Integration with carbohydrate metabolism US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Integration with carbohydrate metabolism. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Integration with carbohydrate metabolism US Medical PG Question 1: A 6-month-old girl is brought to the emergency department by her father after he observed jerking movements of her arms and legs earlier in the day. She appears lethargic. Physical examination shows generalized hypotonia. The liver edge is palpable 3 cm below the right costophrenic angle. Her fingerstick glucose shows hypoglycemia. Serum levels of acetone, acetoacetate, and β-hydroxybutyrate are undetectable. Molecular genetic testing shows a mutation in the carnitine palmitoyltransferase II gene. This patient will most likely benefit from supplementation with which of the following?
- A. Thiamine
- B. Coenzyme A
- C. Methionine
- D. Tetrahydrobiopterin
- E. Medium-chain triglycerides (Correct Answer)
Integration with carbohydrate metabolism Explanation: ***Medium-chain triglycerides***
- This patient presents with symptoms consistent with a **fatty acid oxidation disorder** (lethargy, hypotonia, hepatomegaly, hypoglycemia, and undetectable ketones in the setting of hypoglycemia). A **carnitine palmitoyltransferase II (CPT II) deficiency** is a specific fatty acid oxidation disorder.
- **Medium-chain triglycerides (MCTs)** can be directly used for energy in the mitochondria without requiring the carnitine shuttle system, bypassing the defective CPT II enzyme and providing an alternative energy source.
*Thiamine*
- **Thiamine (vitamin B1)** is a coenzyme important for carbohydrate metabolism (e.g., pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase) and can be deficient in conditions like Wernicke-Korsakoff syndrome.
- It is not directly involved in fatty acid oxidation or carnitine palmitoyltransferase II function.
*Coenzyme A*
- **Coenzyme A (CoA)** is a crucial cofactor in many metabolic pathways, including fatty acid oxidation, but its deficiency is not the primary issue in CPT II deficiency.
- Supplementation with CoA would not address the specific defect in the carnitine shuttle system, which prevents long-chain fatty acids from entering the mitochondria.
*Methionine*
- **Methionine** is an essential amino acid involved in protein synthesis and as a precursor for other molecules like S-adenosylmethionine (SAM).
- It is not directly involved in fatty acid oxidation or CPT II function, and its supplementation would not alleviate the metabolic crisis caused by CPT II deficiency.
*Tetrahydrobiopterin*
- **Tetrahydrobiopterin (BH4)** is a cofactor for several enzymes, including phenylalanine hydroxylase (deficient in PKU) and tyrosine hydroxylase.
- It is not involved in fatty acid metabolism, and its deficiency would not explain the symptoms or the underlying genetic defect in CPT II.
Integration with carbohydrate metabolism US Medical PG Question 2: An investigator is studying severely ill patients who experience hypoglycemia and ketonuria during times of fasting. The investigator determines that during these episodes, amino acids liberated from muscle proteins are metabolized to serve as substrates for gluconeogenesis. Nitrogen from this process is transported to the liver primarily in the form of which of the following molecules?
- A. Glutamate
- B. α-ketoglutarate
- C. Alanine (Correct Answer)
- D. Arginine
- E. Pyruvate
Integration with carbohydrate metabolism Explanation: ***Alanine***
- During prolonged fasting, **muscle proteins are catabolized** to provide amino acids for gluconeogenesis in the liver.
- **Alanine** is the primary amino acid released from muscle into the bloodstream to transport nitrogen to the liver through the **glucose-alanine cycle (Cahill cycle)**.
- In this cycle, pyruvate in muscle accepts an amino group from glutamate to form alanine, which is then transported to the liver, where it is deaminated back to pyruvate (for gluconeogenesis) and ammonia (for the urea cycle).
- **Glutamine** also serves as an important nitrogen transporter, particularly to the kidneys and intestines.
*Glutamate*
- **Glutamate** is an important amino acid in nitrogen metabolism within tissues, but it is not the primary form in which nitrogen is transported from muscle to the liver in significant quantities.
- While glutamate participates in transamination reactions within muscle, its efflux from muscle into the blood is less prominent than alanine for inter-organ nitrogen transport.
*α-ketoglutarate*
- **α-ketoglutarate** is a key intermediate in the **Krebs cycle** and accepts an amino group to form glutamate.
- It is an alpha-keto acid, not an amino acid, and therefore does not directly transport nitrogen in the form of an amino group to the liver via the bloodstream.
*Arginine*
- **Arginine** is primarily involved in the **urea cycle** within the liver, where it helps in the detoxification of ammonia, but it is not a major transporter of nitrogen from peripheral tissues to the liver for gluconeogenesis.
- Its role is mainly within the liver for urea synthesis, not for inter-organ nitrogen transport in this context.
*Pyruvate*
- **Pyruvate** is a keto acid that can be converted to alanine via transamination.
- While pyruvate is a precursor to alanine and a substrate for gluconeogenesis, it transports carbon skeletons and not nitrogen itself; **alanine is the actual nitrogen carrier** in this cycle.
Integration with carbohydrate metabolism US Medical PG Question 3: 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
Integration with carbohydrate metabolism 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.
Integration with carbohydrate metabolism US Medical PG Question 4: An investigator is studying a hereditary defect in the mitochondrial enzyme succinyl-CoA synthetase. In addition to succinate, the reaction catalyzed by this enzyme produces a molecule that is utilized as an energy source for protein translation. This molecule is also required for which of the following conversion reactions?
- A. Oxaloacetate to phosphoenolpyruvate (Correct Answer)
- B. Pyruvate to acetyl-CoA
- C. Acetaldehyde to acetate
- D. Glucose-6-phosphate to 6-phosphogluconolactone
- E. Fructose-6-phosphate to fructose-1,6-bisphosphate
Integration with carbohydrate metabolism Explanation: ***Oxaloacetate to phosphoenolpyruvate***
- The reaction catalyzed by **succinyl-CoA synthetase** (also known as succinate thiokinase) produces **GTP** (guanosine triphosphate) from GDP and Pi, in addition to succinate.
- **GTP** is required for the conversion of **oxaloacetate** to **phosphoenolpyruvate** in gluconeogenesis, catalyzed by **PEP carboxykinase**.
*Pyruvate to acetyl-CoA*
- This reaction is catalyzed by the **pyruvate dehydrogenase complex** and produces NADH, not GTP.
- It is an irreversible step linking glycolysis to the citric acid cycle.
*Acetaldehyde to acetate*
- This reaction is catalyzed by **aldehyde dehydrogenase** and uses **NAD+** as a cofactor, producing NADH.
- It is involved in alcohol metabolism.
*Glucose-6-phosphate to 6-phosphogluconolactone*
- This is the first committed step of the **pentose phosphate pathway**, catalyzed by **glucose-6-phosphate dehydrogenase**.
- It uses **NADP+** as a cofactor, producing NADPH.
*Fructose-6-phosphate to fructose-1,6-bisphosphate*
- This reaction is a key regulatory step in **glycolysis**, catalyzed by **phosphofructokinase-1 (PFK-1)**.
- It consumes **ATP**, rather than producing GTP or utilizing it as a cofactor in the context of this question.
Integration with carbohydrate metabolism US Medical PG Question 5: A 22-year-old medical student decides to fast for 24 hours after reading about the possible health benefits of fasting. She read that blood glucose levels are maintained by metabolic processes such as hepatic glycogenolysis and hepatic gluconeogenesis during the initial 3 days of fasting. During the day, she did not suffer from the symptoms of hypoglycemia. Which of the following signaling molecules most likely stimulated the reaction which maintained her blood glucose after all her stored glucose was broken down and used up?
- A. Adenosine diphosphate
- B. Acetyl CoA (Correct Answer)
- C. Acetate
- D. Citrate
- E. Adenosine monophosphate
Integration with carbohydrate metabolism Explanation: ***Acetyl CoA***
- **Acetyl CoA** is the key **allosteric activator of pyruvate carboxylase**, the first committed enzyme of gluconeogenesis that converts pyruvate to oxaloacetate.
- During prolonged fasting after glycogen stores are depleted, the body shifts to **fatty acid oxidation** (β-oxidation), which produces large amounts of **Acetyl CoA**.
- High **Acetyl CoA** levels signal that fat is being oxidized for energy, and simultaneously **activate gluconeogenesis** to maintain blood glucose for glucose-dependent tissues (brain, RBCs).
- This is the primary signaling mechanism that directly stimulates the gluconeogenic pathway after glycogen is exhausted.
*Adenosine monophosphate (AMP)*
- **AMP** levels rise during energy depletion and activate **AMP-activated protein kinase (AMPK)**.
- However, AMPK **inhibits gluconeogenesis** (not stimulates it) because gluconeogenesis is an **ATP-consuming** anabolic process (requires 6 ATP per glucose).
- AMPK promotes ATP-generating catabolic processes like fatty acid oxidation, but suppresses ATP-consuming processes like gluconeogenesis and fatty acid synthesis.
*Adenosine diphosphate (ADP)*
- **ADP** accumulates when ATP is hydrolyzed and signals moderate energy deficit.
- ADP is primarily a substrate for ATP regeneration via oxidative phosphorylation and does not directly regulate gluconeogenesis.
- Its role in metabolic regulation is less specific than allosteric activators like Acetyl CoA.
*Acetate*
- **Acetate** can be converted to Acetyl CoA but is not a direct signaling molecule for gluconeogenesis.
- It is a minor metabolite that may be produced in specific conditions (e.g., alcohol metabolism, ketoacidosis) but does not play a primary role in fasting-induced glucose homeostasis.
*Citrate*
- **Citrate** is a Krebs cycle intermediate that inhibits **phosphofructokinase-1 (PFK-1)** in glycolysis, thus reducing glucose breakdown.
- While citrate inhibition of glycolysis indirectly favors gluconeogenesis by preventing futile cycling, citrate does not **directly activate** gluconeogenic enzymes.
- Citrate primarily signals energy sufficiency and promotes fatty acid synthesis in the fed state, not fasting gluconeogenesis.
Integration with carbohydrate metabolism US Medical PG Question 6: Which hormone most strongly stimulates gluconeogenesis during prolonged fasting?
- A. Insulin
- B. Epinephrine
- C. Cortisol
- D. Glucagon (Correct Answer)
Integration with carbohydrate metabolism 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.
Integration with carbohydrate metabolism US Medical PG Question 7: During normal respiration in the lungs, oxygen is absorbed into the bloodstream and carbon dioxide is released. The oxygen is used in cells as the final electron acceptor during oxidative phosphorylation, and carbon dioxide is generated during each turn of the tricarboxylic citric acid cycle (TCA). Which of the following steps in the TCA cycle represents the first decarboxylation reaction that generates carbon dioxide?
- A. Isocitrate to alpha ketoglutarate (Correct Answer)
- B. Fumarate to Malate
- C. Citrate to isocitrate
- D. Malate to oxaloacetate
- E. Alpha-ketoglutarate to Succinyl-CoA
Integration with carbohydrate metabolism Explanation: ***Isocitrate to alpha ketoglutarate***
- This is the **first decarboxylation reaction** in the TCA cycle, catalyzed by **isocitrate dehydrogenase**.
- During this reaction, **isocitrate** is oxidized and a molecule of **carbon dioxide** is released, along with the reduction of NAD+ to NADH.
- This is one of the three irreversible steps in the TCA cycle and a key regulatory point.
*Fumarate to Malate*
- This step involves the **hydration** of **fumarate** to **malate** by the enzyme **fumarase**.
- There is no release of carbon dioxide in this reaction; it's a simple addition of water.
*Citrate to isocitrate*
- This is an **isomerization** reaction, catalyzed by **aconitase**, where **citrate** is rearranged into its isomer, **isocitrate**.
- This step does not involve the removal of carbon atoms or the production of carbon dioxide.
*Malate to oxaloacetate*
- In this step, **malate** is oxidized to **oxaloacetate** by **malate dehydrogenase**, which produces NADH.
- This is an **oxidation** reaction, not a decarboxylation reaction, and no carbon dioxide is released.
*Alpha-ketoglutarate to Succinyl-CoA*
- This is the **second decarboxylation** step in the TCA cycle, catalyzed by the **alpha-ketoglutarate dehydrogenase complex**.
- While this step also produces carbon dioxide and reduces NAD+ to NADH, it occurs after the isocitrate to alpha-ketoglutarate step, making it the second rather than the first decarboxylation reaction.
Integration with carbohydrate metabolism US Medical PG Question 8: A 34-year-old male visits the clinic with complaints of intermittent diarrhea over the past 6 months. He has lost 6.8 kg (15 lb) over that time period. His frequent bowel movements are affecting his social life and he would like definitive treatment. Past medical history is significant for chronic type 2 diabetes that is well controlled with insulin. No other family member has a similar condition. He does not smoke tobacco and drinks alcohol only on weekends. Today, his vitals are within normal limits. On physical exam, he appears gaunt and anxious. His heart has a regular rate and rhythm and his lungs are clear to auscultation bilaterally. Additionally, the patient has a red-purple rash on his lower abdomen, groin, and the dorsum of both hands. The rash consists of pruritic annular lesions. He is referred to a dermatologist for core biopsy which is consistent with necrolytic migratory erythema. Further workup reveals a large hormone secreting mass in the tail of his pancreas. Which of the following is the action of the hormone that is in excess in this patient?
- A. Activation of glycogen synthase
- B. Inhibition of gluconeogenesis
- C. Inhibition of insulin secretion
- D. Stimulation of lipolysis (Correct Answer)
- E. Inhibition of acetone production
Integration with carbohydrate metabolism Explanation: ***Stimulation of lipolysis***
- The patient's symptoms (diarrhea, weight loss, necrolytic migratory erythema, and a pancreatic mass) are highly suggestive of a **glucagonoma**.
- **Glucagon** is a catabolic hormone that **stimulates gluconeogenesis and glycogenolysis** to raise blood glucose, and it also promotes **lipolysis** and **ketogenesis**.
*Activation of glycogen synthase*
- **Glycogen synthase** is activated by **insulin**, which promotes glycogen synthesis, storing glucose.
- Glucagon opposes the action of insulin; thus, it would **inactivate glycogen synthase**, not activate it.
*Inhibition of gluconeogenesis*
- **Glucagon's primary role** is to increase blood glucose levels, and it does so by **stimulating gluconeogenesis and glycogenolysis**.
- **Inhibition of gluconeogenesis** would occur with insulin, not glucagon.
*Inhibition of insulin secretion*
- While glucagon can have complex effects on insulin, its primary metabolic actions are independent of direct inhibition of insulin secretion.
- Pancreatic somatostatin inhibits both insulin and glucagon secretion.
*Inhibition of acetone production*
- Glucagon promotes **ketogenesis**, which can lead to the production of **ketone bodies**, including acetone, especially in states of insulin deficiency.
- Therefore, glucagon would **stimulate**, not inhibit, processes that can lead to acetone production.
Integration with carbohydrate metabolism US Medical PG Question 9: A patient came to the hospital with severe abdominal pain, and lipase levels were elevated. On imaging, a stone is found in the common bile duct (CBD). Which enzyme is most likely elevated in this condition?
- A. ALT
- B. GGT
- C. LDH
- D. AST
- E. ALP (Correct Answer)
Integration with carbohydrate metabolism Explanation: ***ALP (Alkaline Phosphatase)***
- **ALP** is the **most characteristic enzyme elevation** in **biliary obstruction** from a CBD stone.
- ALP is found in high concentrations in the **bile duct epithelium** and hepatocytes adjacent to bile ducts, and rises dramatically with **cholestasis** and **obstructive jaundice**.
- In CBD stone obstruction, ALP typically rises **3-10 times normal**, making it the hallmark biochemical marker of this condition.
- While lipase is elevated due to associated pancreatitis, **ALP elevation specifically indicates the biliary obstruction**.
*GGT (Gamma-Glutamyl Transferase)*
- **GGT** is also elevated in **cholestasis** and **bile duct obstruction**.
- GGT often rises in parallel with ALP and helps confirm the hepatobiliary origin of ALP elevation (vs. bone source).
- However, **ALP is more specific** and typically shows greater magnitude of elevation in acute CBD obstruction, making it the **most likely** elevated enzyme in this clinical context.
*ALT (Alanine Aminotransferase)*
- **ALT** may be **mildly to moderately elevated** if there is secondary hepatocellular injury from biliary obstruction.
- However, ALT primarily indicates **hepatocyte damage** rather than cholestasis, and its elevation is typically **less pronounced** than ALP in obstructive biliary disease.
- The pattern in CBD obstruction is **cholestatic** (high ALP) rather than **hepatocellular** (high ALT).
*AST (Aspartate Aminotransferase)*
- **AST** can be elevated in various conditions including liver, heart, and muscle damage.
- Like ALT, it may show mild elevation in biliary obstruction but is **not the primary marker**.
- AST is less specific than ALP for diagnosing CBD stone obstruction.
*LDH (Lactate Dehydrogenase)*
- **LDH** is a **non-specific marker** of tissue damage found in multiple organs.
- While it may be elevated, it provides little diagnostic value when specific markers like **ALP and lipase** are available.
- LDH does not help differentiate biliary obstruction from other causes of abdominal pain.
Integration with carbohydrate metabolism US Medical PG Question 10: A patient came to the emergency room with severe abdominal pain. The serum triglyceride level was $1500 \mathrm{mg} / \mathrm{dL}$. What is the most likely defect?
- A. Apo B-48
- B. Apo B-100
- C. Apo C-II (Correct Answer)
- D. LDL receptor
- E. Lipoprotein lipase
Integration with carbohydrate metabolism Explanation: ***Apo C-II***
- **Apo C-II** is an essential cofactor for **lipoprotein lipase (LPL)**, which is responsible for hydrolyzing triglycerides from chylomicrons and VLDL.
- A defect in Apo C-II leads to severely impaired triglyceride clearance, resulting in **chylomicronemia** and extremely high serum triglyceride levels (e.g., 1500 mg/dL), which can cause acute pancreatitis.
- Both Apo C-II deficiency and LPL deficiency present similarly, but Apo C-II deficiency is the more specific answer when considering the **"defect"** terminology, as it represents the regulatory cofactor rather than the enzyme itself.
*Apo B-48*
- **Apo B-48** is a structural protein uniquely found on **chylomicrons**, synthesized in the intestine, and is essential for their formation and secretion.
- A defect in Apo B-48 (e.g., in abetalipoproteinemia) would lead to the **absence of chylomicrons**, resulting in very low or undetectable triglyceride levels after a fat-containing meal, not high levels.
*Apo B-100*
- **Apo B-100** is the primary apolipoprotein of **VLDL, IDL, and LDL**, and it is crucial for VLDL assembly in the liver and for LDL receptor binding.
- Defects in Apo B-100 leading to hyperlipidemia typically cause elevated LDL cholesterol (e.g., familial defective Apo B-100), rather than severe hypertriglyceridemia associated with chylomicronemia.
*LDL receptor*
- The **LDL receptor** is responsible for the uptake of **LDL particles** from the bloodstream, primarily in the liver.
- A defect in the LDL receptor (e.g., in familial hypercholesterolemia) primarily causes **elevated LDL cholesterol** levels, but typically does not lead to the extreme hypertriglyceridemia seen in this patient.
*Lipoprotein lipase*
- **Lipoprotein lipase (LPL)** is the enzyme that hydrolyzes triglycerides in chylomicrons and VLDL particles.
- A primary deficiency of LPL itself (Type I familial chylomicronemia) would also cause severe hypertriglyceridemia similar to Apo C-II deficiency.
- However, Apo C-II deficiency is the more specific answer as it represents the **cofactor defect** that impairs LPL function, while direct LPL deficiency is a separate genetic entity.
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