafficking signals. Using the concept of enzyme active-site mutations, which explanation best fits the data?
    Answer: The mutation disrupts a catalytic residue, reducing $k_{cat}$ despite normal enzyme abundance and localization This question examines the effects of active-site mutations in lysosomal storage disorders. The principle is that mutations altering catalytic residues impair enzyme function without affecting protein synthesis or localization, leading to substrate accumulation in compartments. The point mutation in the catalytic residue reduces the lysosomal enzyme's k_cat, causing substrate buildup despite normal protein levels and trafficking. This fits the neurodegeneration from lysosomal storage, as activity is low but abundance is normal. Choice A is incorrect because it implies increased affinity accelerating breakdown, confusing catalytic efficiency with substrate binding. To solve similar problems, distinguish between mutations affecting catalysis versus stability. Confirm by evaluating if symptoms align with compartment-specific accumulation.
14. In an experiment, two cell lines are compared: wild-type and a mutant lacking functional glucose-6-phosphatase. Both are cultured with ample glucose, then switched to a medium lacking glucose but containing lactate and alanine. After 4 hours, wild-type cells release free glucose into the medium, while mutant cells do not; mutant cells accumulate intracellular glucose-6-phosphate. The biochemical concept is a mutation that blocks the terminal step of gluconeogenesis/glycogenolysis required for glucose export. Based on the setup, which outcome is most likely given the enzyme defect?
    Answer: Impaired ability to maintain blood glucose during fasting with accumulation of glucose-6-phosphate and increased glycolytic flux to lactate This question tests understanding of glucose-6-phosphatase deficiency (von Gierke disease, GSD Ia). Glucose-6-phosphatase catalyzes the final step in both gluconeogenesis and glycogenolysis, converting glucose-6-phosphate to free glucose for export from hepatocytes; its absence prevents glucose release despite intact upstream pathways. The mutation creates a metabolic trap where glucose-6-phosphate accumulates intracellularly but cannot be released as free glucose, impairing fasting glucose homeostasis and causing the accumulated G6P to be shunted into glycolysis, producing excess lactate. The correct answer B accurately describes impaired glucose maintenance during fasting with G6P accumulation and increased glycolytic flux. Answer A incorrectly suggests increased glucose release when the defect prevents it, while D describes a different gluconeogenic enzyme defect. Understanding terminal pathway steps is crucial - blocking the final common step affects all upstream pathways converging at that point.
15. A child has recurrent vomiting and encephalopathy during infections. Labs show hyperammonemia with low plasma citrulline and elevated orotic acid. Genetic testing reveals an X-linked mutation that reduces ornithine transcarbamylase (OTC) activity. Based on the biochemical concept of urea cycle blockage and shunting, which outcome is most consistent with this defect?
    Answer: Increased mitochondrial carbamoyl phosphate that spills into cytosol and increases orotic acid production This question tests recognition of urea cycle disorders from transcarbamylase mutations. The principle involves metabolic blocks in the urea cycle causing ammonia accumulation and shunting of intermediates like carbamoyl phosphate into pyrimidine synthesis. The OTC mutation reduces formation of citrulline, leading to mitochondrial carbamoyl phosphate buildup that diffuses to the cytosol, driving orotic acid production. This results in hyperammonemia, low citrulline, and elevated orotic acid, explaining the vomiting and encephalopathy. Choice C is misleading because it claims increased urea production, ignoring that the block impairs ammonia detoxification. For similar cases, differentiate urea cycle defects by intermediate patterns like orotic acid levels. Cross-check with clinical triggers such as infections increasing protein catabolism.
16. A newborn is screened and found to have elevated medium-chain acylcarnitines. The infant later develops hypoketotic hypoglycemia during a viral illness. Sequencing reveals a loss-of-function mutation in medium-chain acyl-CoA dehydrogenase (MCAD). The biochemical concept is an enzyme defect in $β$-oxidation that limits acetyl-CoA production and ketogenesis during fasting. What physiological effect would be expected from the described genetic mutation?
    Answer: Hypoketotic hypoglycemia during fasting because reduced $β$-oxidation decreases acetyl-CoA and ATP needed to support gluconeogenesis This question tests understanding of medium-chain acyl-CoA dehydrogenase (MCAD) deficiency, the most common fatty acid oxidation disorder. MCAD catalyzes the first dehydrogenation step in β-oxidation of C6-C12 fatty acids; its deficiency prevents efficient fatty acid oxidation during fasting, limiting acetyl-CoA production needed for both ketogenesis and gluconeogenesis energy requirements. The metabolic block causes accumulation of medium-chain acylcarnitines (diagnostic marker) and manifests as hypoketotic hypoglycemia during metabolic stress because inadequate β-oxidation cannot generate sufficient acetyl-CoA for ketone body synthesis or ATP for gluconeogenesis. The correct answer B accurately describes this hypoketotic hypoglycemia phenotype resulting from impaired β-oxidation. Answer A incorrectly suggests increased ketogenesis when it's actually impaired, while C and D describe unrelated metabolic defects. Recognition of fatty acid oxidation defects requires understanding the dual role of β-oxidation in providing both ketogenic substrate and energy for gluconeogenesis.
17. A patient has recurrent episodes of confusion after high-protein meals. Plasma ammonia is elevated, while blood glucose is normal. Urine orotic acid is increased. Sequencing identifies an X-linked loss-of-function mutation in ornithine transcarbamylase (OTC). The biochemical concept is a metabolic block in the urea cycle leading to upstream substrate accumulation and diversion into pyrimidine synthesis. Which of the following best explains the metabolic consequence of the mutation described?
    Answer: Carbamoyl phosphate accumulates and is shunted to pyrimidine synthesis, increasing orotic acid while ammonia rises This question tests understanding of ornithine transcarbamylase (OTC) deficiency, the most common urea cycle disorder. OTC normally catalyzes the condensation of carbamoyl phosphate with ornithine to form citrulline; its deficiency causes carbamoyl phosphate accumulation in mitochondria, which then leaks to cytoplasm and enters pyrimidine synthesis via carbamoyl phosphate synthetase II, producing excess orotic acid. The metabolic block prevents efficient ammonia detoxification through the urea cycle, causing hyperammonemia particularly after protein intake increases the nitrogen load. The correct answer B accurately describes both the carbamoyl phosphate shunting to pyrimidine synthesis (explaining elevated orotic acid) and the concurrent hyperammonemia. Answer A incorrectly states decreased carbamoyl phosphate when it actually accumulates, while C impossibly suggests the deficiency increases urea production. When analyzing urea cycle defects, elevated orotic acid specifically indicates OTC deficiency due to this unique metabolic shunt.
18. A 6-week-old infant presents with poor feeding, vomiting, and lethargy after switching from breast milk to standard formula. Labs show metabolic acidosis and markedly elevated plasma leucine; urine organic acids reveal increased branched-chain $α$-ketoacids. A genetic test identifies a missense mutation in the E1 subunit of branched-chain $α$-ketoacid dehydrogenase (BCKDH) that lowers $V_{max}$ without changing $K_m$ for substrate. The biochemical concept is a mutation-caused enzyme defect producing a metabolic block and toxic intermediate accumulation. Based on the scenario, which outcome is most consistent with the enzyme defect?
    Answer: Accumulation of branched-chain amino acids and their $α$-ketoacids with neurologic toxicity, worsened by high-protein intake This question tests understanding of maple syrup urine disease (MSUD), an inborn error of branched-chain amino acid metabolism. MSUD results from deficiency of branched-chain α-ketoacid dehydrogenase (BCKDH), which normally catalyzes the oxidative decarboxylation of α-ketoacids derived from leucine, isoleucine, and valine. The mutation described reduces Vmax (maximum enzyme velocity) without changing Km, indicating decreased catalytic efficiency that causes accumulation of both branched-chain amino acids and their corresponding α-ketoacids upstream of the metabolic block. The correct answer B accurately describes this accumulation pattern and the neurologic toxicity that worsens with increased protein intake (providing more substrate). Answer A incorrectly describes pyruvate dehydrogenase deficiency rather than BCKDH deficiency, while C incorrectly suggests increased conversion back to amino acids would prevent ketoacidosis when actually the block prevents further metabolism. When analyzing enzyme defects, focus on what accumulates upstream of the block and what becomes deficient downstream.
19. A 3-year-old develops muscle cramps and dark urine after short bursts of exercise. During an ischemic forearm test, lactate fails to rise but ammonia increases. Muscle biopsy shows excess glycogen. A mutation reduces myophosphorylase (glycogen phosphorylase in muscle) activity. Based on the biochemical concept of impaired glycogenolysis in muscle, which outcome is most consistent?
    Answer: Reduced ability to generate glucose-1-phosphate from glycogen in muscle, limiting glycolysis during anaerobic exercise This question assesses understanding of glycogen storage diseases specific to muscle metabolism. The principle is that tissue-specific enzyme defects impair local glycogen breakdown, limiting energy production during demand without systemic glucose effects. The myophosphorylase mutation blocks glycogen to glucose-1-phosphate conversion in muscle, restricting glycolytic ATP during anaerobic exercise, causing cramps and myoglobinuria. This explains the failed lactate rise in the ischemic test, as glycogenolysis is impaired. Choice C is wrong because it suggests elevated lactate from enhanced glycolysis, overlooking the block preventing glycogen access. In comparable questions, specify the affected tissue and energy pathway. Validate by noting if symptoms are exercise-induced rather than fasting-related.
20. A 3-month-old infant has poor growth, recurrent vomiting, and a musty body odor. Plasma phenylalanine is markedly elevated with low-normal tyrosine. A point mutation in PAH reduces catalytic activity. The biochemical concept is an enzyme defect causing buildup of a substrate and diversion into alternative metabolites. Which of the following best explains the metabolic consequence of the mutation described?
    Answer: Accumulated phenylalanine is transaminated to phenylpyruvate and related metabolites, contributing to neurotoxicity This question tests understanding of classical phenylketonuria (PKU) and its metabolic consequences. PAH deficiency causes phenylalanine accumulation, which exceeds the normal transamination capacity and gets diverted into alternative pathways producing phenylpyruvate, phenyllactate, and phenylacetate - compounds responsible for the characteristic musty odor. These alternative metabolites, particularly phenylpyruvate, contribute to neurotoxicity by interfering with amino acid transport across the blood-brain barrier and disrupting myelin synthesis. The correct answer B accurately describes this metabolic diversion and its toxic consequences. Answer A incorrectly states decreased phenylalanine when it actually accumulates, while C wrongly attributes symptoms to tyrosine accumulation rather than deficiency. When substrate accumulates due to enzyme deficiency, always consider alternative metabolic fates that may produce toxic byproducts.

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