Home

Tutoring

Subjects

Live Classes

Study Coach

Essay Review

On-Demand Courses

Colleges

Games

Sign up

Log in

Opening subject page...

Loading your content

Practice

  • All Subjects
  • Algebra Flashcards
  • SAT Math Practice Tests
  • Math Question of the Day
  • Live Classes
  • On-Demand Courses

Varsity Tutors

  • Find a Tutor
  • Test Prep
  • Online Classes
  • K-12 Learning
  • College Search
  • VarsityTutors.com

© 2026 Varsity Tutors. All rights reserved.

← Back to quizzes

USMLE Step 1 Quiz

USMLE Step 1 Quiz: Pentose Phosphate Pathway And Redox Balance

Practice Pentose Phosphate Pathway And Redox Balance in USMLE Step 1 with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

Question 1 / 20

0 of 20 answered

A 24-year-old man of Mediterranean descent presents to the emergency department with a 2-day history of fatigue, jaundice, and dark-colored urine. He was diagnosed with a urinary tract infection 4 days ago and was started on trimethoprim-sulfamethoxazole. He has no other significant past medical history. Physical examination reveals scleral icterus and pallor. Laboratory studies show a hemoglobin of 8.9 g/dL (normal: 13.5-17.5 g/dL), an elevated reticulocyte count, and increased indirect bilirubin.

A deficiency of which of the following enzymes is the most likely underlying cause of this patient's presentation?

Select an answer to continue

What this quiz covers

This quiz focuses on Pentose Phosphate Pathway And Redox Balance, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 1.

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.

All questions

Question 1

A 24-year-old man of Mediterranean descent presents to the emergency department with a 2-day history of fatigue, jaundice, and dark-colored urine. He was diagnosed with a urinary tract infection 4 days ago and was started on trimethoprim-sulfamethoxazole. He has no other significant past medical history. Physical examination reveals scleral icterus and pallor. Laboratory studies show a hemoglobin of 8.9 g/dL (normal: 13.5-17.5 g/dL), an elevated reticulocyte count, and increased indirect bilirubin.

A deficiency of which of the following enzymes is the most likely underlying cause of this patient's presentation?

  1. Glucose-6-phosphate dehydrogenase (correct answer)
  2. Pyruvate kinase
  3. Aldolase B
  4. Phenylalanine hydroxylase

Explanation: This patient's presentation of acute hemolytic anemia (fatigue, jaundice, dark urine) triggered by an oxidative stressor (sulfonamide drug) in a person of Mediterranean descent is classic for glucose-6-phosphate dehydrogenase (G6PD) deficiency. G6PD is the rate-limiting enzyme of the pentose phosphate pathway, which produces NADPH. NADPH is crucial for protecting red blood cells from oxidative damage.

Question 2

A 30-year-old African American man is prescribed primaquine as prophylaxis before traveling to a malaria-endemic region. Ten days later, he develops pallor, scleral icterus, and back pain. A peripheral blood smear is prepared and stained with crystal violet.

The precipitation of oxidized, denatured hemoglobin within this patient's erythrocytes would most likely be observed as which of the following findings on the specially stained smear?

  1. Howell-Jolly bodies
  2. Basophilic stippling
  3. Pappenheimer bodies
  4. Heinz bodies (correct answer)

Explanation: In G6PD deficiency, oxidative stress (from drugs like primaquine) leads to the oxidation of sulfhydryl groups on hemoglobin, causing it to denature and precipitate within red blood cells. These intracellular precipitates are known as Heinz bodies. They are not visible on a standard Wright-Giemsa stain but can be seen with supravital stains like crystal violet. Macrophages in the spleen pluck out these inclusions, creating 'bite cells'.

Question 3

A researcher is studying the metabolic regulation of the pentose phosphate pathway (PPP) in cultured hepatocytes. The rate-limiting enzyme of this pathway, glucose-6-phosphate dehydrogenase, is subject to allosteric regulation.

The activity of the rate-limiting enzyme of the pentose phosphate pathway is most directly stimulated by an increased intracellular concentration of which of the following?

  1. NADPH
  2. NADP+ (correct answer)
  3. ATP
  4. Citrate

Explanation: The primary regulator of the pentose phosphate pathway is the intracellular concentration of NADP+. Glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme, is allosterically activated by its substrate NADP+ and strongly inhibited by its product, NADPH. A high NADP+/NADPH ratio signifies a need for more NADPH, thus stimulating the pathway.

Question 4

A rapidly dividing malignant melanoma is found to have significantly upregulated metabolic pathways to support its growth and proliferation. These cells have a high demand for both energy and biosynthetic precursors.

In addition to providing reducing equivalents for biosynthesis, the pentose phosphate pathway directly contributes to tumor proliferation by producing which of the following essential precursors for nucleic acid synthesis?

  1. Ribose-5-phosphate (correct answer)
  2. Deoxyadenosine monophosphate
  3. Fructose-1,6-bisphosphate
  4. Acetyl-CoA

Explanation: The pentose phosphate pathway has two major products: NADPH and ribose-5-phosphate. While NADPH is used for reductive biosynthesis and antioxidant defense, ribose-5-phosphate is a critical precursor for the synthesis of purine and pyrimidine nucleotides, which are required for DNA and RNA synthesis. This is essential for rapidly dividing cells, such as those in a malignant tumor.

Question 5

A 4-year-old boy presents with a history of recurrent pneumonia caused by Staphylococcus aureus and skin abscesses caused by Serratia marcescens. A dihydrorhodamine (DHR) flow cytometry test shows an absence of the phagocytic oxidative burst. This condition is caused by a defect in NADPH oxidase.

The underlying defect in this patient's phagocytes involves the inability to utilize which of the following molecules to generate superoxide radicals?

  1. NADH
  2. FADH2
  3. ATP
  4. NADPH (correct answer)

Explanation: This patient has chronic granulomatous disease (CGD), an immunodeficiency caused by a defect in NADPH oxidase. This enzyme is crucial for the phagocytic oxidative (respiratory) burst, where it transfers an electron from NADPH to molecular oxygen to form the superoxide radical (O2-). This is the first step in generating reactive oxygen species used to kill ingested pathogens. The NADPH required for this process is generated primarily by the pentose phosphate pathway.

Question 6

A medical student is reviewing the metabolic activity of various human tissues and notes that the pentose phosphate pathway (PPP) is highly active in certain cell types to support their specific functions.

Which of the following tissues would be expected to have the most highly active pentose phosphate pathway?

  1. Skeletal muscle at rest
  2. Adrenal cortex (correct answer)
  3. Mature erythrocytes
  4. Cardiac muscle

Explanation: The pentose phosphate pathway is most active in tissues involved in reductive biosynthesis or those exposed to high levels of oxidative stress. The adrenal cortex has a very high rate of PPP activity because it requires large amounts of NADPH for the synthesis of steroid hormones from cholesterol. Other examples include the liver (fatty acid and cholesterol synthesis) and red blood cells (antioxidant defense). While erythrocytes (Choice C) rely exclusively on the PPP for NADPH, the overall flux is lower than in biosynthetic tissues like the adrenal cortex.

Question 7

A patient is found to have a rare genetic defect in the enzyme glutathione reductase. This enzyme plays a critical role in the cellular antioxidant defense system.

This enzyme's deficiency would most directly impair the regeneration of reduced glutathione, leading to an accumulation of which of the following molecules, especially under conditions of oxidative stress?

  1. Oxidized glutathione (GSSG) (correct answer)
  2. NADPH
  3. Hydrogen peroxide
  4. Ribulose-5-phosphate

Explanation: Glutathione reductase catalyzes the reduction of glutathione disulfide (oxidized glutathione, GSSG) to sulfhydryl-form glutathione (reduced glutathione, GSH). This reaction requires NADPH as a reducing agent. A deficiency in glutathione reductase would prevent the recycling of GSSG back to GSH, leading to the accumulation of GSSG and a depletion of the cell's primary antioxidant, GSH.

Question 8

A 28-year-old woman with dermatitis herpetiformis is started on dapsone. Two weeks later, she presents to her physician with fatigue and back pain. Laboratory studies reveal a normocytic anemia, an elevated reticulocyte count, and increased indirect bilirubin. A peripheral smear shows red blood cells with portions of their cytoplasm removed, creating 'bite cells'.

The patient's symptoms are most likely due to an inherited deficiency that impairs the production of which of the following?

  1. ATP
  2. Heme
  3. NADPH (correct answer)
  4. Pyruvate

Explanation: This patient is experiencing drug-induced hemolytic anemia, characteristic of G6PD deficiency. Dapsone is a well-known oxidative stressor. The underlying defect is the inability to produce sufficient NADPH via the pentose phosphate pathway. Without adequate NADPH, erythrocytes cannot maintain a supply of reduced glutathione to protect against oxidative damage from the drug, leading to hemoglobin denaturation (Heinz bodies), and subsequent hemolysis as splenic macrophages remove these inclusions ('bite cells').

Question 9

In a hepatocyte, there is a high demand for NADPH to be used in fatty acid synthesis, but the need for ribose-5-phosphate for nucleotide synthesis is low. The cell must adjust its metabolic flux to meet these specific demands.

Under these metabolic conditions, the non-oxidative reactions of the pentose phosphate pathway would be primarily utilized to convert its products into which of the following pairs of glycolytic intermediates?

  1. Fructose-6-phosphate and glyceraldehyde-3-phosphate (correct answer)
  2. Pyruvate and acetyl-CoA
  3. Glucose-6-phosphate and 6-phosphogluconate
  4. 2-phosphoglycerate and phosphoenolpyruvate

Explanation: When the cell needs NADPH but not ribose-5-phosphate, the oxidative phase of the PPP runs to produce NADPH. The resulting ribose-5-phosphate is then converted by the non-oxidative reactions (transketolase and transaldolase) into fructose-6-phosphate and glyceraldehyde-3-phosphate. These intermediates can then re-enter the glycolytic pathway to be recycled back to glucose-6-phosphate, allowing for continued production of NADPH.

Question 10

A healthy couple has a son who experiences a severe hemolytic episode after being treated with a sulfonamide antibiotic. The mother mentions that her brother had similar episodes as a child after eating fava beans. The father and his family have no such history.

Given this family history, what is the most likely mode of inheritance for this son's condition?

  1. Autosomal dominant
  2. Autosomal recessive
  3. X-linked recessive (correct answer)
  4. Mitochondrial inheritance

Explanation: The family history is classic for an X-linked recessive disorder, such as G6PD deficiency. The condition affects males (son, mother's brother) and is passed down from an unaffected carrier mother. Since males have only one X chromosome, they are more likely to be affected by recessive mutations on that chromosome. The father's negative history is consistent with this pattern, as he passes his Y chromosome to his son.

Question 11

An experimental drug is found to be a potent and specific inhibitor of glutathione peroxidase in all cell types. This enzyme is selenium-dependent and plays a central role in protecting cells from oxidative damage.

Inhibition of this enzyme would lead to the toxic accumulation of which of the following molecules within the cell?

  1. Superoxide anion
  2. Hydrogen peroxide (correct answer)
  3. Reduced glutathione
  4. NADP+

Explanation: Glutathione peroxidase catalyzes the reduction of hydrogen peroxide (H2O2) and other organic peroxides to water, using reduced glutathione (GSH) as the electron donor. By inhibiting this enzyme, the cell loses a primary mechanism for detoxifying H2O2, leading to its accumulation. This buildup of H2O2 can cause significant oxidative damage to lipids, proteins, and DNA.

Question 12

A 22-year-old male soldier is being screened for G6PD deficiency before deployment to a malaria-endemic area where he will receive primaquine. He reports having a severe viral illness with fever two weeks ago, from which he has fully recovered. His blood count is now normal.

A quantitative G6PD enzyme activity assay performed at this time may yield a falsely normal result due to which of the following factors?

  1. Presence of a young erythrocyte population with higher enzyme levels (correct answer)
  2. Inhibition of the enzyme by circulating antibodies from the recent infection
  3. Rapid degradation of the screening test substrate by plasma enzymes
  4. Downregulation of G6PD gene expression during convalescence

Explanation: G6PD activity naturally declines as red blood cells age. During an acute hemolytic episode or any event that stimulates reticulocytosis (like recovery from an infection), older, G6PD-deficient erythrocytes are destroyed and replaced by a younger population of red blood cells and reticulocytes. These younger cells have higher, often near-normal, levels of G6PD activity. Therefore, testing for G6PD deficiency shortly after a hemolytic event or period of high RBC turnover can yield a false-negative result. Testing should be delayed for 2-3 months.

Question 13

A 5-year-old boy of Greek descent is brought to the emergency department with sudden onset of weakness, pallor, and jaundice. His mother states that he ate a large portion of a fava bean stew for the first time the previous day. His hemoglobin is 7.5 g/dL, and a peripheral smear shows bite cells.

The compounds present in fava beans precipitate hemolysis in susceptible individuals by generating a high level of reactive oxygen species, overwhelming the antioxidant capacity of a system dependent on which metabolic pathway?

  1. Heme synthesis pathway
  2. Embden-Meyerhof pathway
  3. Pentose phosphate pathway (correct answer)
  4. Urea cycle

Explanation: This clinical picture describes favism, a severe hemolytic anemia triggered by fava beans in individuals with G6PD deficiency. Fava beans contain oxidants (divicine, isouramil) that generate reactive oxygen species. In healthy individuals, the pentose phosphate pathway produces sufficient NADPH to maintain reduced glutathione levels and neutralize this oxidative stress. In G6PD-deficient individuals, this protective mechanism is impaired, leading to massive, acute hemolysis.

Question 14

Endothelial cells lining blood vessels produce nitric oxide (NO), a potent vasodilator that plays a key role in regulating blood pressure. The synthesis of NO from the amino acid arginine is catalyzed by nitric oxide synthase (NOS).

In addition to molecular oxygen and arginine, which of the following molecules is an essential reducing cofactor for the nitric oxide synthase reaction?

  1. NADH
  2. ATP
  3. NADPH (correct answer)
  4. Coenzyme A

Explanation: The synthesis of nitric oxide from arginine is an oxidative reaction that requires electron input. Nitric oxide synthase utilizes NADPH as the electron donor to catalyze this conversion. This is another example of the crucial role of NADPH, generated by the pentose phosphate pathway, in important physiological processes beyond antioxidant defense and macromolecule synthesis.

Question 15

The liver is the primary site of de novo fatty acid synthesis, a process that occurs in the cytosol. This anabolic pathway requires a significant supply of reducing equivalents to convert acetyl-CoA into palmitate.

The primary source of the cytosolic NADPH required for this reductive biosynthetic process is which of the following pathways?

  1. Glycolysis
  2. Beta-oxidation
  3. Citric acid cycle
  4. Pentose phosphate pathway (correct answer)

Explanation: Fatty acid synthesis is a reductive process that requires NADPH as the electron donor. The main pathway for generating cytosolic NADPH is the pentose phosphate pathway (PPP). The liver, being a major site of fatty acid synthesis, has a highly active PPP to supply the necessary reducing power. The citric acid cycle produces NADH and FADH2 in the mitochondria, while beta-oxidation is the catabolic process of breaking down fatty acids.

Question 16

A biochemist is outlining the reactions of the pentose phosphate pathway, dividing it into an oxidative phase and a non-oxidative phase.

The irreversible, oxidative reactions of this pathway are physiologically dedicated to the production of which of the following key molecules?

  1. ATP and NADH
  2. FADH2 and acetyl-CoA
  3. NADPH and a pentose phosphate (correct answer)
  4. Pyruvate and lactate

Explanation: The oxidative phase of the pentose phosphate pathway consists of two irreversible steps catalyzed by glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. For each molecule of glucose-6-phosphate that enters this phase, two molecules of NADPH are produced, along with one molecule of the pentose phosphate, ribulose-5-phosphate. This phase serves the cell's need for reducing power (NADPH) and precursors for nucleotide synthesis.

Question 17

A 19-year-old man presents with a lifelong history of mild to moderate fatigue and jaundice, which worsen during infections. Physical examination reveals splenomegaly. Laboratory tests show a chronic normocytic, hemolytic anemia with a high reticulocyte count. A peripheral smear shows numerous echinocytes (burr cells). He denies any episodes of acute, severe hemolysis after exposure to specific drugs or foods.

The chronic nature of this patient's hemolysis and the specific red blood cell morphology make a deficiency in which of the following enzymes more likely than a deficiency in glucose-6-phosphate dehydrogenase?

  1. Pyruvate kinase (correct answer)
  2. Fumarase
  3. Spectrin
  4. Catalase

Explanation: This patient's presentation of chronic, congenital, non-spherocytic hemolytic anemia with splenomegaly and echinocytes is classic for pyruvate kinase (PK) deficiency. PK deficiency is a defect in glycolysis, leading to insufficient ATP production. Lack of ATP impairs ion pumps (e.g., Na+/K+-ATPase), causing red blood cells to become dehydrated and rigid (forming echinocytes), leading to their premature destruction in the spleen. This contrasts with G6PD deficiency, which typically causes episodic, trigger-induced hemolysis related to oxidative stress, not a chronic state of hemolysis from ATP depletion.

Question 18

A student is contrasting the metabolic roles of the nicotinamide adenine dinucleotide coenzymes, NADH and NADPH. Although structurally similar, their primary functions in the cell are distinct.

While NADH is primarily oxidized by the electron transport chain to generate ATP, NADPH is predominantly used for which of the following cellular processes?

  1. Oxidative catabolic reactions
  2. Substrate-level phosphorylation
  3. Proton gradient formation
  4. Reductive biosynthetic reactions (correct answer)

Explanation: Cellular metabolism maintains a high NAD+/NADH ratio to favor oxidative reactions (catabolism), whereas it maintains a high NADPH/NADP+ ratio to favor reductive reactions (anabolism). NADPH serves as the primary electron donor in reductive biosynthetic pathways, such as fatty acid synthesis, cholesterol synthesis, and steroid synthesis. It is also essential for antioxidant defense systems (e.g., glutathione reductase).

Question 19

A patient with a known deficiency in glucose-6-phosphate dehydrogenase (G6PD) develops acute hemolytic anemia after exposure to an oxidative stressor. Erythrocytes are particularly vulnerable in this condition due to their reliance on the pentose phosphate pathway for protection against oxidative damage.

The increased susceptibility of this patient's erythrocytes to oxidative damage is primarily due to a decreased ability to regenerate which of the following molecules?

  1. 2,3-Bisphosphoglycerate
  2. Methemoglobin
  3. Reduced glutathione (correct answer)
  4. Pyruvate

Explanation: G6PD deficiency leads to insufficient production of NADPH. NADPH is required by the enzyme glutathione reductase to convert oxidized glutathione (GSSG) back to reduced glutathione (GSH). GSH is essential for the function of glutathione peroxidase, which neutralizes reactive oxygen species like hydrogen peroxide. Without adequate GSH regeneration, erythrocytes cannot defend against oxidative damage, leading to hemolysis.

Question 20

A 55-year-old man with a history of chronic alcohol use disorder is brought to the emergency department with confusion, ataxia, and nystagmus. He is diagnosed with Wernicke encephalopathy due to thiamine (vitamin B1) deficiency. Thiamine pyrophosphate is a necessary cofactor for several key enzymes in carbohydrate metabolism.

The activity of transketolase, an enzyme dependent on this patient's deficient vitamin, is a critical component for which of the following processes?

  1. Irreversible oxidation of glucose-6-phosphate
  2. Interconversion of sugar phosphates in the non-oxidative PPP (correct answer)
  3. Decarboxylation of pyruvate to acetyl-CoA
  4. Synthesis of glycogen from glucose-1-phosphate

Explanation: Transketolase is a key enzyme in the non-oxidative (reversible) branch of the pentose phosphate pathway (PPP). It requires thiamine pyrophosphate (TPP) as a cofactor and catalyzes the transfer of two-carbon units between sugar phosphates, allowing for their interconversion (e.g., xylulose-5-P + ribose-5-P <=> glyceraldehyde-3-P + sedoheptulose-7-P). This links the PPP with glycolysis. While pyruvate dehydrogenase (Choice C) also requires thiamine, the question specifically asks about transketolase.