Lipid Metabolism — MCQs
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Which of the following contains the highest protein content?
An individual with hypertriglyceridemia has a triglyceride level of 350 mg/dL. The patient switches to a low-fat, low-protein, high-carbohydrate diet while maintaining the same caloric intake. Three months later, his triglyceride level has increased to 375 mg/dL. Which component of his new diet is responsible for this increase in lipid content?
All of the following are diseases occurring due to disturbance in lipid metabolism except?
Which of the following statements about Thromboxane A2 (TXA2) is false?
What enzyme hydrolyzes storage triacylglycerols?
All of the following are derived from glycerol 3-phosphate, EXCEPT:
Abetalipoproteinemia results in the absence of which of the following?
The conversion of a fatty acid to an active fatty acid is catalysed by which of the following enzymes?
A patient is prescribed a medication to deter alcohol consumption due to repeated DUI citations. This medication induces nausea and vomiting upon alcohol ingestion. The drug-induced illness is caused by the buildup of which one of the following metabolites?
Which of the following are bile acids?
Lipid Metabolism Indian Medical PG Practice Questions and MCQs
Question 541: Which of the following contains the highest protein content?
- A. Triglycerides
- B. HDL (Correct Answer)
- C. LDL
- D. VLDL
Explanation: **Explanation:** The classification of lipoproteins is based on their **density**, which is determined by the ratio of proteins to lipids. Proteins are significantly denser than lipids; therefore, a higher protein content results in a higher density. **1. Why HDL is Correct:** **High-Density Lipoprotein (HDL)** contains the highest percentage of protein (approximately **40–55%**) and the lowest percentage of lipids among all lipoproteins. Because it has the most protein relative to its size, it is the densest particle. Functionally, HDL is involved in "Reverse Cholesterol Transport," carrying cholesterol from peripheral tissues back to the liver. **2. Why the Other Options are Incorrect:** * **Triglycerides (Option A):** These are a type of lipid, not a lipoprotein. They are the primary cargo carried by chylomicrons and VLDL. * **VLDL (Option D):** Very Low-Density Lipoprotein has a very high lipid content (mainly endogenous triglycerides) and only about **8–10% protein**. * **LDL (Option C):** Low-Density Lipoprotein is the primary carrier of cholesterol to peripheral tissues. It contains more protein than VLDL (about **20–25%**) but significantly less than HDL. **High-Yield NEET-PG Pearls:** * **Density Order (Highest to Lowest):** HDL > LDL > IDL > VLDL > Chylomicrons. * **Size Order (Largest to Smallest):** Chylomicrons > VLDL > LDL > HDL (Density and size are inversely related). * **Apolipoprotein Markers:** * **HDL:** Apo A-I (activates LCAT). * **LDL:** Apo B-100 (ligand for LDL receptor). * **Chylomicrons:** Apo B-48. * **Electrophoretic Mobility:** On electrophoresis (at pH 8.6), the migration order from origin is: **HDL (Alpha) > VLDL (Pre-beta) > LDL (Beta) > Chylomicrons (Origin).**
Question 542: An individual with hypertriglyceridemia has a triglyceride level of 350 mg/dL. The patient switches to a low-fat, low-protein, high-carbohydrate diet while maintaining the same caloric intake. Three months later, his triglyceride level has increased to 375 mg/dL. Which component of his new diet is responsible for this increase in lipid content?
- A. Phospholipids
- B. Triglycerides
- C. Amino acids
- D. Carbohydrates (Correct Answer)
Explanation: ### Explanation **1. Why Carbohydrates are Correct:** The phenomenon observed in this patient is known as **Carbohydrate-Induced Hypertriglyceridemia**. When a diet is high in refined carbohydrates (especially fructose and glucose), the liver undergoes increased **de novo lipogenesis**. * Excess glucose is converted into Acetyl-CoA, which serves as the precursor for fatty acid synthesis. * These fatty acids are esterified into triglycerides and packaged into **Very Low-Density Lipoproteins (VLDL)** for secretion into the blood. * High carbohydrate intake also stimulates insulin release, which promotes fatty acid synthesis and inhibits fatty acid oxidation, further elevating plasma triglyceride levels. **2. Why Other Options are Incorrect:** * **Phospholipids (A):** These are structural components of cell membranes and do not significantly contribute to the bulk of plasma triglyceride levels. * **Triglycerides (B):** The patient switched to a *low-fat* diet. If dietary triglycerides were the cause, the levels should have decreased, as dietary fats are transported via chylomicrons. * **Amino acids (C):** The patient switched to a *low-protein* diet. While carbon skeletons of amino acids can enter lipogenesis, the reduction in protein intake makes them an unlikely cause for the rise in lipids in this specific scenario. **3. Clinical Pearls for NEET-PG:** * **VLDL vs. Chylomicrons:** Endogenous triglycerides (from carbs) are carried by **VLDL** (Apo B-100), while exogenous triglycerides (from diet) are carried by **Chylomicrons** (Apo B-48). * **Rate-limiting enzyme:** Acetyl-CoA Carboxylase (ACC) is the key enzyme in fatty acid synthesis, stimulated by insulin. * **High-Yield Fact:** Fructose is more lipogenic than glucose because it bypasses the major regulatory step of glycolysis (Phosphofructokinase-1), leading to rapid VLDL production.
Question 543: All of the following are diseases occurring due to disturbance in lipid metabolism except?
- A. Gaucher’s Disease
- B. Niemann-Pick Disease
- C. Hurler syndrome (Correct Answer)
- D. Letterer-Siwe Disease
Explanation: **Explanation:** The core of this question lies in distinguishing between **Sphingolipidoses** (Lipid Storage Disorders) and **Mucopolysaccharidoses** (GAG Storage Disorders). **Why Hurler Syndrome is the correct answer:** Hurler Syndrome (MPS Type I) is a **Mucopolysaccharidosis**. It is caused by a deficiency of the enzyme **$\alpha$-L-iduronidase**, leading to the systemic accumulation of **Glycosaminoglycans (GAGs)**, specifically Dermatan sulfate and Heparan sulfate. It is characterized by coarse facial features, corneal clouding, and hepatosplenomegaly, but it is fundamentally a disorder of carbohydrate (GAG) metabolism, not lipid metabolism. **Analysis of Incorrect Options:** * **Gaucher’s Disease:** The most common lysosomal storage disorder. It is a lipid metabolism disturbance caused by **Glucocerebrosidase** deficiency, leading to the accumulation of Glucocerebroside. * **Niemann-Pick Disease:** A lipid storage disorder caused by **Sphingomyelinase** deficiency, resulting in the accumulation of Sphingomyelin. It classically presents with a "cherry-red spot" on the macula and hepatosplenomegaly. * **Letterer-Siwe Disease:** This is the most severe, systemic form of **Langerhans Cell Histiocytosis (LCH)**. It involves the proliferation of Langerhans cells which are laden with lipids (cholesterol), historically classifying it under "Hand-Schüller-Christian complex" or lipid-related histiocytosis. **NEET-PG High-Yield Pearls:** * **Corneal Clouding:** Present in Hurler Syndrome (MPS I) but **absent** in Hunter Syndrome (MPS II). * **Inheritance:** Most lysosomal storage diseases are Autosomal Recessive, EXCEPT **Hunter Syndrome** and **Fabry Disease**, which are **X-linked Recessive**. * **Gaucher Cells:** Characterized by a "wrinkled tissue paper" appearance of the cytoplasm.
Question 544: Which of the following statements about Thromboxane A2 (TXA2) is false?
- A. It is formed in platelets.
- B. It is formed from PGG2/H2.
- C. It possesses platelet anti-aggregatory reactivity. (Correct Answer)
- D. Aspirin can inhibit its production.
Explanation: ### Explanation **Why Option C is the Correct (False) Statement:** Thromboxane A2 (TXA2) is a potent **pro-aggregatory** agent. It is released by activated platelets to promote platelet aggregation and induce vasoconstriction. Its primary physiological role is to facilitate the formation of a platelet plug during hemostasis. The statement that it possesses "anti-aggregatory" reactivity is incorrect; that property belongs to **Prostacyclin (PGI2)**, which is produced by vascular endothelial cells to inhibit aggregation and cause vasodilation. **Analysis of Incorrect Options:** * **Option A:** TXA2 is synthesized primarily in **platelets** via the enzyme Thromboxane synthase. * **Option B:** The precursor for all prostanoids is Arachidonic acid, which is converted by Cyclooxygenase (COX) into the cyclic endoperoxides **PGG2 and PGH2**. TXA2 is then derived specifically from PGH2. * **Option C:** **Aspirin** irreversibly inhibits the **COX-1 enzyme** by acetylation. Since platelets are anuclear and cannot synthesize new enzymes, aspirin effectively shuts down TXA2 production for the lifespan of the platelet (7–10 days). **NEET-PG Clinical Pearls:** * **The "Prostacyclin-Thromboxane Balance":** Normal vascular health depends on the balance between PGI2 (anti-aggregatory/vasodilator) and TXA2 (pro-aggregatory/vasoconstrictor). * **Low-dose Aspirin:** Used for cardioprotection because it inhibits platelet TXA2 more significantly than endothelial PGI2 (as endothelial cells can regenerate COX enzymes). * **Half-life:** TXA2 is highly unstable with a very short half-life (approx. 30 seconds), rapidly hydrating to the biologically inactive **TXB2**.
Question 545: What enzyme hydrolyzes storage triacylglycerols?
- A. Pancreatic lipase
- B. Lipoprotein lipase
- C. Lysosomal lipase
- D. Hormone-sensitive lipase (Correct Answer)
Explanation: **Explanation:** The mobilization of stored fat from adipose tissue is a tightly regulated process known as lipolysis. **1. Why Hormone-sensitive lipase (HSL) is correct:** HSL is the key regulatory enzyme responsible for the hydrolysis of **storage triacylglycerols** (TAGs) within adipocytes into free fatty acids and glycerol. It is called "hormone-sensitive" because its activity is regulated by covalent modification: it is activated by **Glucagon and Epinephrine** (via cAMP-mediated phosphorylation) during fasting or stress, and inhibited by **Insulin** (via dephosphorylation) in the well-fed state. **2. Why other options are incorrect:** * **Pancreatic lipase:** Responsible for the hydrolysis of **dietary TAGs** in the small intestine. It requires colipase for its function. * **Lipoprotein lipase (LPL):** Located on the capillary endothelium. It hydrolyzes TAGs found in **circulating lipoproteins** (Chylomicrons and VLDL) to provide fatty acids to extrahepatic tissues. * **Lysosomal lipase:** Involved in the degradation of lipids internalized via endocytosis; it is not the primary enzyme for mobilizing bulk adipose stores. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** While HSL is the major regulated enzyme, **Adipose Triglyceride Lipase (ATGL)** initiates the process by converting TAG to Diacylglycerol (DAG). * **Inhibitor:** Niacin (Vitamin B3) inhibits HSL, reducing the mobilization of fatty acids, which is why it is used to treat hyperlipidemia. * **Product Fate:** Glycerol released by HSL cannot be reused by adipocytes (due to lack of *glycerol kinase*); it travels to the liver for gluconeogenesis.
Question 546: All of the following are derived from glycerol 3-phosphate, EXCEPT:
- A. Phosphatidylcholine
- B. Phosphatidylethanolamine
- C. Phosphatidylinositol
- D. None of the above (Correct Answer)
Explanation: **Explanation:** The synthesis of most phospholipids, including those listed in the options, begins with the common precursor **Glycerol 3-phosphate**. This molecule is primarily derived from the reduction of dihydroxyacetone phosphate (DHAP) in glycolysis or via the phosphorylation of glycerol by glycerol kinase [1]. **1. Why "None of the above" is correct:** All three phospholipids mentioned—Phosphatidylcholine (PC), Phosphatidylethanolamine (PE), and Phosphatidylinositol (PI)—are synthesized from **Phosphatidic acid**. Phosphatidic acid itself is formed by the sequential acylation of Glycerol 3-phosphate [1]. Since all these lipids share Glycerol 3-phosphate as their initial structural backbone, none of the options are "exceptions." **2. Analysis of Options:** * **Phosphatidylcholine (Lecithin) & Phosphatidylethanolamine (Cephalin):** These are synthesized via the **Kennedy pathway**. After Glycerol 3-phosphate is converted to 1,2-diacylglycerol (DAG), the activated head groups (CDP-choline or CDP-ethanolamine) are added [2]. * **Phosphatidylinositol:** This is synthesized by the reaction of **CDP-diacylglycerol** (derived from phosphatidic acid/glycerol 3-phosphate) with inositol [2]. **Clinical Pearls for NEET-PG:** * **Lecithin/Sphingomyelin (L/S) Ratio:** A ratio >2 in amniotic fluid indicates fetal lung maturity. Lecithin is a major component of pulmonary surfactant [3]. * **Dipalmitoylphosphatidylcholine (DPPC):** The specific type of lecithin that acts as the primary lung surfactant [3]. * **Cardiolipin:** The only phospholipid that is antigenic; it is also derived from glycerol 3-phosphate (via CDP-diacylglycerol) and is found exclusively in the inner mitochondrial membrane [2]. * **Phosphatidylinositol 4,5-bisphosphate (PIP2):** Acts as a precursor for second messengers IP3 and DAG in cell signaling [3].
Question 547: Abetalipoproteinemia results in the absence of which of the following?
- A. Chylomicrons
- B. Low-density lipoprotein (LDL)
- C. Very low-density lipoprotein (VLDL)
- D. All of the above (Correct Answer)
Explanation: **Explanation:** **Abetalipoproteinemia** (Bassen-Kornzweig syndrome) is an autosomal recessive disorder caused by a mutation in the **Microsomal Triglyceride Transfer Protein (MTP)** gene. MTP is essential for loading lipids onto **Apolipoprotein B (Apo B)**. 1. **Why "All of the above" is correct:** Apo B is the structural scaffolding for several lipoproteins. There are two primary isoforms: * **Apo B-48:** Required for the assembly and secretion of **Chylomicrons** in the enterocytes (intestine). * **Apo B-100:** Required for the assembly and secretion of **VLDL** in the hepatocytes (liver). Since **LDL** is the metabolic end-product of VLDL (VLDL → IDL → LDL), the absence of VLDL inevitably leads to the absence of LDL. In Abetalipoproteinemia, the failure of MTP means neither Apo B-48 nor Apo B-100 can be lipidated; consequently, Chylomicrons, VLDL, and LDL are all virtually absent from the plasma. 2. **Analysis of Options:** * **Options A, B, and C** are individually correct but incomplete. Because the defect affects the common assembly mechanism for all Apo B-containing lipoproteins, the deficiency is global across these classes. 3. **High-Yield Clinical Pearls for NEET-PG:** * **Lipid Profile:** Extremely low cholesterol and triglycerides from birth. * **Biopsy Findings:** Intestinal biopsy shows **lipid-laden enterocytes** (steatosis) because dietary fats are absorbed but cannot be exported as chylomicrons. * **Clinical Presentation:** Malabsorption (steatorrhea), failure to thrive, and deficiencies of fat-soluble vitamins (A, D, E, K). * **Hematology:** Presence of **Acanthocytes** (spur cells) on peripheral smear due to altered RBC membrane lipids. * **Neurological:** Vitamin E deficiency leads to spinocerebellar degeneration and retinitis pigmentosa.
Question 548: The conversion of a fatty acid to an active fatty acid is catalysed by which of the following enzymes?
- A. Acyl-CoA synthetase (Correct Answer)
- B. Enoyl CoA hydratase
- C. Thiolase
- D. Acyl CoA dehydrogenase
Explanation: ### Explanation **1. Why Acyl-CoA Synthetase is Correct:** The "activation" of a fatty acid is the essential first step of fatty acid oxidation (β-oxidation). Fatty acids are chemically inert and must be converted into an active form, **Acyl-CoA**, to participate in metabolic pathways. This reaction is catalyzed by **Acyl-CoA synthetase** (also known as **Thiokinase**). * **Mechanism:** The enzyme attaches a Coenzyme A (CoA) group to the fatty acid. This process is energy-dependent, requiring the hydrolysis of **ATP to AMP and inorganic pyrophosphate (PPi)**. The subsequent hydrolysis of PPi by pyrophosphatase makes the reaction irreversible. This activation occurs in the **outer mitochondrial membrane** or the cytosol. **2. Why Other Options are Incorrect:** * **B. Enoyl CoA hydratase:** This is the second enzyme of the β-oxidation cycle. It adds water across the double bond of *trans*-Δ²-enoyl-CoA to form 3-hydroxyacyl-CoA. * **C. Thiolase:** This is the final enzyme of the β-oxidation cycle. It performs thiolytic cleavage, releasing one molecule of Acetyl-CoA and a fatty acid chain shortened by two carbons. * **D. Acyl CoA dehydrogenase:** This is the first enzyme of the β-oxidation cycle (after activation and transport). It creates a double bond between the α and β carbons, producing FADH₂. **3. Clinical Pearls & High-Yield Facts:** * **Location:** While activation occurs at the outer mitochondrial membrane, β-oxidation occurs in the **mitochondrial matrix**. * **The Carnitine Shuttle:** Long-chain Acyl-CoA cannot cross the inner mitochondrial membrane directly; it requires the **Carnitine shuttle** (CPT-I and CPT-II) for transport. * **Energy Cost:** The conversion of ATP to AMP is energetically equivalent to consuming **2 high-energy phosphate bonds** (2 ATP). * **Inhibitor:** CPT-I (the rate-limiting step of oxidation) is inhibited by **Malonyl-CoA**, preventing simultaneous synthesis and breakdown of fatty acids.
Question 549: A patient is prescribed a medication to deter alcohol consumption due to repeated DUI citations. This medication induces nausea and vomiting upon alcohol ingestion. The drug-induced illness is caused by the buildup of which one of the following metabolites?
- A. Ethanol
- B. Acetaldehyde (Correct Answer)
- C. Acetate
- D. Acetyl-CoA
Explanation: **Explanation:** The medication described is **Disulfiram**, which is used in the treatment of chronic alcoholism. It acts as a deterrent by causing an extremely unpleasant physiological reaction when alcohol is consumed. **Why Acetaldehyde is correct:** Alcohol metabolism primarily occurs in the liver via two steps: 1. **Ethanol** is converted to **Acetaldehyde** by the enzyme *Alcohol Dehydrogenase*. 2. **Acetaldehyde** is then converted to **Acetate** by the enzyme **Aldehyde Dehydrogenase (ALDH)**. Disulfiram irreversibly inhibits **Aldehyde Dehydrogenase**. When a patient on Disulfiram drinks alcohol, acetaldehyde cannot be converted to acetate, leading to a 5–10 fold increase in blood acetaldehyde levels. This "Acetaldehyde Syndrome" manifests as flushing, tachycardia, hypotension, nausea, and severe vomiting. **Why incorrect options are wrong:** * **A. Ethanol:** This is the parent compound. While its levels remain high, it is the toxic metabolite (acetaldehyde), not ethanol itself, that causes the acute illness. * **C & D. Acetate and Acetyl-CoA:** These are the subsequent products of normal alcohol metabolism. Because ALDH is inhibited, the pathway is blocked *before* these metabolites can be formed in significant quantities. **NEET-PG High-Yield Pearls:** * **Disulfiram-like reaction:** Several other drugs inhibit ALDH and cause similar reactions. Mnemonic: **"PM GCT"** (**P**rocarbazine, **M**etronidazole, **G**riseofulvin, **C**ephalosporins [Cefoperazone/Cefotetan], **T**olbutamide). * **Fomepizole:** Inhibits *Alcohol Dehydrogenase*; used as an antidote for Methanol or Ethylene Glycol poisoning. * **Rate-limiting step:** Alcohol metabolism follows **zero-order kinetics** (a constant amount of drug is eliminated per unit time).
Question 550: Which of the following are bile acids?
- A. Cholic acid
- B. Lithocholic acid
- C. Deoxycholic acid
- D. All of the above (Correct Answer)
Explanation: **Explanation:** Bile acids are steroid acids synthesized from **cholesterol** in the liver. They are essential for the emulsification and absorption of dietary lipids and fat-soluble vitamins (A, D, E, K). They are categorized into two types based on their site of synthesis: 1. **Primary Bile Acids:** Synthesized directly from cholesterol in the **liver**. These include **Cholic acid** and Chenodeoxycholic acid. 2. **Secondary Bile Acids:** Formed in the **intestine** by the action of bacterial enzymes (dehydroxylation) on primary bile acids. These include **Deoxycholic acid** (derived from cholic acid) and **Lithocholic acid** (derived from chenodeoxycholic acid). Since Cholic acid (Primary), Deoxycholic acid (Secondary), and Lithocholic acid (Secondary) are all members of the bile acid family, the correct answer is **All of the above**. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** The conversion of cholesterol to bile acids is regulated by **7-alpha-hydroxylase** (inhibited by bile acids via feedback inhibition). * **Conjugation:** Before secretion into bile, bile acids are conjugated with **Glycine or Taurine** to form bile salts (e.g., Glycocholic acid), which are more polar and better emulsifiers. * **Enterohepatic Circulation:** Approximately 95% of bile salts are reabsorbed in the **terminal ileum** and returned to the liver via the portal vein. * **Clinical Correlation:** Malabsorption of bile acids (e.g., in Crohn’s disease affecting the terminal ileum) leads to **steatorrhea** and gallstone formation.
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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