Lipid Metabolism — MCQs

A clinical study involves subjects from families experiencing complications of atherosclerotic cardiovascular disease and tendinous xanthomas before age 30. Some children in these families exhibit early atheroma formation and benefit from treatment with pharmacologic agents that inhibit HMG-CoA reductase. Affected individuals in these families are most likely to have a mutation in a gene encoding a cell surface receptor for which of the following?

Q65Easy

Increased level of lipoprotein(a) predisposes to which of the following conditions?

Q66Easy

Lipogenesis is stimulated by which hormone?

Q67Easy

The main apoprotein in HDL is:

Q68Easy

Which cholesterol ester is found in maximum concentration in plasma?

Q69Easy

Mitochondria is involved in all of the following functions, except:

Q70Medium

Acetyl-CoA generated from the metabolism of fatty acids can be converted into all of the following, except:

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 61: What is the rate-limiting step in cholesterol synthesis?

A. HMG CoA synthetase
B. HMG CoA lyase
C. HMG CoA reductase (Correct Answer)
D. Mevalonate synthetase

Explanation: ### Explanation **Correct Answer: C. HMG CoA reductase** Cholesterol synthesis occurs primarily in the liver and involves the conversion of Acetyl-CoA into cholesterol. The **rate-limiting and committed step** of this pathway is the conversion of **3-hydroxy-3-methylglutaryl-CoA (HMG CoA)** to **Mevalonate**. This reaction is catalyzed by the enzyme **HMG CoA reductase**, which is located in the endoplasmic reticulum and requires NADPH as a reducing agent. Because this step is the slowest in the pathway, the entire rate of cholesterol production depends on its activity. **Analysis of Incorrect Options:** * **A. HMG CoA synthetase:** This enzyme catalyzes the formation of HMG CoA from Acetoacetyl-CoA and Acetyl-CoA. While it is an early step, it is not the primary regulatory point for cholesterol synthesis. (Note: A mitochondrial isoform of this enzyme is involved in ketogenesis). * **B. HMG CoA lyase:** This enzyme is involved in **ketogenesis** (breaking down HMG CoA into Acetoacetate) and leucine catabolism, rather than cholesterol synthesis. * **D. Mevalonate synthetase:** This is a distractor term; the enzyme that produces mevalonate is HMG CoA reductase. **High-Yield Clinical Pearls for NEET-PG:** * **Pharmacology Link:** **Statins** (e.g., Atorvastatin) are competitive inhibitors of HMG CoA reductase, used to treat hypercholesterolemia. * **Regulation:** HMG CoA reductase is inhibited by high levels of intracellular cholesterol (feedback inhibition) and stimulated by **Insulin**, while **Glucagon** and **AMP** (via AMPK) inhibit it. * **Subcellular Location:** The pathway begins in the **cytosol**, but HMG CoA reductase is anchored in the **ER membrane**.

Question 62: Where does ketogenesis take place?

A. Hepatic mitochondria (Correct Answer)
B. Hepatic cytoplasm
C. Hepatic microsome
D. Hepatic lysosome

Explanation: **Explanation:** Ketogenesis is the metabolic pathway by which ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone) are produced. This process occurs exclusively in the **liver** (hepatic) because it is the only organ that contains significant quantities of the rate-limiting enzyme, **HMG-CoA synthase**. 1. **Why Hepatic Mitochondria is Correct:** The synthesis of ketone bodies occurs within the **mitochondrial matrix** of hepatocytes. This is strategically significant because ketogenesis utilizes **Acetyl-CoA** derived from the beta-oxidation of fatty acids, which also takes place inside the mitochondria. The key regulatory enzyme, mitochondrial HMG-CoA synthase, converts Acetyl-CoA and Acetoacetyl-CoA into HMG-CoA, which is then cleaved to form ketone bodies. 2. **Why Other Options are Incorrect:** * **Hepatic Cytoplasm:** While HMG-CoA is also synthesized in the cytoplasm, it is used there for **cholesterol synthesis** via the enzyme cytosolic HMG-CoA reductase. These two pathways are spatially separated to prevent metabolic interference. * **Hepatic Microsomes:** Microsomes (Smooth ER) are primarily involved in lipid synthesis (triacylglycerols, phospholipids) and drug detoxification (Cytochrome P450 system), not ketogenesis. * **Hepatic Lysosomes:** Lysosomes are involved in the degradation of macromolecules and autophagy, not the synthesis of metabolic fuels. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** Mitochondrial HMG-CoA synthase. * **The "Liver Paradox":** The liver produces ketone bodies but **cannot utilize them** because it lacks the enzyme **Thiophorase** (Succinyl-CoA:3-ketoacid CoA transferase). * **Stimulus:** Ketogenesis is stimulated by high glucagon, low insulin, and high levels of circulating free fatty acids (e.g., starvation, uncontrolled Diabetes Mellitus). * **Ketone Bodies:** Acetone is a non-metabolizable side product excreted via the lungs, giving the characteristic "fruity breath" in ketoacidosis.

Question 63: In which form are bile acids primarily present in the body?

A. Weak acids
B. In ionized form
C. In salt form (Correct Answer)
D. In esterified form

Explanation: ### Explanation **Correct Answer: C. In salt form** Bile acids (primary and secondary) are synthesized from cholesterol in the liver. Before being secreted into the bile, they are conjugated with either **glycine** or **taurine**. This conjugation significantly lowers their pKa (from approximately 6.0 to 2.0–4.0). At the physiological pH of the duodenum and gallbladder (pH ~7.0–8.0), these conjugated bile acids exist almost entirely in their **ionized, negatively charged state**. To maintain electrical neutrality, they associate with cations like **Sodium (Na⁺)** or **Potassium (K⁺)**, forming **bile salts**. Bile salts are much more efficient detergents than bile acids because they are more amphipathic and water-soluble, which is essential for the emulsification of dietary fats. **Why other options are incorrect:** * **A & B (Weak acids / Ionized form):** While bile acids are technically weak acids, they do not remain as free acids in the body. Although they exist in an "ionized form," the term **"salt form"** is the more accurate physiological description of their existence in bile and the intestine when paired with cations. * **D (Esterified form):** Cholesterol is often stored or transported in an esterified form (cholesterol esters), but bile acids are **conjugated** (via amide bonds), not esterified. --- ### High-Yield Clinical Pearls for NEET-PG: * **Rate-limiting enzyme:** Cholesterol 7α-hydroxylase (inhibited by bile salts; stimulated by cholesterol). * **Primary Bile Acids:** Cholic acid and Chenodeoxycholic acid (synthesized in the liver). * **Secondary Bile Acids:** Deoxycholic acid and Lithocholic acid (formed by bacterial action in the colon). * **Enterohepatic Circulation:** 95% of bile salts are reabsorbed in the **terminal ileum**; a deficiency in this process (e.g., Crohn’s disease) leads to steatorrhea and malabsorption.

Question 64: A clinical study involves subjects from families experiencing complications of atherosclerotic cardiovascular disease and tendinous xanthomas before age 30. Some children in these families exhibit early atheroma formation and benefit from treatment with pharmacologic agents that inhibit HMG-CoA reductase. Affected individuals in these families are most likely to have a mutation in a gene encoding a cell surface receptor for which of the following?

A. Cortisol
B. Insulin
C. LDL cholesterol (Correct Answer)
D. Leptin

Explanation: The clinical presentation of **tendinous xanthomas** and **premature atherosclerosis** (before age 30) is a classic hallmark of **Familial Hypercholesterolemia (Type IIa Hyperlipoproteinemia)**. ### Why LDL Cholesterol is Correct Familial Hypercholesterolemia is most commonly caused by an autosomal dominant mutation in the **LDL receptor (LDLR) gene**. * **Mechanism:** Under normal conditions, the LDL receptor on the liver cell surface binds and internalizes LDL particles via receptor-mediated endocytosis. A defect in this receptor leads to decreased clearance of LDL from the plasma. * **Pathology:** Elevated LDL levels lead to cholesterol deposition in tendons (xanthomas) and arteries (atheromas). * **Treatment:** **HMG-CoA reductase inhibitors (Statins)** are the first-line treatment. They inhibit endogenous cholesterol synthesis, which triggers a compensatory **upregulation of LDL receptors** (in those with at least one functional allele), thereby increasing the clearance of circulating LDL. ### Why Other Options are Incorrect * **A. Cortisol:** Mutations in glucocorticoid receptors lead to Primary Generalized Glucocorticoid Resistance, presenting with hypertension and hirsutism, not xanthomas. * **B. Insulin:** Insulin receptor mutations (e.g., Donohue syndrome) cause severe insulin resistance, acanthosis nigricans, and growth retardation. * **D. Leptin:** Leptin or its receptor mutations lead to early-onset hyperphagia and morbid obesity, but not isolated premature tendinous xanthomas. ### High-Yield Clinical Pearls for NEET-PG * **Inheritance:** Autosomal Dominant. Homozygotes are more severely affected, often presenting with myocardial infarction in childhood. * **Key Finding:** **Achilles tendon xanthoma** is the most specific physical sign. * **Lipid Profile:** Isolated elevation of **LDL** and **Total Cholesterol**; Triglycerides are usually normal. * **Statins Mechanism:** They work by increasing the *expression* of LDL receptors on hepatocytes.

Question 65: Increased level of lipoprotein(a) predisposes to which of the following conditions?

A. Liver cirrhosis
B. Atherosclerosis (Correct Answer)
C. Nephrotic syndrome
D. Pancreatitis

Explanation: **Explanation:** **Lipoprotein(a) [Lp(a)]** is a specialized lipoprotein consisting of an LDL-like particle and a specific protein called **apolipoprotein(a)**, which is covalently linked to apolipoprotein B-100. **Why Atherosclerosis is the Correct Answer:** Lp(a) is highly atherogenic and thrombogenic due to two primary mechanisms: 1. **Structural Similarity to Plasminogen:** Apo(a) has a high degree of structural homology with plasminogen. It competes with plasminogen for binding sites on fibrin, thereby **inhibiting fibrinolysis** and promoting clot formation (thrombogenesis). 2. **LDL-like properties:** Like LDL, Lp(a) undergoes oxidative modification and is taken up by macrophages to form **foam cells**, leading to the development of atherosclerotic plaques. Elevated levels are an independent risk factor for coronary artery disease and stroke. **Analysis of Incorrect Options:** * **Liver Cirrhosis:** Chronic liver disease typically leads to *decreased* synthesis of lipoproteins (hypolipoproteinemia), not an increase in Lp(a). * **Nephrotic Syndrome:** While nephrotic syndrome causes generalized hyperlipidemia (increased LDL and VLDL), Lp(a) is not the primary diagnostic or causative marker for this condition. * **Pancreatitis:** Acute pancreatitis is classically associated with severe **Hypertriglyceridemia** (Type I, IV, or V hyperlipoproteinemia), specifically elevated Chylomicrons, rather than Lp(a). **High-Yield Clinical Pearls for NEET-PG:** * **Niacin** is one of the few pharmacological agents that significantly lowers Lp(a) levels. * Lp(a) levels are largely **genetically determined** and are not significantly affected by diet or exercise. * **Kringles:** The repeating structural units in Apo(a) are called "Kringle domains" (specifically Kringle IV), which mediate its interference with plasminogen.

Question 66: Lipogenesis is stimulated by which hormone?

A. Insulin (Correct Answer)
B. Glucagon
C. Cortisol
D. None of the above

Explanation: **Explanation:** **Lipogenesis** is the metabolic process of synthesizing fatty acids and triglycerides, primarily occurring in the liver and adipose tissue during the "fed state" when energy supply exceeds demand. **1. Why Insulin is Correct:** Insulin is the primary **anabolic hormone** of the body. It stimulates lipogenesis through several mechanisms: * **Substrate Availability:** It increases glucose uptake into cells (via GLUT-4), providing Acetyl-CoA and NADPH (via the HMP shunt) required for fatty acid synthesis. * **Enzyme Activation:** It dephosphorylates and activates **Acetyl-CoA Carboxylase (ACC)**, the rate-limiting enzyme of fatty acid synthesis. * **Gene Expression:** It induces the expression of Fatty Acid Synthase (FAS). * **Inhibition of Breakdown:** It inhibits Hormone-Sensitive Lipase (HSL), preventing lipolysis. **2. Why Other Options are Incorrect:** * **Glucagon:** This is a catabolic hormone released during fasting. It stimulates lipolysis (breakdown of fats) and inhibits ACC via cAMP-dependent phosphorylation, effectively shutting down lipogenesis. * **Cortisol:** While cortisol has complex effects, its primary role in lipid metabolism is stimulating **lipolysis** in the extremities to provide substrates for gluconeogenesis, although it may promote fat deposition in the trunk (Cushingoid features). **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Acetyl-CoA Carboxylase (ACC). * **Cofactor required:** Biotin (for ACC) and NADPH (from HMP Shunt). * **Key Regulator:** Citrate acts as an allosteric activator of ACC, while Palmitoyl-CoA (end product) acts as an inhibitor. * **Location:** Occurs in the **Cytosol** (unlike Beta-oxidation, which occurs in the Mitochondria).

Question 67: The main apoprotein in HDL is:

A. ApoB-100
B. ApoB-48
C. ApoA-1 (Correct Answer)
D. ApoC-1

Explanation: **Explanation:** **ApoA-1** is the primary structural protein of **High-Density Lipoprotein (HDL)**, accounting for approximately 70% of its protein content. It is synthesized in the liver and intestine. Its critical physiological role is the activation of the enzyme **Lecithin-Cholesterol Acyltransferase (LCAT)**, which esterifies free cholesterol into cholesterol esters. This process allows HDL to sequester cholesterol within its core, facilitating **Reverse Cholesterol Transport** (carrying cholesterol from peripheral tissues back to the liver). **Analysis of Incorrect Options:** * **ApoB-100:** The primary structural protein for **VLDL, IDL, and LDL**. It serves as the ligand for the LDL receptor. * **ApoB-48:** Found exclusively in **Chylomicrons**. It is a truncated version of ApoB-100 (produced via mRNA editing) and is essential for the secretion of chylomicrons from the intestine. * **ApoC-1:** A minor apoprotein found in VLDL and HDL; it primarily acts as an activator of LCAT, but it is not the "main" structural apoprotein. **High-Yield Clinical Pearls for NEET-PG:** * **HDL** is known as "Good Cholesterol" because of its anti-atherogenic properties. * **Tangier Disease:** A rare genetic disorder caused by a mutation in the **ABCA1 transporter**, leading to extremely low levels of HDL and ApoA-1. * **ApoE:** Essential for the hepatic uptake of chylomicron remnants and IDL via the LDL receptor-related protein (LRP). * **ApoC-II:** The obligatory co-factor for **Lipoprotein Lipase (LPL)**; deficiency leads to Type I Hyperlipoproteinemia.

Question 68: Which cholesterol ester is found in maximum concentration in plasma?

A. HDL
B. LDL (Correct Answer)
C. VLDL
D. Chylomicron

Explanation: ### Explanation The correct answer is **LDL (Low-Density Lipoprotein)**. **Why LDL is the correct answer:** Cholesterol exists in the plasma in two forms: free cholesterol and esterified cholesterol (cholesterol esters). Approximately **70% of the total plasma cholesterol** is transported within LDL particles. LDL is the final product of the VLDL → IDL → LDL pathway and functions primarily to deliver cholesterol to peripheral tissues. Because LDL has the highest percentage of cholesterol (about 50% of its weight is cholesterol/cholesterol esters) and a longer half-life in circulation compared to other lipoproteins, it contains the maximum concentration of cholesterol esters in the plasma. **Why the other options are incorrect:** * **HDL (High-Density Lipoprotein):** While HDL is involved in "Reverse Cholesterol Transport" and contains the enzyme LCAT (which creates cholesterol esters), it serves as a carrier to move cholesterol back to the liver. Its total concentration of cholesterol esters is lower than that of LDL. * **VLDL (Very Low-Density Lipoprotein):** VLDL is primarily composed of **endogenous triglycerides** (approx. 60%). While it contains some cholesterol, its main role is triglyceride transport. * **Chylomicrons:** These are the largest lipoproteins but are composed almost entirely (**90%**) of **exogenous (dietary) triglycerides**. They contain the least amount of cholesterol among all lipoproteins. **High-Yield Clinical Pearls for NEET-PG:** * **Apo-B100** is the characteristic apoprotein for LDL, VLDL, and IDL. * **Friedewald Equation:** LDL Cholesterol = Total Cholesterol – [HDL + (Triglycerides/5)]. (Note: This is invalid if TG >400 mg/dL). * **Rate-limiting enzyme** of cholesterol synthesis: HMG-CoA Reductase (inhibited by Statins). * **LCAT (Lecithin-Cholesterol Acyltransferase):** The enzyme responsible for forming cholesterol esters in the plasma (associated with HDL).

Question 69: Mitochondria is involved in all of the following functions, except:

A. ATP production
B. Apoptosis
C. Tricarboxylic acid cycle
D. Fatty acid biosynthesis (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Fatty acid biosynthesis**. In biochemistry, the "compartmentalization" of metabolic pathways is a high-yield concept for NEET-PG. 1. **Why Fatty Acid Biosynthesis is the correct answer:** Fatty acid synthesis (Lipogenesis) occurs primarily in the **cytosol**. The process requires NADPH and Acetyl-CoA. While Acetyl-CoA is produced in the mitochondria, it must be transported to the cytosol via the "Citrate Shuttle" because the mitochondrial membrane is impermeable to it. In contrast, **Fatty acid oxidation (Beta-oxidation)** occurs within the mitochondria. 2. **Analysis of Incorrect Options:** * **A. ATP Production:** Known as the "powerhouse of the cell," mitochondria are the primary site for oxidative phosphorylation and the Electron Transport Chain (ETC), which generates the majority of cellular ATP. * **B. Apoptosis:** Mitochondria play a central role in the intrinsic pathway of apoptosis. The release of **Cytochrome c** from the mitochondrial intermembrane space into the cytosol activates caspases, leading to programmed cell death. * **C. Tricarboxylic acid (TCA) cycle:** All enzymes of the Krebs cycle (except succinate dehydrogenase, which is on the inner membrane) are located in the mitochondrial matrix. **High-Yield Clinical Pearls for NEET-PG:** * **Dual-site pathways:** Heme synthesis, Urea cycle, and Gluconeogenesis occur in **both** the mitochondria and cytosol (Mnemonic: **HUG**). * **Mitochondrial DNA:** It is circular, double-stranded, and inherited exclusively from the **mother**. * **Marker Enzyme:** **Succinate dehydrogenase** is the marker enzyme for the inner mitochondrial membrane and is also part of Complex II of the ETC.

Question 70: Acetyl-CoA generated from the metabolism of fatty acids can be converted into all of the following, except:

A. Glucose (Correct Answer)
B. Fatty Acids
C. Cholesterol
D. Ketone bodies

Explanation: **Explanation:** The conversion of Acetyl-CoA to glucose is impossible in humans because the **Pyruvate Dehydrogenase (PDH) complex reaction is irreversible**. 1. **Why Glucose is the correct answer:** In the metabolic pathway, Pyruvate is converted to Acetyl-CoA by the PDH complex. However, there is no enzyme in human tissues that can convert Acetyl-CoA back into Pyruvate or Oxaloacetate (net gain). While Acetyl-CoA enters the TCA cycle by condensing with Oxaloacetate, two carbons are lost as $CO_2$ during the cycle. Consequently, there is **no net synthesis of glucose** from Acetyl-CoA. This is why fatty acids (which break down into Acetyl-CoA) cannot be used for gluconeogenesis. 2. **Analysis of Incorrect Options:** * **Fatty Acids:** Acetyl-CoA is the primary substrate for fatty acid synthesis (Lipogenesis) via its conversion to Malonyl-CoA in the cytoplasm. * **Cholesterol:** Acetyl-CoA is the precursor for the entire steroid synthesis pathway. Two molecules of Acetyl-CoA form Acetoacetyl-CoA, eventually leading to HMG-CoA and then Mevalonate (the rate-limiting step of cholesterol synthesis). * **Ketone Bodies:** During starvation or uncontrolled diabetes, excess Acetyl-CoA from $\beta$-oxidation is diverted to Ketogenesis in the liver mitochondria to form Acetoacetate and $\beta$-hydroxybutyrate. **High-Yield Clinical Pearls for NEET-PG:** * **Odd-chain fatty acids** are the exception: Their metabolism yields **Propionyl-CoA**, which enters the TCA cycle as Succinyl-CoA and *can* be converted to glucose. * **Leucine and Lysine** are purely ketogenic amino acids because they are metabolized directly to Acetyl-CoA or Acetoacetate. * The **PDH complex** requires five cofactors: Thiamine ($B_1$), Riboflavin ($B_2$), Niacin ($B_3$), Pantothenic acid ($B_5$), and Lipoic acid.

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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