Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 621: How does insulin affect lipid metabolism?

A. Stimulates ketogenesis in adipose tissue
B. Increases fatty acid oxidation
C. Inhibits lipolysis (Correct Answer)
D. Activates hormone-sensitive lipase (HSL)

Explanation: ***Inhibits lipolysis*** - Insulin **downregulates hormone-sensitive lipase (HSL)** activity, a key enzyme responsible for breaking down stored triglycerides into fatty acids and glycerol. - By inhibiting lipolysis, insulin promotes the storage of energy and prevents the release of fatty acids into circulation. *Stimulates ketogenesis in adipose tissue* - Ketogenesis primarily occurs in the **liver** during periods of low insulin and high glucagon, not typically in adipose tissue. - Insulin's primary role is to **reduce** ketone body production by promoting glucose utilization and fatty acid storage. *Increases fatty acid oxidation* - Insulin **reduces fatty acid oxidation** (beta-oxidation) by stimulating malonyl-CoA production, which inhibits carnitine palmitoyltransferase 1 (CPT1), preventing fatty acid entry into mitochondria. - Its main effect is to promote **fatty acid synthesis and storage**, not their breakdown for energy. *Activates hormone-sensitive lipase (HSL)* - Insulin has an **inhibitory effect on HSL** through dephosphorylation, while glucagon and catecholamines activate HSL. - The activation of HSL would lead to the **breakdown of triglycerides**, which is contrary to insulin's role in promoting energy storage.

Question 622: Which lysosomal enzyme deficiency is associated with a disease characterized by the accumulation of glucocerebrosides in cells?

A. Alpha-galactosidase deficiency
B. Hexosaminidase A deficiency
C. Sphingomyelinase deficiency
D. Glucocerebrosidase deficiency (Correct Answer)

Explanation: **Correct: Glucocerebrosidase deficiency** - This deficiency leads to **Gaucher disease**, specifically characterized by the accumulation of **glucocerebrosides** in macrophages within the lysosome. - The accumulated glucocerebrosides impede normal cellular function, particularly in the **spleen, liver, bone marrow**, and sometimes the nervous system. *Incorrect: Alpha-galactosidase deficiency* - This deficiency causes **Fabry disease**, leading to the accumulation of **globotriaosylceramide (Gb3)**, primarily affecting the kidneys, heart, and nervous system. - Clinical features include **neuropathic pain**, angiokeratomas, and renal failure, which are distinct from the symptoms of glucocerebroside accumulation. *Incorrect: Hexosaminidase A deficiency* - This enzyme deficiency is responsible for **Tay-Sachs disease**, marked by the accumulation of **GM2 gangliosides**, predominantly in neuronal lysosomes. - It results in rapidly progressive neurodegeneration, **developmental regression**, and a characteristic **cherry-red spot** on the retina. *Incorrect: Sphingomyelinase deficiency* - This deficiency is associated with **Niemann-Pick disease**, types A and B, causing the accumulation of **sphingomyelin** in various tissues. - Niemann-Pick disease typically presents with hepatosplenomegaly, **neurodegeneration** (Type A), and often has foam cells.

Question 623: A patient with muscle weakness and hypoketotic hypoglycemia is diagnosed with CPT II deficiency. How does this deficiency affect fatty acid metabolism?

A. Enhances FA synthesis; lipid accumulation
B. Increases ketogenesis; ketoacidosis
C. Inhibits FA transport into mitochondria; reduced β-oxidation (Correct Answer)
D. Reduces lipolysis; decreased free FAs

Explanation: ***Inhibits FA transport into mitochondria; reduced β-oxidation*** - **Carnitine palmitoyltransferase II (CPT II)** is an enzyme located on the inner mitochondrial membrane that converts **acylcarnitine** back to **acyl-CoA**, allowing fatty acids to enter the mitochondrial matrix for **β-oxidation**. - A deficiency in CPT II prevents the transport of long-chain fatty acids into the mitochondria, consequently **reducing their β-oxidation** and leading to impaired energy production from fats, causing **hypoketotic hypoglycemia** and **muscle weakness**. *Enhances FA synthesis; lipid accumulation* - CPT II deficiency does not directly enhance **fatty acid synthesis**; instead, it impairs their breakdown. - While there may be **lipid accumulation** due to the inability to metabolize fatty acids, it is not due to enhanced synthesis but rather a critical block in degradation. *Increases ketogenesis; ketoacidosis* - **Ketogenesis** relies on the breakdown of fatty acids via β-oxidation to produce **acetyl-CoA**, which then forms ketone bodies. - CPT II deficiency **impairs β-oxidation**, leading to **hypoketotic hypoglycemia** (low ketone bodies), not increased ketogenesis or ketoacidosis. *Reduces lipolysis; decreased free FAs* - **Lipolysis** is the breakdown of triglycerides into free fatty acids and glycerol, which primarily occurs in adipose tissue. - CPT II deficiency affects the **subsequent mitochondrial oxidation of fatty acids** after they have been released, not their initial release from triglycerides.

Question 624: In a patient with CPT I deficiency, what best describes the primary consequence on fatty acid metabolism?

A. Inhibits FA transport to mitochondria (Correct Answer)
B. Enhances FA synthesis
C. Increases ketogenesis
D. Reduces lipolysis

Explanation: ***Inhibits FA transport to mitochondria*** - **CPT I (Carnitine Palmitoyltransferase I)** is the rate-limiting enzyme in the **carnitine shuttle system**, which is essential for transporting long-chain fatty acids into the mitochondrial matrix for **beta-oxidation**. - A deficiency in CPT I directly prevents the formation of **acylcarnitine**, thereby blocking the entry of fatty acids into the mitochondria for energy production. *Enhances FA synthesis* - Fatty acid synthesis and fatty acid oxidation are distinct metabolic pathways that are generally regulated inversely. - A CPT I deficiency would lead to an accumulation of fatty acids in the cytoplasm, which might *downregulate* synthesis through feedback mechanisms, rather than enhance it. *Increases ketogenesis* - Ketogenesis is the process of producing ketone bodies from fatty acids in the liver, primarily when glucose is scarce. - Since CPT I deficiency impairs the transport of fatty acids into the mitochondria, it would *decrease* the availability of substrates for beta-oxidation, thus reducing ketogenesis. *Reduces lipolysis* - Lipolysis is the breakdown of stored triglycerides into free fatty acids and glycerol, primarily occurring in adipose tissue. - CPT I deficiency affects the *utilization* of fatty acids, not their release from storage, so it would not directly reduce lipolysis.

Question 625: Which of the following enzymes is inhibited by malonyl-CoA to prevent simultaneous fatty acid synthesis and degradation?

A. Acetyl-CoA carboxylase
B. Carnitine palmitoyltransferase I (Correct Answer)
C. HMG-CoA reductase
D. Fatty acid synthase

Explanation: ***Carnitine palmitoyltransferase I*** - **Malonyl-CoA** inhibits **carnitine palmitoyltransferase I (CPT1)**, which is crucial for transporting fatty acyl-CoAs into the mitochondria for **beta-oxidation**. - This inhibition ensures that when fatty acid synthesis is active (producing malonyl-CoA), fatty acid degradation is simultaneously suppressed, preventing a futile cycle. *Acetyl-CoA carboxylase* - **Acetyl-CoA carboxylase (ACC)** is the enzyme responsible for synthesizing **malonyl-CoA** from acetyl-CoA, not inhibited by it. - ACC is the committed step in **fatty acid synthesis** and is subject to allosteric regulation by citrate (activator) and long-chain fatty acids (inhibitor). *HMG-CoA reductase* - **HMG-CoA reductase** is a key enzyme in the synthesis of **cholesterol**, not directly involved in the primary regulation of fatty acid synthesis and degradation by malonyl-CoA. - It is inhibited by cholesterol and statin drugs. *Fatty acid synthase* - **Fatty acid synthase** is a multi-enzyme complex responsible for the elongation of fatty acids using **malonyl-CoA** as a substrate. - It is activated, not inhibited, by malonyl-CoA, which serves as a building block for fatty acid chains.

Question 626: A patient with high levels of LDL and normal levels of HDL and triglycerides is likely suffering from which type of familial dyslipidemia?

A. Familial hypercholesterolemia (Correct Answer)
B. Familial combined hyperlipidemia
C. Dysbetalipoproteinemia
D. Familial hypertriglyceridemia

Explanation: ***Familial hypercholesterolemia*** - This condition is characterized by significantly **elevated LDL-C levels** due to defects in the LDL receptor pathway, leading to impaired clearance of LDL from the blood. - **HDL and triglyceride levels typically remain normal**, which aligns perfectly with the patient's lipid profile. *Familial combined hyperlipidemia* - This disorder involves **elevated levels of both cholesterol (primarily LDL-C) and triglycerides**, often with low HDL-C levels. - The patient's **normal triglyceride levels** make this diagnosis less likely. *Dysbetalipoproteinemia* - This condition is characterized by increased levels of **chylomicron remnants** and **VLDL remnants (IDL)**, leading to elevated cholesterol and triglycerides. - It typically results in **elevated triglycerides and cholesterol**, which is not seen here as triglycerides are normal. *Familial hypertriglyceridemia* - This condition primarily involves **markedly elevated triglyceride levels**, often with normal or only slightly elevated cholesterol and characteristically low HDL-C levels. - The patient's **normal triglyceride levels** directly rule out this diagnosis.

Question 627: What is the primary cause of ketoacidosis in uncontrolled diabetes mellitus?

A. High blood glucose levels
B. Fat metabolism producing ketone bodies (Correct Answer)
C. Excessive insulin production
D. Lack of glucagon

Explanation: ***Fat metabolism producing ketone bodies*** - In uncontrolled diabetes, the body cannot use **glucose** for energy due to insulin deficiency or resistance, leading to excessive breakdown of **fats** (lipolysis) for energy. - Free fatty acids undergo **β-oxidation** in the liver, producing excess acetyl-CoA that enters the **ketogenesis pathway**. - This produces large amounts of **ketone bodies** (acetoacetate, β-hydroxybutyrate, and acetone), which are acidic and accumulate faster than peripheral tissues can utilize them. - The accumulation of these organic acids lowers blood pH, causing **metabolic ketoacidosis** - the hallmark of diabetic ketoacidosis (DKA). *High blood glucose levels* - While hyperglycemia is characteristic of uncontrolled diabetes, it is a **consequence** of the underlying metabolic derangement, not the direct cause of the acidosis. - Hyperglycemia contributes to osmotic diuresis, dehydration, and electrolyte imbalances, but the **acidosis component is specifically due to ketone accumulation**. *Excessive insulin production* - **Diabetic ketoacidosis (DKA)** is characterized by an **absolute or relative deficiency of insulin**, not excessive production. - High insulin levels would promote glucose uptake into cells and inhibit lipolysis, preventing fat breakdown and thus preventing ketone body formation. *Lack of glucagon* - Glucagon levels are typically **elevated** in uncontrolled diabetes, as the body perceives a state of cellular starvation despite high blood glucose. - **Glucagon promotes glycogenolysis, gluconeogenesis, and lipolysis**, contributing to both hyperglycemia and enhanced ketone body formation, not preventing it.

Question 628: A patient with hypercholesterolemia has low LDL receptor activity. Which gene mutation is most likely responsible?

A. APOB
B. LDLR (Correct Answer)
C. PCSK9
D. APOE

Explanation: ***LDLR*** - Mutations in the **LDLR (low-density lipoprotein receptor) gene** directly lead to reduced or dysfunctional LDL receptors, preventing efficient clearance of LDL from the blood. - This is the **most common genetic cause** of **Familial Hypercholesterolemia (FH)**, accounting for **~85-90% of cases**, characterized by very high LDL-C levels and premature cardiovascular disease. - **Most likely answer** when low LDL receptor activity is described. *APOB* - Mutations in the **APOB gene (apolipoprotein B)** can affect the binding of LDL to its receptor through **defective apolipoprotein B-100**, which is the ligand for the LDL receptor. - This causes a rare form of FH (~5-10% of cases) but represents a **ligand defect** rather than a primary **receptor defect**. - The receptor itself has normal structure but cannot bind LDL effectively. *PCSK9* - **PCSK9 (proprotein convertase subtilisin/kexin type 9)** regulates LDL receptor degradation, and **gain-of-function mutations** lead to **increased degradation of LDL receptors**, resulting in **reduced LDL receptor numbers and activity** on cell surfaces. - This does cause hypercholesterolemia and low functional LDL receptor activity, but accounts for only **~1-3% of FH cases**, making it **much less likely** than LDLR mutations. - Loss-of-function PCSK9 mutations conversely lead to **increased LDL receptors** and lower LDL levels (protective). *APOE* - **APOE (apolipoprotein E)** plays a crucial role in the metabolism of **chylomicron and VLDL remnants**, primarily binding to hepatic receptors for remnant clearance. - While APOE polymorphisms (E2, E3, E4) influence lipid levels and cardiovascular risk, they do not directly cause **reduced LDL receptor activity** or classic familial hypercholesterolemia. - APOE2 homozygosity can cause **Type III hyperlipoproteinemia** (dysbetalipoproteinemia), not the LDL receptor defect described.

Question 629: What is the role of lecithin-cholesterol acyltransferase (LCAT) in lipid metabolism?

A. Converts free cholesterol into cholesteryl esters (Correct Answer)
B. Hydrolyzes triglycerides in lipoproteins
C. Synthesizes triglycerides in the liver
D. Breaks down cholesterol into bile acids

Explanation: ***Converts free cholesterol into cholesteryl esters*** - **LCAT** is an enzyme synthesized in the liver and secreted into the bloodstream, where it associates with **HDL** particles. - Its primary role is to catalyze the esterification of **free cholesterol** in the surface of lipoproteins, especially HDL, into **cholesteryl esters** using a fatty acid from lecithin. This traps cholesterol within the lipoprotein core, facilitating cholesterol transport. *Hydrolyzes triglycerides in lipoproteins* - The hydrolysis of triglycerides in lipoproteins, particularly **chylomicrons** and **VLDLs**, is primarily carried out by **lipoprotein lipase (LPL)**, an enzyme found on the endothelial surface of capillaries. - **Hepatic lipase (HL)** also contributes to triglyceride hydrolysis, especially in remnants and HDL, but LCAT's role is specifically cholesterol esterification. *Synthesizes triglycerides in the liver* - The synthesis of triglycerides in the liver, a process known as **lipogenesis**, involves enzymes such as **glycerol-3-phosphate acyltransferase** and **diacylglycerol acyltransferase (DGAT)**. - These enzymes are involved in assembling fatty acids and glycerol-3-phosphate into triglycerides, which are then packaged into **VLDL** particles for transport. *Breaks down cholesterol into bile acids* - The breakdown of cholesterol into **bile acids** (bile acid synthesis) occurs in the liver and is a major pathway for cholesterol elimination. - The rate-limiting enzyme in this process is **cholesterol 7-alpha-hydroxylase**, which initiates the conversion of cholesterol into primary bile acids.

Question 630: Which enzyme is responsible for converting dihydroxyacetone phosphate to glycerol-3-phosphate during triglyceride synthesis?

A. Glycerol kinase
B. Glycerol-3-phosphate dehydrogenase (Correct Answer)
C. Acyl-CoA synthetase
D. Phosphatidate phosphatase

Explanation: ***Glycerol-3-phosphate dehydrogenase*** - This enzyme **catalyzes the reduction** of **dihydroxyacetone phosphate (DHAP)**, a glycolysis intermediate, to **glycerol-3-phosphate**. - **NADH** serves as the **reducing agent** in this reaction, converting DHAP to glycerol-3-phosphate, which is the backbone for triglyceride synthesis. *Glycerol kinase* - This enzyme catalyzes the phosphorylation of **free glycerol** to **glycerol-3-phosphate**, using **ATP**. - It is primarily active in the **liver** and kidney, as adipocytes lack this enzyme, meaning they cannot directly use free glycerol for triglyceride synthesis. *Acyl-CoA synthetase* - This enzyme is responsible for **activating fatty acids** by converting them into **acyl-CoA**, using **ATP** and **CoA**. - This activation step is essential before fatty acids can be incorporated into triglycerides or phospholipids. *Phosphatidate phosphatase* - This enzyme **removes a phosphate group** from **phosphatidic acid** to form **diacylglycerol (DAG)**. - DAG is then acylated to form triglycerides, making this enzyme crucial in the final steps of triglyceride synthesis.

Practice by Chapter

Lipid Classification and Chemistry

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Fatty Acid Oxidation

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Ketone Body Metabolism

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Fatty Acid Synthesis

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Metabolism of Triacylglycerols

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Phospholipid Metabolism

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Cholesterol Metabolism and Biosynthesis

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Bile Acids and Bile Salts

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Lipoprotein Metabolism and Transport

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Dyslipidemias and Atherosclerosis

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Prostaglandins and Eicosanoids

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Fatty Liver and Lipotropic Factors

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