MCAT Biological and Biochemical Foundations of Living Systems Quiz: 1c Mutations Inborn Errors

What this quiz covers

This quiz focuses on 1c Mutations Inborn Errors, giving you a quick way to practice the rules, question types, and explanations that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Elevated homogentisic acid due to impaired tyrosine catabolism
2. Decreased urinary cystine due to increased transporter affinity for cystine
3. Hyperammonemia due to impaired urea cycle transport of ornithine into mitochondria
4. Increased urinary cystine due to impaired reabsorption, promoting cystine stone formation (correct answer)

Explanation: This question tests renal transport disorders from amino acid transporter mutations. The principle is that loss-of-function in reabsorptive transporters increases urinary excretion, leading to precipitation and stones. The dibasic amino acid transporter mutation impairs cystine reabsorption, elevating urinary cystine and forming stones. This accounts for the kidney stones and hexagonal crystals. Choice B is incorrect as it suggests decreased cystine from increased affinity, confusing loss-of-function with gain. For similar cases, link transport defects to solute loss. Verify with urinalysis findings like crystals.

Question 2

1. Elevated branched-chain ketoacids due to impaired mitochondrial dehydrogenase activity
2. Impaired cytosolic glycolysis due to inability to convert glucose-6-phosphate to fructose-6-phosphate
3. Increased hepatic ketone production due to excessive lysosomal glycogen breakdown
4. Impaired lysosomal glycogen degradation leading to glycogen-filled lysosomes and muscle dysfunction (correct answer)

Explanation: This question tests lysosomal storage diseases from glucosidase mutations. The principle is that compartment-specific enzyme defects impair degradation, causing substrate-filled lysosomes and organ dysfunction. The acid alpha-glucosidase deficiency blocks lysosomal glycogen breakdown, leading to accumulation and muscle weakness. This explains cardiomyopathy and weakness in Pompe disease. Choice B is wrong because it confuses lysosomal with cytosolic pathways. In analogous scenarios, emphasize subcellular sites. Verify with biopsy accumulation patterns.

Question 3

1. The mutation increases enzyme affinity for substrate, accelerating substrate breakdown
2. The mutation disrupts a catalytic residue, reducing $k_{cat}$ despite normal enzyme abundance and localization (correct answer)
3. The mutation reverses the reaction direction, converting product back to substrate in lysosomes
4. The mutation causes autosomal dominant inheritance because catalytic residues determine dominance

Explanation: 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.

Question 4

1. Inability to release free glucose from the liver, causing accumulation of glucose-6-phosphate and increased glycolytic flux to lactate (correct answer)
2. Impaired glycogen synthesis due to inability to form UDP-glucose, leading to low liver glycogen
3. Increased hepatic ketogenesis due to excessive export of glucose into blood during fasting
4. Reduced lactate production because glucose-6-phosphate cannot enter glycolysis

Explanation: This question assesses knowledge of glycogen storage diseases caused by mutations in gluconeogenic enzymes. The key principle is that defects in enzymes required for glucose release from the liver lead to intracellular trapping of glucose precursors, shunting them into alternative metabolic routes. Here, the glucose-6-phosphatase deficiency prevents dephosphorylation of glucose-6-phosphate, causing its accumulation in the liver and increased flux through glycolysis to lactate. This explains the hepatomegaly, hypoglycemia, and elevated lactate, as the liver cannot export glucose during fasting. Choice D is wrong because it assumes glucose-6-phosphate cannot enter glycolysis, ignoring that the defect is downstream, allowing glycolytic entry but blocking glucose release. For similar problems, locate the metabolic bottleneck and predict buildup of intermediates. Consider compensatory pathways like increased lactate production to confirm the diagnosis.

Question 5

1. Increased urea production due to enhanced amino acid catabolism as the primary defect
2. Increased succinyl-CoA formation from acetyl-CoA, causing TCA cycle overload
3. Direct blockage of pyruvate dehydrogenase, leading to isolated lactic acidosis without organic acids
4. Decreased conversion of propionyl-CoA to methylmalonyl-CoA, reducing succinyl-CoA replenishment and causing organic acid accumulation (correct answer)

Explanation: This question tests anaplerotic pathway defects from carboxylase mutations. The principle is that blocks in converting odd-chain precursors to TCA intermediates cause organic acid accumulation and metabolic acidosis. The propionyl-CoA carboxylase deficiency impairs conversion to methylmalonyl-CoA, reducing succinyl-CoA entry and building up propionyl-derived acids. This results in failure to thrive, diarrhea, and anion gap acidosis. Choice B is wrong because it claims increased succinyl-CoA from acetyl-CoA, confusing the pathway direction. In analogous cases, trace carbon flow to the TCA cycle. Confirm with elevated specific metabolites like propionate.

Question 6

1. Decreased galactose-1-phosphate with increased UDP-galactose due to enhanced GALT activity
2. Accumulation of galactose and galactose-1-phosphate with diversion of galactose to galactitol in the lens (correct answer)
3. Increased conversion of galactose-1-phosphate to glucose-6-phosphate by reversal of GALT
4. Isolated elevation of methylmalonic acid due to impaired odd-chain fatty acid oxidation

Explanation: This question tests knowledge of carbohydrate metabolism disorders from transferase mutations. The principle is that blocks in sugar conversion pathways cause accumulation of phosphorylated intermediates and reduction to polyols in tissues like the lens. The GALT deficiency prevents galactose-1-phosphate exchange to UDP-glucose, leading to galactose and its phosphate buildup, with aldose reductase converting galactose to galactitol causing cataracts. This accounts for the neonatal symptoms after milk ingestion. Choice C is incorrect as it proposes reversal of GALT, which is not possible due to thermodynamic barriers. For similar disorders, identify toxic accumulations and alternative pathways. Check consistency with dietary triggers like lactose.

Question 7

1. Primary elevation of phenylalanine due to impaired aromatic amino acid hydroxylation
2. Increased NADPH production enhances detoxification of reactive oxygen species, preventing hemolysis
3. Direct inhibition of hemoglobin synthesis causing microcytic anemia without hemolysis
4. Reduced NADPH availability limits regeneration of reduced glutathione, increasing susceptibility to hemolysis (correct answer)

Explanation: This question evaluates redox metabolism defects from dehydrogenase mutations. The principle is that impaired NADPH production reduces glutathione regeneration, increasing oxidative damage susceptibility. The G6PD mutation limits NADPH in RBCs, impairing antioxidant defenses and causing hemolysis under stress. This explains drug-triggered symptoms in affected individuals. Choice B is wrong because it claims increased NADPH, misstating the deficiency effect. In comparable scenarios, connect NADPH to redox buffering. Confirm with triggers like oxidants for verification.

Question 8

1. Primary neurologic crises due to acute GABA deficiency as the direct result of this enzyme defect
2. Decreased porphyrins with increased heme synthesis, causing polycythemia
3. Increased conversion of heme to porphyrins due to reversal of decarboxylase activity
4. Accumulation of porphyrin intermediates that generate reactive species upon light exposure, contributing to cutaneous photosensitivity (correct answer)

Explanation: 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.

Question 9

1. Primary hypoketotic hypoglycemia due to impaired medium-chain fatty acid oxidation
2. Increased conversion of succinyl-CoA to methylmalonyl-CoA due to reversal of the mutase reaction
3. Isolated elevation of orotic acid due to direct inhibition of OTC by methylmalonic acid
4. Accumulation of methylmalonyl-CoA-derived metabolites with reduced entry of propionyl units into the TCA cycle as succinyl-CoA (correct answer)

Explanation: 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.

Question 10

1. Decreased conversion of galactose-1-phosphate to UDP-galactose, leading to accumulation of galactose-1-phosphate in hepatocytes (correct answer)
2. Increased conversion of UDP-galactose to galactose-1-phosphate, causing depletion of UDP-glucose pools
3. Impaired transport of galactose across the intestinal brush border, leading to osmotic diarrhea without hepatomegaly
4. Reduced hepatic gluconeogenesis due to direct inhibition of fructose-1,6-bisphosphatase by galactose

Explanation: This question tests understanding of enzyme defects causing metabolic blocks with upstream metabolite accumulation. Galactose-1-phosphate uridyltransferase (GALT) catalyzes the conversion of galactose-1-phosphate to UDP-galactose in the Leloir pathway of galactose metabolism. When GALT activity is reduced due to a missense mutation, galactose-1-phosphate cannot be efficiently converted to UDP-galactose, causing it to accumulate in tissues, particularly hepatocytes. The correct answer (A) accurately describes this metabolic block and accumulation pattern, which explains the hepatomegaly and liver enzyme elevation seen in classic galactosemia. Choice B incorrectly reverses the reaction direction, as GALT catalyzes the forward reaction, not the reverse. To identify the correct answer in similar questions, focus on the directionality of the enzymatic reaction and recognize that enzyme defects cause substrate accumulation upstream of the block, not downstream product accumulation.

Question 11

1. Increased ketone body production due to shunting of fatty acids into mitochondrial oxidation
2. Decreased acetyl-CoA generation reduces ketogenesis and limits activation of pyruvate carboxylase, worsening hypoglycemia (correct answer)
3. Increased urea cycle activity causes hyperammonemia and elevated orotic acid after protein meals
4. Defective glycogen branching enzyme causes hepatosplenomegaly and cirrhosis in early childhood

Explanation: This question tests understanding of medium-chain acyl-CoA dehydrogenase (MCAD) deficiency and the metabolic consequences of impaired β-oxidation. MCAD catalyzes the first dehydrogenation step in β-oxidation of medium-chain fatty acids (C6-C12), and its deficiency blocks fatty acid oxidation at this step, preventing acetyl-CoA generation from these substrates. During fasting, when fatty acid oxidation normally provides acetyl-CoA for ketogenesis and allosteric activation of pyruvate carboxylase (the rate-limiting enzyme of gluconeogenesis), MCAD deficiency limits both processes, resulting in hypoketotic hypoglycemia. The correct answer B accurately describes how decreased acetyl-CoA generation impairs both ketogenesis and gluconeogenesis support through reduced pyruvate carboxylase activation. Answer A is incorrect because it suggests increased ketone production, when actually ketogenesis is impaired due to insufficient acetyl-CoA substrate from blocked β-oxidation. To identify β-oxidation defects, look for hypoketotic hypoglycemia during fasting with elevated acylcarnitines corresponding to the chain length where the block occurs. Remember that acetyl-CoA from β-oxidation serves dual roles: substrate for ketogenesis and allosteric activator of gluconeogenesis.

Question 12

1. Defective glucose-6-phosphatase prevents glycogen breakdown in muscle, causing rhabdomyolysis after exercise
2. Reduced aldolase B activity directly increases pyruvate kinase flux, accelerating hepatic ATP production
3. Fructose-1-phosphate is efficiently converted to glucose-6-phosphate, causing hyperglycemia
4. Accumulated fructose-1-phosphate traps Pi, decreasing ATP and impairing gluconeogenesis and glycogenolysis (correct answer)

Explanation: This question tests understanding of hereditary fructose intolerance and the concept of metabolic trapping. Aldolase B catalyzes the cleavage of fructose-1-phosphate to dihydroxyacetone phosphate and glyceraldehyde in the liver, and its deficiency causes accumulation of fructose-1-phosphate after fructose ingestion. The accumulated fructose-1-phosphate sequesters inorganic phosphate (Pi), depleting the free Pi pool needed for ATP synthesis and as a substrate for glucose-6-phosphatase in gluconeogenesis and glycogenolysis, resulting in hypoglycemia. The correct answer D accurately describes this Pi trapping mechanism and its consequences on hepatic glucose production. Answer B is incorrect because it suggests increased ATP production, when actually ATP is depleted due to Pi sequestration and impaired oxidative phosphorylation. When evaluating metabolic blocks, consider secondary effects on cofactor pools (Pi, NAD+, CoA) that can explain seemingly unrelated symptoms. The key concept is that substrate accumulation can deplete essential cofactors, causing dysfunction in other pathways.

Question 13

1. Impaired fructose-1,6-bisphosphatase increases ketone production and causes hyperglycemia after fasting
2. Increased hepatic export of glucose reduces peripheral glycolysis, causing lactate accumulation in blood
3. Defective pyruvate dehydrogenase prevents acetyl-CoA formation, causing isolated lactic acidosis without hypoglycemia
4. Blocked conversion of glucose-6-phosphate to glucose increases diversion of glucose-6-phosphate into glycolysis, raising lactate (correct answer)

Explanation: This question tests understanding of von Gierke disease (GSD type Ia) and the metabolic consequences of glucose-6-phosphatase deficiency. Glucose-6-phosphatase catalyzes the final step of both gluconeogenesis and glycogenolysis in liver, converting glucose-6-phosphate to free glucose for export; its deficiency traps glucose-6-phosphate in hepatocytes. The accumulated glucose-6-phosphate cannot exit the cell and is diverted into glycolysis, producing pyruvate that is converted to lactate by lactate dehydrogenase to regenerate NAD+ for continued glycolysis, leading to lactic acidosis even during fasting when gluconeogenesis should predominate. The correct answer D accurately describes how blocked glucose export increases diversion into glycolysis and lactate production. Answer B is incorrect because it suggests increased glucose export, when the mutation specifically prevents glucose release from hepatocytes. To understand glucose-6-phosphatase deficiency, remember it affects the final common pathway of hepatic glucose production, causing fasting hypoglycemia, lactic acidosis, hyperuricemia, and hepatomegaly from glycogen accumulation. The key concept is that metabolic blocks force substrate diversion into alternative pathways.

Question 14

1. Inability to generate reactive oxygen species in phagolysosomes, leading to impaired killing of catalase-positive organisms (correct answer)
2. Increased oxidative burst with excessive hypochlorite formation, causing severe tissue necrosis after minor infections
3. Defective urea cycle enzyme causing elevated ammonia and increased orotic acid in urine
4. Reduced synthesis of heme due to impaired ALA synthase, causing microcytic anemia and basophilic stippling

Explanation: This question tests understanding of chronic granulomatous disease and the role of NADPH oxidase in phagocyte function. NADPH oxidase is a multi-subunit enzyme complex that generates superoxide radicals from molecular oxygen using NADPH as an electron donor, initiating the respiratory burst that produces reactive oxygen species (ROS) for microbial killing. When NADPH oxidase cannot assemble properly, phagocytes cannot generate superoxide and downstream ROS like hydrogen peroxide and hypochlorite, making them unable to kill catalase-positive organisms that can neutralize any residual hydrogen peroxide from other sources. The correct answer A accurately describes the inability to generate ROS and the specific susceptibility to catalase-positive organisms like Staphylococcus aureus. Answer B is incorrect because it suggests increased oxidative burst, when the mutation actually prevents ROS generation entirely. To identify phagocyte defects, remember that chronic granulomatous disease specifically impairs the oxidative burst, while myeloperoxidase deficiency affects only hypochlorite production but maintains some antimicrobial activity. The key diagnostic test is the nitroblue tetrazolium (NBT) or dihydrorhodamine (DHR) test showing absent oxidative burst.

Question 15

1. Decreased cytosolic carbamoyl phosphate causes reduced pyrimidine synthesis and low orotic acid
2. Accumulation of mitochondrial carbamoyl phosphate increases cytosolic orotic acid production via pyrimidine synthesis (correct answer)
3. Increased conversion of ornithine to citrulline lowers ammonia and prevents neurologic symptoms
4. Primary impairment of phenylalanine hydroxylase leads to elevated phenylketones and musty odor

Explanation: This question tests understanding of urea cycle enzyme deficiencies and their metabolic consequences. Ornithine transcarbamylase (OTC) catalyzes the conversion of ornithine and carbamoyl phosphate to citrulline in the mitochondria, and its deficiency causes the most common urea cycle disorder. When OTC activity is reduced, carbamoyl phosphate accumulates in the mitochondria and leaks into the cytosol, where it enters the pyrimidine synthesis pathway via carbamoyl phosphate synthetase II, leading to increased orotic acid production. The correct answer B accurately describes this pathophysiology, explaining both the elevated ammonia (due to impaired urea cycle) and elevated orotic acid (due to shunting into pyrimidine synthesis). Answer A is incorrect because it suggests decreased cytosolic carbamoyl phosphate and low orotic acid, which is opposite to what occurs in OTC deficiency. To identify urea cycle defects, remember that OTC deficiency uniquely causes both hyperammonemia and orotic aciduria, while carbamoyl phosphate synthetase I deficiency causes hyperammonemia without orotic aciduria.

Question 16

1. Increased glycolytic flux with increased lactate production because PFK-1 is less regulated by ATP
2. Decreased glycolytic flux leading to reduced ATP generation from substrate-level phosphorylation and early fatigue (correct answer)
3. Increased conversion of lactate to glucose in muscle, causing hyperglycemia during exercise
4. Primary impairment of $\beta$-oxidation causes hypoketotic hypoglycemia during fasting, unrelated to exercise

Explanation: 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.

Question 17

1. Decreased phenylalanine causes reduced neurotransmitter synthesis due to lack of aromatic amino acids
2. Accumulated phenylalanine is transaminated to phenylpyruvate and related metabolites, contributing to neurotoxicity (correct answer)
3. Tyrosine accumulation directly inhibits glycolysis, causing primary lactic acidosis
4. Defective PAH prevents conversion of tyrosine to DOPA, causing isolated hypopigmentation without elevated phenylalanine

Explanation: 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.

Question 18

1. Increased melanin synthesis due to excess tyrosine production from phenylalanine
2. Accumulation of phenylalanine with reduced tyrosine availability, potentially decreasing catecholamine synthesis downstream (correct answer)
3. Enhanced urea cycle flux due to increased conversion of phenylalanine nitrogen into urea
4. Increased conversion of tyrosine back to phenylalanine, preventing phenylalanine accumulation

Explanation: This question tests understanding of phenylketonuria (PKU) pathophysiology at the molecular level. Phenylalanine hydroxylase (PAH) requires tetrahydrobiopterin (BH4) as a cofactor to convert phenylalanine to tyrosine; reduced cofactor affinity decreases the enzyme's effective activity even with normal expression levels. The mutation creates a functional deficiency where phenylalanine accumulates because it cannot be efficiently hydroxylated, while tyrosine becomes conditionally essential due to reduced synthesis from phenylalanine. The correct answer B accurately describes this accumulation pattern and the potential downstream effect on catecholamine synthesis, which requires tyrosine as a precursor. Answer A incorrectly suggests increased melanin when actually tyrosine deficiency would decrease it, while D impossibly suggests reversing an irreversible reaction. Understanding cofactor-dependent enzymes requires recognizing that mutations affecting cofactor binding can be as deleterious as those affecting the active site.

Question 19

1. Decreased homogentisate levels with increased conversion to maleylacetoacetate, preventing pigment formation
2. Accumulation of homogentisate with increased urinary excretion that darkens upon oxidation (correct answer)
3. Primary hyperammonemia due to impaired nitrogen disposal from tyrosine catabolism
4. Increased melanin synthesis due to shunting of homogentisate into the catecholamine pathway

Explanation: This question tests understanding of enzyme deficiency leading to accumulation of a pathway intermediate that polymerizes and deposits in tissues. Homogentisate 1,2-dioxygenase cleaves the aromatic ring of homogentisate in the tyrosine degradation pathway. When this enzyme is deficient in alkaptonuria, homogentisate accumulates and is excreted in urine, where it polymerizes upon oxidation (exposure to air) to form dark pigments. The correct answer (B) accurately describes this accumulation and excretion pattern, explaining the characteristic darkening urine. Choice A incorrectly suggests decreased homogentisate levels, when the enzyme deficiency actually causes homogentisate accumulation upstream of the block. When analyzing metabolic defects with visible phenotypes, connect the chemical properties of accumulating metabolites (like oxidation-prone aromatic compounds) to the observed clinical signs.

Question 20

1. Increased lactate production in muscle due to accelerated anaerobic glycolysis from glycogen-derived glucose-1-phosphate
2. Decreased ability to generate glucose-6-phosphate from muscle glycogen, reducing glycolytic flux and predisposing to rhabdomyolysis (correct answer)
3. Increased hepatic ketogenesis as the primary cause of myoglobinuria during sprinting
4. Improved endurance due to preferential oxidation of fatty acids replacing the need for glycogen in anaerobic conditions

Explanation: 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.