Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 631: How does a defect in the LDL receptor gene contribute to the development of familial hypercholesterolemia?

A. Decreased clearance of LDL from the bloodstream (Correct Answer)
B. Reduced conversion of cholesterol to bile acids
C. Increased synthesis of cholesterol in the liver due to elevated LDL levels
D. Enhanced uptake of HDL by liver cells

Explanation: ***Decreased clearance of LDL from the bloodstream*** - A defect in the **LDL receptor gene** leads to fewer functional LDL receptors on the surface of liver cells. - These receptors are responsible for binding to and internalizing **LDL particles** from the bloodstream, so a reduction in their number results in **elevated circulating LDL**. *Reduced conversion of cholesterol to bile acids* - The conversion of cholesterol to bile acids is a primary mechanism for cholesterol excretion, mainly regulated by the enzyme **cholesterol 7α-hydroxylase**. - This process is largely independent of **LDL receptor function** and not the primary defect in familial hypercholesterolemia. *Increased synthesis of cholesterol in the liver due to elevated LDL levels* - High LDL levels typically **inhibit hepatic cholesterol synthesis** through a feedback mechanism involving HMG-CoA reductase, rather than increasing it. - The primary defect in familial hypercholesterolemia is impaired **LDL catabolism**, not increased *de novo* cholesterol synthesis. *Enhanced uptake of HDL by liver cells* - **HDL (high-density lipoprotein)** uptake by the liver is mediated by receptors like **scavenger receptor class B type 1 (SR-B1)**, which is distinct from the LDL receptor. - A defect in the **LDL receptor gene** does not directly affect HDL metabolism or its uptake by the liver.

Question 632: Which metabolic disorder is characterized by the accumulation of sphingomyelin in lysosomes?

A. Niemann-Pick disease (Correct Answer)
B. Tay-Sachs disease
C. Gaucher disease
D. Fabry disease

Explanation: ***Niemann-Pick disease*** - This lysosomal storage disorder is specifically characterized by the accumulation of **sphingomyelin** due to a deficiency in the enzyme **sphingomyelinase**. - Clinical manifestations include **hepatosplenomegaly**, neurodegeneration, and a **cherry-red spot** on the retina in some types. *Tay-Sachs disease* - This disorder involves the accumulation of **GM2 gangliosides** in lysosomes due to a deficiency of **hexosaminidase A**. - It primarily affects the central nervous system, leading to **progressive neurodegeneration** and a cherry-red spot in the macula. *Gaucher disease* - Characterized by the buildup of **glucocerebroside** in lysosomes, caused by a deficiency in the enzyme **glucocerebrosidase**. - Key features include **hepatosplenomegaly**, bone pain, and neurologic manifestations in severe forms. *Fabry disease* - This X-linked disorder results from a deficiency of **alpha-galactosidase A**, leading to the accumulation of **globotriaosylceramide (Gb3)**. - Clinical signs include **acroparesthesias**, angiokeratomas, renal failure, and cardiac involvement.

Question 633: In lipid metabolism, what is the primary role of lipoprotein lipase?

A. To synthesize triglycerides in adipose tissue
B. To hydrolyze triglycerides in chylomicrons and VLDLs (Correct Answer)
C. To convert cholesterol into bile acids
D. To package triglycerides into VLDLs in the liver

Explanation: ***To hydrolyze triglycerides in chylomicrons and VLDLs*** - **Lipoprotein lipase (LPL)** is an enzyme located on the endothelial surface of capillaries, where it acts to break down **triglycerides** carried in **chylomicrons** and **very-low-density lipoproteins (VLDLs)**. - This hydrolysis releases **fatty acids** and **glycerol**, which can then be taken up by surrounding tissues (e.g., adipose tissue, muscle) for energy or storage. *To synthesize triglycerides in adipose tissue* - The synthesis of **triglycerides** in adipose tissue is primarily carried out by enzymes within the adipocytes, such as **acyl-CoA:diacylglycerol acyltransferase (DGAT)**, using fatty acids and glycerol-3-phosphate. - LPL’s role is on the *outside* of the cell, making fatty acids available for uptake, not *synthesizing* within the cell. *To convert cholesterol into bile acids* - The conversion of **cholesterol** into **bile acids** occurs in the **liver** and is catalyzed by a series of enzymes, with **cholesterol 7α-hydroxylase** being the rate-limiting step. - This process is distinct from the function of LPL, which is involved in triglyceride metabolism. *To package triglycerides into VLDLs in the liver* - The packaging of **triglycerides** into **VLDLs** takes place within **hepatocytes** (liver cells) and involves the action of **microsomal triglyceride transfer protein (MTP)** and other enzymes. - LPL does not participate in this intracellular process but rather acts on VLDLs once they are secreted into the bloodstream.

Question 634: Which of the following metabolic changes is characteristic of diabetic ketoacidosis?

A. Hypoglycemia
B. Hypoketonemia
C. Hyperketonemia (Correct Answer)
D. Hyperinsulinemia

Explanation: ***Hyperketonemia*** - Diabetic ketoacidosis (DKA) is characterized by **uncontrolled ketone body production** due to severe insulin deficiency and increased counter-regulatory hormones. - This leads to a significant increase in blood **ketone levels**, such as beta-hydroxybutyrate and acetoacetate. *Hypoglycemia* - DKA is fundamentally a state of **absolute or relative insulin deficiency**, leading to **hyperglycemia**, not hypoglycemia. - The lack of insulin prevents glucose uptake by cells, causing high blood sugar. *Hypoketonemia* - **Hypoketonemia** means abnormally low levels of ketones, which is the opposite of what occurs in DKA. - DKA is defined by the **overproduction and accumulation of ketones**, leading to metabolic acidosis. *Hyperinsulinemia* - DKA is caused by an **absolute or relative deficiency of insulin**, leading to high blood glucose and ketone production. - **Hyperinsulinemia** would inhibit lipolysis and ketogenesis, thus preventing DKA.

Question 635: A 54-year-old male presents with exercise intolerance, weakness, and myalgia following fasting. Laboratory results show elevated creatine kinase and elevated acylcarnitines. What is the likely enzyme deficiency?

A. Glucose-6-phosphate dehydrogenase
B. Carnitine palmitoyltransferase II (Correct Answer)
C. Pyruvate dehydrogenase
D. Phosphofructokinase

Explanation: ***Carnitine palmitoyltransferase II*** * **Carnitine palmitoyltransferase II (CPT II)** deficiency is a disorder of **fatty acid oxidation** and is typically triggered by **fasting** or intense exercise due to increased reliance on fat as an energy source. * The presentation of **exercise intolerance**, **weakness**, **myalgia**, **elevated creatine kinase (CK)**, and **elevated acylcarnitines** perfectly matches the clinical picture of CPT II deficiency. *Glucose-6-phosphate dehydrogenase (G6PD)* * **G6PD deficiency** primarily affects **red blood cells**, leading to **hemolytic anemia** in response to oxidative stress (e.g., fava beans, certain medications). * It does **not typically cause myalgia**, exercise intolerance, or elevated CK and acylcarnitines as its primary symptoms. *Pyruvate dehydrogenase (PDH)* * **Pyruvate dehydrogenase (PDH)** deficiency affects the conversion of pyruvate to acetyl-CoA, impairing **carbohydrate metabolism** and leading to **lactic acidosis**. * While it can cause exercise intolerance and neurological symptoms, the constellation of elevated CK and acylcarnitines, especially in response to fasting, points away from PDH deficiency as the primary cause. *Phosphofructokinase (PFK)* * **Phosphofructokinase (PFK)** deficiency (Tarui disease) is a disorder of **glycogenolysis** that primarily impairs the breakdown of glycogen for energy. * This condition typically results in **exercise intolerance**, **hemolytic anemia**, and **rhabdomyolysis**, but it would not typically cause elevated acylcarnitines, which are characteristic of fatty acid oxidation defects.

Question 636: A patient with hyperlipidemia is found to have elevated levels of chylomicrons. Which apolipoprotein deficiency is most likely?

A. ApoB100
B. ApoCII (Correct Answer)
C. ApoE
D. ApoA1

Explanation: ***Correct Answer: ApoCII*** - **ApoCII** is a crucial activator of **lipoprotein lipase (LPL)**, an enzyme responsible for hydrolyzing triglycerides within chylomicrons and VLDL. - A deficiency in **ApoCII** leads to impaired LPL activity, causing **chylomicrons** to accumulate in the bloodstream after a meal, resulting in severe hypertriglyceridemia (Type I hyperlipoproteinemia or familial chylomicronemia syndrome). *Incorrect: ApoB100* - **ApoB100** is primarily found on **LDL** and **VLDL** particles, where it serves as the ligand for the **LDL receptor**. - Deficiency in **ApoB100** (or defective **ApoB100**) typically leads to **hypobetalipoproteinemia** or **familial hypocholesterolemia**, not elevated chylomicrons. *Incorrect: ApoE* - **ApoE** is essential for the receptor-mediated uptake of **chylomicron remnants** and **VLDL remnants** by the liver. - A deficiency or defective **ApoE** results in **Type III hyperlipoproteinemia** (familial dysbetalipoproteinemia), characterized by the accumulation of remnants, not intact chylomicrons. *Incorrect: ApoA1* - **ApoA1** is the primary apolipoprotein of **HDL** and plays a critical role in **reverse cholesterol transport** by activating lecithin-cholesterol acyltransferase (LCAT). - Deficiency in **ApoA1** leads to very low **HDL** levels and impaired cholesterol efflux, but it does not directly cause an accumulation of chylomicrons.

Question 637: In familial hypercholesterolemia, what is the effect of defective LDL receptors on lipid metabolism and cardiovascular risk?

A. Increased plasma LDL levels; atherosclerosis (Correct Answer)
B. Increased HDL levels; no change in cardiovascular health
C. Decreased VLDL production; increased triglycerides
D. Decreased plasma LDL levels; increased atherosclerosis

Explanation: ***Increased plasma LDL levels; atherosclerosis*** - Defective **LDL receptors** prevent the efficient uptake of **LDL cholesterol** from the bloodstream by cells, leading to its accumulation in the plasma. - Chronically elevated **plasma LDL levels** promote the formation of **atherosclerotic plaques** in arteries, significantly increasing cardiovascular risk. *Increased HDL levels; no change in cardiovascular health* - **Familial hypercholesterolemia** is primarily characterized by elevated **LDL** due to receptor defects, not increased **HDL**. **High-density lipoprotein (HDL)** levels are generally not directly affected by LDL receptor function. - While high HDL is generally considered protective, the severe elevation in LDL overrides any potential protective effect of unchanged or slightly altered HDL, leading to a substantial increase in cardiovascular risk. *Decreased VLDL production; increased triglycerides* - **VLDL (very-low-density lipoprotein)** production is mainly regulated by the liver's synthesis of triglycerides, which is not directly impaired by defective **LDL receptors**. - While hypertriglyceridemia can occur in some dyslipidemias, it is not the primary feature or direct consequence of defective LDL receptors in **familial hypercholesterolemia**, which is fundamentally about LDL clearance. *Decreased plasma LDL levels; increased atherosclerosis* - Defective **LDL receptors** lead to *increased*, not decreased, plasma **LDL levels**, as the receptors are responsible for clearing LDL from circulation. - Increased plasma LDL levels are the direct cause of increased **atherosclerosis** in familial hypercholesterolemia, making this option contradictory.

Question 638: What is the primary consequence of carnitine palmitoyltransferase I (CPT I) deficiency in fatty acid metabolism?

A. Decreased fatty acid oxidation (Correct Answer)
B. Increased gluconeogenesis
C. Accumulation of fatty acyl-CoA
D. Increased production of ketone bodies

Explanation: ***Decreased fatty acid oxidation*** - CPT I is crucial for transporting **long-chain fatty acids** into the mitochondrial matrix. A deficiency impairs this transport, preventing fatty acids from undergoing **beta-oxidation**. - Without proper entry into the mitochondria, fatty acids cannot be broken down to produce **acetyl-CoA** and energy, leading to a shortage of ATP derived from fat metabolism. - This represents the **primary functional defect** — the loss of the enzyme's physiological role in energy production. *Accumulation of fatty acyl-CoA* - While accumulation of fatty acyl-CoA in the cytoplasm does occur as a **direct biochemical consequence**, it represents the "what accumulates" rather than the "loss of function" that defines the primary metabolic impact. - In the context of enzyme deficiencies, the **primary consequence** refers to the loss of the catalytic function (decreased oxidation), while accumulation is a **downstream effect** of that functional loss. - Both occur simultaneously, but decreased oxidation is the pathophysiological hallmark of CPT I deficiency. *Increased gluconeogenesis* - CPT I deficiency actually leads to **decreased gluconeogenesis**, not increased, due to insufficient energy from fatty acid oxidation. - The lack of **acetyl-CoA** reduces the allosteric activation of pyruvate carboxylase, impairing glucose synthesis from non-carbohydrate sources. - Clinically presents as **hypoketotic hypoglycemia**, particularly during fasting states. *Increased production of ketone bodies* - Ketone body production relies on the **beta-oxidation of fatty acids** to produce acetyl-CoA, which is then used in the liver for ketogenesis. - In CPT I deficiency, the impaired fatty acid oxidation directly causes **reduced production of acetyl-CoA** and therefore **decreased ketone body synthesis**, leading to hypoketonemia. - The hypoketotic state distinguishes CPT I deficiency from other metabolic disorders.

Question 639: Which of the following hormones is synthesized from cholesterol and plays a crucial role in the stress response?

A. Insulin
B. Cortisol (Correct Answer)
C. Thyroxine
D. Epinephrine

Explanation: ***Cortisol*** - **Cortisol** is a **glucocorticoid** hormone synthesized from **cholesterol** in the adrenal cortex. - It plays a critical role in the body's **stress response** by increasing blood glucose, suppressing the immune system, and influencing metabolism. *Insulin* - **Insulin** is a peptide hormone, not a steroid, and is synthesized from **proinsulin** in the pancreatic beta cells. - Its primary role is to regulate **blood glucose levels** by facilitating glucose uptake into cells, not directly in the stress response in the same manner as cortisol. *Thyroxine* - **Thyroxine** (T4) is an **amine hormone** derived from the amino acid **tyrosine** and iodine, not cholesterol. - It primarily regulates **metabolism**, growth, and development, playing a more chronic, overarching role rather than an acute stress response. *Epinephrine* - **Epinephrine** (adrenaline) is a **catecholamine** hormone, derived from the amino acid **tyrosine**, not cholesterol. - While it is a key hormone in the "fight-or-flight" **stress response**, its synthesis pathway is different from steroid hormones.

Question 640: What is the role of carnitine in fatty acid oxidation?

A. Activates fatty acids by converting them to acyl-CoA
B. Regulates the rate of β-oxidation by controlling enzyme activity
C. Transports fatty acids into mitochondria, facilitating β-oxidation (Correct Answer)
D. Converts fatty acids into ketone bodies

Explanation: ***Transports fatty acids into mitochondria, facilitating β-oxidation.*** - **Carnitine** is essential for carrying long-chain fatty acids across the **inner mitochondrial membrane** into the mitochondrial matrix. - Once inside, these fatty acids undergo **beta-oxidation** to produce acetyl-CoA, which enters the **Krebs cycle** for energy production. *Converts fatty acids into ketone bodies, leading to ketoacidosis* - The primary role of carnitine is transport, not direct conversion into **ketone bodies**. - While fatty acid oxidation does contribute to ketone body formation during fasting, carnitine deficiency primarily impairs the ability to oxidize fatty acids, leading to their accumulation, not necessarily increased ketoacidosis in the initial stages. *Activates fatty acids for degradation, leading to accumulation of fatty acids* - **Fatty acid activation** (conversion to acyl-CoA) occurs in the cytosol, prior to carnitine's involvement. - A carnitine deficiency would hinder the transport of these *activated* fatty acids into the mitochondria, causing them to accumulate in the cytoplasm. *Regulates the rate of β-oxidation, leading to uncontrolled fatty acid breakdown* - Carnitine's main function is transport, not the direct **regulation of the rate of beta-oxidation**. - A deficiency would *impair* or *reduce* fatty acid breakdown rather than lead to uncontrolled breakdown.

Practice by Chapter

Lipid Classification and Chemistry

Practice Questions

Fatty Acid Oxidation

Practice Questions

Ketone Body Metabolism

Practice Questions

Fatty Acid Synthesis

Practice Questions

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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