Exercise metabolism — MCQs
A 25-year-old male athlete undergoes a cardiopulmonary exercise test. As exercise intensity increases from rest to moderate levels, which of the following best describes the relationship between oxygen consumption and cardiac output?
A 24-year-old professional athlete is advised to train in the mountains to enhance his performance. After 5 months of training at an altitude of 1.5 km (5,000 feet), he is able to increase his running pace while competing at sea-level venues. Which of the following changes would produce the same effect on the oxygen-hemoglobin dissociation curve as this athlete's training did?
A 15-year-old boy is sent from gym class with a chief complaint of severe muscle aches. In class today he was competing with his friends and therefore engaged in weightlifting for the first time. A few hours later he was extremely sore and found that his urine was red when he went to urinate. This concerned him and he was sent to the emergency department for evaluation. Upon further questioning, you learn that since childhood he has always had muscle cramps with exercise. Physical exam was unremarkable. Upon testing, his creatine kinase level was elevated and his urinalysis was negative for blood and positive for myoglobin. Thinking back to biochemistry you suspect that he may be suffering from a hereditary glycogen disorder. Given this suspicion, what would you expect to find upon examination of his cells?
A scientist is trying to design a drug to modulate cellular metabolism in the treatment of obesity. Specifically, he is interested in understanding how fats are processed in adipocytes in response to different energy states. His target is a protein within these cells that catalyzes catabolism of an energy source. The products of this reaction are subsequently used in gluconeogenesis or β-oxidation. Which of the following is true of the most likely protein that is being studied by this scientist?
During exercise, what is the primary mechanism for increased oxygen delivery to active muscles?
During heavy exercise, what is the primary mechanism for maintaining arterial pH despite increased lactic acid production?
Which hormone most strongly stimulates gluconeogenesis during prolonged fasting?
A 52-year-old man undergoes an exercise stress test for a 1-week history of squeezing substernal chest pain that is aggravated by exercise and relieved by rest. During the test, there is a substantial increase in the breakdown of glycogen in the muscle cells. Which of the following changes best explains this intracellular finding?
A 4-year-old boy presents to the ophthalmologist for a down- and inward dislocation of the lens in his left eye. On physical exam, the boy has a marfanoid habitus and intellectual disability. Biochemical tests were performed to locate the exact defect in this boy. It was found that there was a significant deficiency in cystathionine beta-synthase activity with elevated homocysteine levels. Which of the following is the diagnosis?
A 45-year-old patient presents with joint pain and weakness and is known to have homocystinuria. Which vitamin is required in the treatment?
Exercise metabolism US Medical PG Practice Questions and MCQs
Question 1: A 25-year-old male athlete undergoes a cardiopulmonary exercise test. As exercise intensity increases from rest to moderate levels, which of the following best describes the relationship between oxygen consumption and cardiac output?
- A. Linear increase until anaerobic threshold (Correct Answer)
- B. Exponential increase throughout exercise
- C. Plateau at low exercise intensities
- D. No change until anaerobic threshold
Explanation: ***Linear increase until anaerobic threshold*** - During incremental exercise, both **oxygen consumption (VO2)** and **cardiac output (CO)** increase proportionally with work rate. - This **linear relationship** continues until the body reaches the **anaerobic threshold**, beyond which other physiological responses begin to dominate. *Exponential increase throughout exercise* - An **exponential increase** would imply a disproportionately rapid rise in oxygen consumption and cardiac output even at low-to-moderate exercise intensities, which is not physiologically accurate. - While both parameters do increase, the initial increase is typically linear, reflecting the immediate physiological demands. *Plateau at low exercise intensities* - A **plateau** would suggest that the body's demand for oxygen and the heart's pumping capacity stabilize despite an increase in exercise intensity, which contradicts the need for increased energy supply during exercise. - The cardiovascular system actively responds to even low-intensity exercise to meet metabolic demands. *No change until anaerobic threshold* - **No change** would mean that the cardiovascular system is not responding to the increased metabolic demands of exercise, which is incorrect. - Both VO2 and CO begin to rise almost immediately upon starting exercise to meet the muscles' increasing oxygen requirements.
Question 2: A 24-year-old professional athlete is advised to train in the mountains to enhance his performance. After 5 months of training at an altitude of 1.5 km (5,000 feet), he is able to increase his running pace while competing at sea-level venues. Which of the following changes would produce the same effect on the oxygen-hemoglobin dissociation curve as this athlete's training did?
- A. Decreased 2,3-bisphosphoglycerate (Correct Answer)
- B. Increased carbon monoxide inhalation
- C. Decreased temperature
- D. Decreased pH
- E. Increased partial pressure of oxygen
Explanation: ***Decreased 2,3-bisphosphoglycerate*** - This is **NOT** the correct physiological adaptation from altitude training, making this question conceptually flawed. - Altitude training causes **increased erythropoietin → polycythemia → increased total hemoglobin**, which increases oxygen-carrying capacity. - 2,3-BPG is **initially increased** at altitude (right shift) to facilitate O2 release, and remains elevated or returns to normal with acclimatization, **not decreased**. - While decreased 2,3-BPG would cause a left shift (increased O2 affinity), this does NOT replicate altitude training adaptations. *Increased carbon monoxide inhalation* - Carbon monoxide binds hemoglobin with **200-250× higher affinity** than oxygen, forming carboxyhemoglobin. - This **reduces oxygen-carrying capacity** and causes a left shift for remaining hemoglobin. - This is harmful and does NOT replicate beneficial altitude adaptations. *Decreased temperature* - Decreases metabolic rate and causes a **left shift** (increased O2 affinity). - Oxygen is held more tightly and released less readily to tissues. - This does NOT replicate altitude training benefits. *Decreased pH* - Acidosis causes the **Bohr effect**: **right shift** (decreased O2 affinity). - Facilitates O2 release to tissues during exercise. - This is beneficial during exercise but does NOT replicate the chronic altitude adaptation of increased oxygen-carrying capacity. *Increased partial pressure of oxygen* - Higher PO2 increases hemoglobin saturation but does NOT shift the curve. - This increases oxygen availability but does NOT replicate the physiological adaptation (polycythemia) from altitude training. **Note:** This question is conceptually problematic as none of the options accurately replicate the primary altitude training adaptation (increased RBC mass/hemoglobin concentration).
Question 3: A 15-year-old boy is sent from gym class with a chief complaint of severe muscle aches. In class today he was competing with his friends and therefore engaged in weightlifting for the first time. A few hours later he was extremely sore and found that his urine was red when he went to urinate. This concerned him and he was sent to the emergency department for evaluation. Upon further questioning, you learn that since childhood he has always had muscle cramps with exercise. Physical exam was unremarkable. Upon testing, his creatine kinase level was elevated and his urinalysis was negative for blood and positive for myoglobin. Thinking back to biochemistry you suspect that he may be suffering from a hereditary glycogen disorder. Given this suspicion, what would you expect to find upon examination of his cells?
- A. Normal glycogen structure (Correct Answer)
- B. Short outer glycogen chains
- C. Accumulation of glycogen in lysosomes forming dense granules
- D. Glycogen without normal branching pattern
- E. Absence of glycogen in muscles
Explanation: ***Normal glycogen structure*** - The patient's symptoms (exercise-induced muscle cramps, myoglobinuria, and elevated CK) are classic for **McArdle disease** (Glycogen Storage Disease Type V), caused by a deficiency in **muscle glycogen phosphorylase**. - In McArdle disease, the enzyme responsible for breaking down glycogen (glycogen phosphorylase) is deficient, but the enzymes involved in synthesizing glycogen are normal. Therefore, the **structure of glycogen is normal**, but it accumulates in muscle cells because it cannot be catabolized. *Short outer glycogen chains* - **Short outer glycogen chains** are characteristic of **Cori disease** (Glycogen Storage Disease Type III), caused by a deficiency in **debranching enzyme**. - This condition also presents with hypoglycemia and hepatomegaly, which are not described in the patient's presentation. *Accumulation of glycogen in lysosomes forming dense granules* - **Accumulation of glycogen in lysosomes** and the formation of **dense granules** is characteristic of **Pompe disease** (Glycogen Storage Disease Type II), caused by a deficiency in **lysosomal alpha-glucosidase (acid maltase)**. - Pompe disease typically presents as a severe infantile form with cardiomegaly and hypotonia, or a later-onset form with proximal muscle weakness, which differs from the patient's primary complaint of exercise intolerance and myoglobinuria. *Glycogen without normal branching pattern* - **Glycogen without a normal branching pattern** (very long unbranched chains) is characteristic of **Andersen disease** (Glycogen Storage Disease Type IV), caused by a deficiency in **branching enzyme**. - This condition typically leads to cirrhosis and liver failure in infancy, which is not consistent with the patient's presentation. *Absence of glycogen in muscles* - While McArdle disease involves an inability to break down muscle glycogen, it does not result in the **absence of glycogen** in muscles; rather, there is an **over-accumulation** of normal-structured glycogen because it cannot be utilized. - The defect is in **glycogenolysis**, not glycogen synthesis, so glycogen is formed but not broken down.
Question 4: A scientist is trying to design a drug to modulate cellular metabolism in the treatment of obesity. Specifically, he is interested in understanding how fats are processed in adipocytes in response to different energy states. His target is a protein within these cells that catalyzes catabolism of an energy source. The products of this reaction are subsequently used in gluconeogenesis or β-oxidation. Which of the following is true of the most likely protein that is being studied by this scientist?
- A. It is stimulated by epinephrine (Correct Answer)
- B. It is inhibited by glucagon
- C. It is inhibited by acetylcholine
- D. It is inhibited by cortisol
- E. It is stimulated by insulin
Explanation: ***It is stimulated by epinephrine*** - The protein described is likely **hormone-sensitive lipase (HSL)**, which catabolizes **triglycerides** in adipocytes to **glycerol** and **fatty acids**. - **Epinephrine** (and norepinephrine) stimulates HSL activity via a **cAMP-dependent protein kinase A (PKA)** pathway, leading to increased fatty acid release for energy. *It is inhibited by glucagon* - **Glucagon primarily acts on the liver** to promote gluconeogenesis and glycogenolysis, but it does **not directly inhibit HSL** in adipocytes. - While glucagon has a lipolytic effect, it doesn't inhibit the enzyme that releases fatty acids. *It is inhibited by acetylcholine* - **Acetylcholine** is a neurotransmitter involved in the **parasympathetic nervous system**, which generally promotes energy storage. - It does **not directly inhibit HSL**; its effects on lipid metabolism are indirect and typically involve other pathways. *It is inhibited by cortisol* - **Cortisol**, a glucocorticoid, generally **promotes lipolysis** (breakdown of fats) in certain contexts, particularly during stress to provide energy substrates. - Therefore, it would **not inhibit HSL**; rather, it often enhances its activity or provides a permissive effect for other lipolytic hormones. *It is stimulated by insulin* - **Insulin** is an **anabolic hormone** that promotes energy storage, including **lipogenesis** (fat synthesis) and inhibits lipolysis. - Insulin **inhibits HSL activity** by activating phosphodiesterase, which reduces cAMP levels, thus deactivating PKA and preventing HSL phosphorylation.
Question 5: During exercise, what is the primary mechanism for increased oxygen delivery to active muscles?
- A. Decreased blood viscosity
- B. Increased cardiac output (Correct Answer)
- C. Increased hemoglobin affinity
- D. Enhanced oxygen diffusion
Explanation: ***Increased cardiac output*** - During exercise, **cardiac output** increases significantly due to both an elevated **heart rate** and increased **stroke volume**, directly pushing more oxygenated blood to the active muscles. - This augmentation in blood flow is the primary factor ensuring a sufficient supply of oxygen and nutrients to meet the heightened metabolic demands of exercising muscles. *Decreased blood viscosity* - While factors like **hemodilution** can decrease blood viscosity during prolonged exercise, this effect is relatively minor and not the primary mechanism for acute increases in oxygen delivery compared to the dramatic increase in cardiac output. - A decrease in blood viscosity can slightly improve flow efficiency, but it doesn't fundamentally change the amount of blood pumped per minute to the muscles. *Increased hemoglobin affinity* - An *increased* hemoglobin affinity for oxygen would actually make it *harder* for oxygen to unload from hemoglobin to the tissues, which is counterproductive for oxygen delivery during exercise. - In fact, during exercise, local conditions like increased temperature, decreased pH (**Bohr effect**), and increased 2,3-BPG tend to *decrease* hemoglobin's affinity for oxygen, facilitating oxygen release to active muscles. *Enhanced oxygen diffusion* - While exercise does improve the efficiency of oxygen extraction at the tissue level due to a steeper partial pressure gradient and increased capillary recruitment, the *rate* of oxygen diffusion across the capillary membrane isn't the primary modulator of overall oxygen delivery. - The main determinant is the *amount* of oxygenated blood reaching the muscle, which is governed by cardiac output and local blood flow regulation.
Question 6: During heavy exercise, what is the primary mechanism for maintaining arterial pH despite increased lactic acid production?
- A. Increased bicarbonate reabsorption
- B. Phosphate buffering
- C. Increased hydrogen secretion
- D. Hyperventilation (Correct Answer)
Explanation: ***Hyperventilation*** - **Hyperventilation** during heavy exercise increases the expulsion of **carbon dioxide (CO2)**, shifting the **bicarbonate buffer system** equilibrium to the left. - This reduction in **CO2** effectively removes **hydrogen ions (H+)**, thereby helping to maintain **arterial pH** despite rising **lactic acid** levels. *Increased bicarbonate reabsorption* - While the kidneys adapt by increasing **bicarbonate reabsorption** to compensate for acidosis, this is a **slower renal mechanism** for pH regulation, taking hours to days, rather than an immediate response during acute exercise. - The rapid pH regulation during exercise primarily relies on respiratory and chemical buffer systems, not renal function. *Phosphate buffering* - The **phosphate buffer system** is indeed important for intracellular and renal tubular fluid buffering. - However, its buffering capacity in the extracellular fluid and plasma is relatively limited compared to the **bicarbonate system** due to its lower concentration. *Increased hydrogen secretion* - **Increased hydrogen secretion** by the renal tubules is a long-term mechanism for compensating for acidosis, which helps excrete excess **acid** and regenerate **bicarbonate**. - This is a slow, renal regulatory process and not the primary rapid mechanism for maintaining pH during the immediate demands of heavy exercise.
Question 7: Which hormone most strongly stimulates gluconeogenesis during prolonged fasting?
- A. Insulin
- B. Epinephrine
- C. Cortisol
- D. Glucagon (Correct Answer)
Explanation: ***Glucagon*** - **Glucagon** is the primary hormone that promotes **gluconeogenesis** and glycogenolysis to maintain blood glucose during fasting. - Its secretion is strongly stimulated by **low blood glucose levels**, making it critical throughout fasting states. - Glucagon directly stimulates hepatic gluconeogenic enzymes and increases the availability of gluconeogenic substrates. *Insulin* - **Insulin** is an **anabolic hormone** that promotes glucose uptake and storage, thereby decreasing blood glucose levels. - Its levels decrease during fasting, *suppressing* rather than stimulating gluconeogenesis. - Insulin inhibits gluconeogenic enzyme expression and promotes glycolysis instead. *Epinephrine* - **Epinephrine** (adrenaline) is a stress hormone that rapidly increases blood glucose through both **glycogenolysis** and gluconeogenesis. - Its effects are more prominent during **acute stress** or immediate energy demands (fight-or-flight response), rather than sustained fasting. - Its action is rapid but transient compared to glucagon's sustained effect during fasting. *Cortisol* - **Cortisol** is a glucocorticoid that promotes **gluconeogenesis** by providing amino acid substrates through protein catabolysis and inducing gluconeogenic enzymes. - While cortisol becomes increasingly important in **prolonged fasting** (>24-48 hours), **glucagon remains the primary and most potent direct stimulator** of hepatic gluconeogenesis throughout all phases of fasting. - Cortisol's effects are slower in onset but more sustained, working synergistically with glucagon during extended fasting periods.
Question 8: A 52-year-old man undergoes an exercise stress test for a 1-week history of squeezing substernal chest pain that is aggravated by exercise and relieved by rest. During the test, there is a substantial increase in the breakdown of glycogen in the muscle cells. Which of the following changes best explains this intracellular finding?
- A. Activation of phosphorylase kinase (Correct Answer)
- B. Decrease in protein kinase A
- C. Inactivation of glycogen synthase kinase
- D. Activation of protein phosphatase
- E. Increase in glucose-6-phosphate
Explanation: ***Activation of phosphorylase kinase*** - Exercise, particularly in the context of **ischemic heart disease** suggested by the patient's symptoms, triggers a rapid need for energy, leading to **glycogenolysis**. - **Phosphorylase kinase** is the key enzyme that activates **glycogen phosphorylase**, the rate-limiting step in glycogen breakdown, to release glucose-1-phosphate from glycogen stores. *Decrease in protein kinase A* - **Protein kinase A (PKA)** is typically activated during exercise via **epinephrine** signaling, which in turn *activates* phosphorylase kinase and *inhibits* glycogen synthase. - A decrease in PKA activity would lead to *reduced* glycogen breakdown, which contradicts the described increase in glycogen breakdown. *Inactivation of glycogen synthase kinase* - **Glycogen synthase kinase (GSK3)** phosphorylates and inactivates **glycogen synthase**, thereby *inhibiting* glycogen synthesis. - If GSK3 were inactivated, glycogen synthesis would be *promoted*, rather than glycogen breakdown, further contradicting the clinical scenario. *Activation of protein phosphatase* - **Protein phosphatases** generally remove phosphate groups, which would *deactivate* glycogen phosphorylase and *activate* glycogen synthase. - This action would promote glycogen synthesis and inhibit glycogen breakdown, which is the opposite of the observed physiological response during exercise. *Increase in glucose-6-phosphate* - While **glucose-6-phosphate** is an intermediate in glycogen metabolism, an increase in its concentration would primarily signal abundant glucose and tend to *inhibit* glycogen phosphorylase and *activate* glycogen synthase. - This effect would favor glycogen synthesis and inhibit its breakdown, making it an unlikely explanation for increased glycogen breakdown during exercise.
Question 9: A 4-year-old boy presents to the ophthalmologist for a down- and inward dislocation of the lens in his left eye. On physical exam, the boy has a marfanoid habitus and intellectual disability. Biochemical tests were performed to locate the exact defect in this boy. It was found that there was a significant deficiency in cystathionine beta-synthase activity with elevated homocysteine levels. Which of the following is the diagnosis?
- A. Marfan syndrome
- B. Homocystinuria (Correct Answer)
- C. Alkaptonuria
- D. Phenylketonuria
- E. Maple syrup urine disease
Explanation: ***Homocystinuria*** - The combination of **ectopia lentis** (lens dislocation), **marfanoid habitus**, and **intellectual disability** is characteristic of homocystinuria. - The **deficiency in cystathionine beta-synthase** leading to **elevated homocysteine levels** is the biochemical hallmark of this disorder. *Marfan syndrome* - While it presents with **marfanoid habitus** and **ectopia lentis**, the lens dislocation is typically **upward and outward**, unlike the down- and inward dislocation seen here. - Marfan syndrome is caused by a defect in **fibrillin-1**, and biochemical tests would not show elevated homocysteine. *Alkaptonuria* - This disorder is characterized by **dark urine** upon standing, **ochronosis** (darkening of cartilage and connective tissue), and **arthropathy**. - It results from a deficiency in **homogentisate oxidase** and does not present with lens dislocation or intellectual disability. *Phenylketonuria* - PKU is caused by a deficiency in **phenylalanine hydroxylase**, leading to an accumulation of phenylalanine. - It primarily causes **severe intellectual disability**, seizures, and a musty odor, but not ectopia lentis or marfanoid habitus. *Maple syrup urine disease* - This is a metabolic disorder affecting the metabolism of **branched-chain amino acids** (leucine, isoleucine, valine). - It presents with a characteristic **maple syrup odor** in urine, poor feeding, lethargy, and developmental delay, but not the specific features seen in this case.
Question 10: A 45-year-old patient presents with joint pain and weakness and is known to have homocystinuria. Which vitamin is required in the treatment?
- A. Vitamin B6 (Correct Answer)
- B. Vitamin B12
- C. Vitamin B7
- D. Vitamin B1
- E. Vitamin B9
Explanation: ***Vitamin B6*** - **Homocystinuria** is often caused by a deficiency in the enzyme **cystathionine beta-synthase**, which requires **pyridoxal phosphate (active form of B6)** as a cofactor. - Supplementation with high-dose **vitamin B6** can help some patients by increasing the residual activity of the enzyme, thereby reducing **homocysteine levels**. - This is the **primary treatment** for **B6-responsive homocystinuria** (approximately 50% of cases respond to B6 therapy). *Vitamin B12* - Vitamin B12 is a cofactor for the enzyme **methionine synthase**, which converts homocysteine back to methionine. - While it plays a role in homocysteine metabolism, **vitamin B6** is typically the primary treatment for homocystinuria caused by **cystathionine beta-synthase deficiency**. *Vitamin B9* - Vitamin B9 (folic acid) works together with **vitamin B12** as a cofactor in the **remethylation pathway** via methionine synthase. - While folate supplementation may help lower homocysteine levels, it is **not the primary treatment** for classical homocystinuria due to cystathionine beta-synthase deficiency. - **Vitamin B6** remains the first-line vitamin therapy for enzyme deficiency-related homocystinuria. *Vitamin B7* - Vitamin B7, or **biotin**, is a cofactor for carboxylase enzymes and is involved in fatty acid synthesis and gluconeogenesis. - It has no direct role in the metabolism of **homocysteine** or the treatment of homocystinuria. *Vitamin B1* - Vitamin B1, or **thiamine**, is essential for carbohydrate metabolism and nerve function. - It is not involved in the metabolic pathways that regulate **homocysteine levels** or the treatment of homocystinuria.
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