USMLE Step 1 Quiz
Practice Citric Acid Cycle And Oxidative Phosphorylation in USMLE Step 1 with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.
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A 25-year-old sprinter undergoes a muscle biopsy as part of a research study on exercise physiology. The skeletal muscle cells are analyzed for their mechanism of transferring reducing equivalents from glycolysis into the mitochondria. These cells primarily utilize the glycerol-3-phosphate shuttle.
Compared to the malate-aspartate shuttle, the glycerol-3-phosphate shuttle results in a lower net ATP yield from glycolysis because it transfers electrons from cytosolic NADH to:
This quiz focuses on Citric Acid Cycle And Oxidative Phosphorylation, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 1.
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A 25-year-old sprinter undergoes a muscle biopsy as part of a research study on exercise physiology. The skeletal muscle cells are analyzed for their mechanism of transferring reducing equivalents from glycolysis into the mitochondria. These cells primarily utilize the glycerol-3-phosphate shuttle.
Compared to the malate-aspartate shuttle, the glycerol-3-phosphate shuttle results in a lower net ATP yield from glycolysis because it transfers electrons from cytosolic NADH to:
Explanation: The glycerol-3-phosphate shuttle transfers electrons from cytosolic NADH to dihydroxyacetone phosphate (DHAP), forming glycerol-3-phosphate. An inner mitochondrial membrane enzyme then transfers these electrons to FAD, forming FADH₂. This FADH₂ then enters the electron transport chain at Complex II. Because this process generates FADH₂ instead of NADH, it bypasses Complex I, resulting in fewer protons being pumped and a lower ATP yield (~1.5 ATP) compared to the malate-aspartate shuttle.
A 62-year-old man with a history of chronic alcohol use disorder is brought to the emergency department with confusion, ataxia, and nystagmus. His symptoms are attributed to a deficiency in thiamine. Thiamine is a crucial cofactor for an enzyme in the citric acid cycle that is structurally and mechanistically similar to the pyruvate dehydrogenase complex.
A deficiency of thiamine directly impairs the function of which of the following citric acid cycle enzymes?
Explanation: The patient's symptoms are characteristic of Wernicke encephalopathy, caused by thiamine (vitamin B1) deficiency. Thiamine, in its active form thiamine pyrophosphate (TPP), is an essential cofactor for the α-ketoglutarate dehydrogenase complex. This enzyme catalyzes the conversion of α-ketoglutarate to succinyl-CoA. This complex, like the pyruvate dehydrogenase complex, requires five cofactors: TPP, lipoic acid, coenzyme A, FAD, and NAD⁺.
A 24-year-old woman is participating in a prolonged fast for religious reasons. After 24 hours, her liver is actively engaged in gluconeogenesis to maintain blood glucose levels. This process consumes large amounts of oxaloacetate, potentially depleting it from the citric acid cycle.
Which of the following anaplerotic reactions is most critical for replenishing oxaloacetate in the liver under these conditions?
Explanation: Pyruvate carboxylase is the most important anaplerotic ('filling up') enzyme. It catalyzes the irreversible carboxylation of pyruvate to form oxaloacetate. This reaction is particularly crucial in the liver during gluconeogenesis, as it provides the necessary oxaloacetate for both the citric acid cycle to continue and for the synthesis of glucose. Malate dehydrogenase (C) is a reversible step within the TCA cycle itself, not a net source of new intermediates.
A 45-year-old agricultural worker presents to the emergency department with vomiting, watery diarrhea, and a garlic-like odor on his breath. He reports accidentally ingesting a pesticide. Laboratory studies show a severe lactic acidosis. The toxin is known to inactivate enzymes that require lipoic acid as a cofactor.
The patient's metabolic acidosis is primarily due to the inhibition of which two enzyme complexes?
Explanation: This is a classic presentation of arsenic poisoning. Arsenic binds to sulfhydryl groups in lipoic acid, inactivating it. Lipoic acid is a required cofactor for both the pyruvate dehydrogenase (PDH) complex and the α-ketoglutarate dehydrogenase complex. Inhibition of PDH prevents the conversion of pyruvate to acetyl-CoA, shunting pyruvate towards lactate formation and causing a severe lactic acidosis.
A researcher is studying mitochondrial respiration using an oxygen electrode. She adds isolated mitochondria, ADP, phosphate, and a substrate that provides NADH to the reaction chamber. After observing a steady rate of oxygen consumption, she adds rotenone, a pesticide.
The addition of rotenone will most directly lead to which of the following?
Explanation: Rotenone is a specific inhibitor of Complex I (NADH:ubiquinone oxidoreductase) of the electron transport chain. It blocks the transfer of electrons from NADH to coenzyme Q. This directly halts the proton pumping activity of Complex I, which in turn leads to a buildup of NADH (increasing the NADH/NAD⁺ ratio) and a decrease in overall oxygen consumption and ATP synthesis. It does not affect electron flow from FADH₂ (which enters at Complex II) nor does it uncouple the chain.
A laboratory experiment is designed to investigate oxidative phosphorylation. Oligomycin is added to a suspension of actively respiring mitochondria. This results in a sharp decrease in both ATP synthesis and oxygen consumption.
The observed effects of oligomycin are best explained by its direct inhibition of which of the following?
Explanation: Oligomycin is an antibiotic that specifically inhibits ATP synthase (Complex V) by binding to its F₀ subunit and blocking the proton channel. This prevents protons from flowing back into the mitochondrial matrix, thus inhibiting ATP synthesis. Because ATP synthesis is tightly coupled to electron transport, the buildup of the proton gradient creates a back-pressure that also inhibits the electron transport chain and oxygen consumption.
A 19-year-old woman is brought to the emergency department with fever, rapid breathing, and confusion after ingesting a large quantity of aspirin. Laboratory results show a high anion gap metabolic acidosis. The patient's hyperthermia is thought to be a direct consequence of the drug's effect on mitochondrial metabolism.
The fever observed in this patient is most likely caused by which of the following mechanisms?
Explanation: High doses of salicylates (aspirin) act as uncoupling agents. They are lipid-soluble weak acids that can transport protons across the inner mitochondrial membrane, bypassing ATP synthase. This dissipates the proton motive force, uncoupling electron transport from ATP synthesis. The energy that would have been captured in ATP is instead released as heat, causing hyperthermia. The body attempts to compensate by increasing the rate of electron transport and oxygen consumption, which generates even more heat.
A neonate born at term is able to maintain a stable core body temperature when placed in a cool environment, a process known as non-shivering thermogenesis. This is largely mediated by a specific protein found in the mitochondria of brown adipose tissue.
The protein responsible for this physiological process, thermogenin (UCP1), generates heat by which of the following mechanisms?
Explanation: Thermogenin, also known as uncoupling protein 1 (UCP1), is a proton channel in the inner mitochondrial membrane of brown adipocytes. When activated, it allows protons that have been pumped into the intermembrane space to flow back into the matrix, bypassing ATP synthase. This uncouples oxidative phosphorylation, and the energy stored in the proton gradient is released as heat rather than being converted to ATP, thus warming the infant.
An 11-month-old infant presents with profound developmental delay, seizures, and hypotonia. Urine organic acid analysis reveals a marked elevation of fumarate. A deficiency of the enzyme fumarase is suspected.
In addition to being a key component of the citric acid cycle, the reaction catalyzed by fumarase is also important for linking the citric acid cycle with which other metabolic pathway?
Explanation: The urea cycle produces fumarate as a byproduct when argininosuccinate is cleaved to form arginine. This fumarate can then enter the citric acid cycle and be converted to malate by fumarase, and subsequently to oxaloacetate. This links the two cycles, allowing for the disposal of nitrogen from amino acid catabolism while regenerating intermediates that can be used for energy or gluconeogenesis. A deficiency in fumarase disrupts this link.
A medical student is creating a study guide on heme synthesis. The first step of this pathway, which occurs in the mitochondria, is the condensation of glycine and an intermediate derived from the citric acid cycle.
Which of the following citric acid cycle intermediates is a direct precursor for heme synthesis?
Explanation: The first and rate-limiting step of heme synthesis is the formation of δ-aminolevulinic acid (ALA), catalyzed by the enzyme ALA synthase. This mitochondrial enzyme catalyzes the condensation of one molecule of glycine with one molecule of succinyl-CoA, which is an intermediate of the citric acid cycle. This illustrates another key role of the TCA cycle in providing building blocks for other molecules.
A 58-year-old rancher presents with seizures and severe metabolic acidosis after eating plants contaminated with fluoroacetate, a potent toxin. Laboratory analysis reveals a markedly elevated serum citrate level.
Fluoroacetate toxicity results from the 'lethal synthesis' of fluorocitrate, which is a potent noncompetitive inhibitor of which of the following enzymes?
Explanation: Fluoroacetate is converted in vivo to fluoroacetyl-CoA, which is then used by citrate synthase to produce fluorocitrate. Fluorocitrate is a structural analog of citrate and acts as a potent inhibitor of aconitase, the enzyme that converts citrate to isocitrate. This inhibition blocks the citric acid cycle at this step, leading to a massive accumulation of citrate and a profound energy deficit.
A 66-year-old man suffers an acute myocardial infarction due to a complete occlusion of his left anterior descending artery. The affected cardiomyocytes switch to anaerobic glycolysis, but this is insufficient to meet their energy demands, leading to cell death.
The primary reason for the failure of the citric acid cycle and oxidative phosphorylation in these ischemic cells is the absence of:
Explanation: In ischemia, the lack of blood flow means a lack of oxygen delivery. Oxygen is the final electron acceptor for the electron transport chain. Without oxygen, electrons cannot be passed from Complex IV, causing the entire chain to back up. This prevents the re-oxidation of NADH and FADH₂ to NAD⁺ and FAD. The lack of these oxidized cofactors, in turn, halts the citric acid cycle. Therefore, the absence of oxygen is the root cause of the metabolic collapse.
A 28-year-old man with type 1 diabetes presents with diabetic ketoacidosis. His liver is producing large quantities of ketone bodies from the acetyl-CoA generated by fatty acid oxidation. This process is favored over the entry of acetyl-CoA into the citric acid cycle.
The shunting of acetyl-CoA towards ketogenesis in the liver is primarily caused by the depletion of which citric acid cycle intermediate for gluconeogenesis?
Explanation: In diabetic ketoacidosis, the low insulin/glucagon ratio promotes gluconeogenesis in the liver to maintain blood glucose. Gluconeogenesis consumes oxaloacetate (OAA). The massive rate of beta-oxidation produces an abundance of acetyl-CoA, but this acetyl-CoA cannot enter the citric acid cycle because its partner molecule, OAA, has been depleted. The excess acetyl-CoA is therefore converted into ketone bodies (acetoacetate and β-hydroxybutyrate).
A 34-year-old man is rescued from a house fire and brought to the emergency department. He is hypotensive, tachycardic, and has a severe metabolic acidosis. His venous blood is noted to be bright red, suggesting that tissues are not extracting oxygen. Poisoning with cyanide is suspected.
Cyanide exerts its toxic effect by binding to the ferric (Fe³⁺) iron of cytochrome a₃. This action directly inhibits which component of cellular respiration?
Explanation: Cyanide has a high affinity for the ferric (Fe³⁺) form of heme iron in cytochrome a₃, which is a component of Complex IV (cytochrome c oxidase). This binding prevents the final transfer of electrons to oxygen, the ultimate electron acceptor. This effectively halts the entire electron transport chain, leading to a rapid cessation of aerobic ATP production and cytotoxic hypoxia.
A 22-year-old man presents with progressive bilateral vision loss. His family history is significant for a maternal uncle who went blind in his twenties. A muscle biopsy shows ragged red fibers, and genetic testing reveals a mutation in a gene encoding a subunit of Complex I of the electron transport chain. This defect impairs the oxidation of NADH.
Due to this defect, which of the following metabolic consequences is most likely to be observed in this patient's affected tissues?
Explanation: The patient's presentation is consistent with Leber hereditary optic neuropathy (LHON), a mitochondrial disease. A defect in Complex I impairs the re-oxidation of NADH to NAD⁺. This causes NADH to accumulate, leading to a significantly elevated NADH/NAD⁺ ratio. The lack of NAD⁺ regeneration inhibits NAD⁺-dependent enzymes, including those in the citric acid cycle and glycolysis (e.g., pyruvate dehydrogenase), and shunts pyruvate to lactate to regenerate some NAD⁺.
A rapidly dividing cancer cell is cultured in a medium rich in glucose. To support the synthesis of new membranes, the cell must produce large quantities of fatty acids. This process requires a supply of acetyl-CoA in the cytoplasm.
Which citric acid cycle intermediate is transported out of the mitochondria to provide the acetyl-CoA required for cytoplasmic fatty acid synthesis?
Explanation: Acetyl-CoA produced in the mitochondria cannot directly cross the inner mitochondrial membrane. To be used for fatty acid synthesis in the cytoplasm, it first condenses with oxaloacetate to form citrate via citrate synthase. Citrate is then transported out to the cytoplasm via a specific transporter. In the cytoplasm, the enzyme ATP-citrate lyase cleaves citrate back into acetyl-CoA and oxaloacetate, making acetyl-CoA available for lipid synthesis.
A biochemistry student is calculating the total ATP yield from the complete oxidation of a glucose molecule. She notes that the reducing equivalents FADH₂ and NADH, which are generated in the citric acid cycle, yield different amounts of ATP when they donate electrons to the electron transport chain.
The oxidation of one molecule of FADH₂ yields fewer ATPs than the oxidation of one molecule of NADH because electrons from FADH₂:
Explanation: FADH₂ is generated by succinate dehydrogenase (Complex II) within the citric acid cycle. It donates its electrons directly to Complex II, which then passes them to coenzyme Q. This entry point bypasses Complex I, which is a major site of proton pumping. Because electrons from FADH₂ contribute to the proton gradient only at Complexes III and IV, fewer protons are pumped compared to NADH (which utilizes Complexes I, III, and IV). This results in a lower ATP yield (~1.5 ATP for FADH₂ vs. ~2.5 ATP for NADH).
A researcher treats isolated mitochondria with antimycin A, an inhibitor of Complex III. She then analyzes the oxidation state of the various components of the electron transport chain.
Following treatment with antimycin A, which of the following electron carriers would accumulate in its reduced form?
Explanation: Antimycin A inhibits Complex III, blocking the transfer of electrons from coenzyme Q (ubiquinone) to cytochrome c. This creates an electronic 'dam'. All components upstream of the block (NADH, Complex I, FADH₂, Complex II, and Coenzyme Q) will be unable to pass off their electrons and will accumulate in a reduced state. All components downstream of the block (cytochrome c, Complex IV, and oxygen) will be unable to accept electrons and will remain in an oxidized state.
A researcher is studying metabolism in cultured hepatocytes. These cells rely on the malate-aspartate shuttle to transport reducing equivalents from cytosolic NADH, produced during glycolysis, into the mitochondria for oxidative phosphorylation.
The primary purpose of the malate-aspartate shuttle is to:
Explanation: The inner mitochondrial membrane is impermeable to NADH. The malate-aspartate shuttle is a complex mechanism that transfers the electrons (reducing equivalents) from NADH in the cytosol to NAD⁺ inside the mitochondrial matrix, forming mitochondrial NADH. This allows the energy from cytosolic NADH to be efficiently captured by the electron transport chain, yielding approximately 2.5 ATP per molecule.