Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 441: NADPH is used in which of the following metabolic pathways?

A. Fatty acid synthesis (Correct Answer)
B. Ketone synthesis
C. Gluconeogenesis
D. Glycolysis

Explanation: **Explanation:** **1. Why Fatty Acid Synthesis is Correct:** Fatty acid synthesis (Lipogenesis) is a reductive anabolic process that occurs in the cytosol. The enzyme complex **Fatty Acid Synthase (FAS)** requires **NADPH** as a reducing agent during two critical steps: the reduction of the ketoacyl group and the reduction of the enoyl group. NADPH provides the high-energy electrons necessary to build long-chain hydrocarbons from acetyl-CoA units. **2. Why the Other Options are Incorrect:** * **Ketone Synthesis:** This process involves the condensation of acetyl-CoA units in the mitochondria. It is primarily regulated by substrate availability and does not require NADPH. * **Gluconeogenesis:** This is the synthesis of glucose from non-carbohydrate precursors. It requires **NADH** (at the glyceraldehyde-3-phosphate dehydrogenase step) and ATP/GTP, but not NADPH. * **Glycolysis:** This is a catabolic pathway that breaks down glucose. It generates **NADH** (at the glyceraldehyde-3-phosphate dehydrogenase step) and ATP; it does not consume NADPH. **3. High-Yield Clinical Pearls for NEET-PG:** * **Sources of NADPH:** The primary source is the **Hexose Monophosphate (HMP) Shunt** (via G6PD and 6-phosphogluconate dehydrogenase). Other sources include the **Malic Enzyme** and Isocitrate Dehydrogenase. * **Key Uses of NADPH:** 1. **Reductive Biosynthesis:** Fatty acids, Cholesterol, and Steroid hormone synthesis. 2. **Antioxidant Defense:** Maintaining **Reduced Glutathione** to protect RBCs against reactive oxygen species (ROS). 3. **Phagocytosis:** Used by **NADPH Oxidase** in neutrophils for the respiratory burst to kill bacteria. 4. **Detoxification:** Used by the Cytochrome P450 system in the liver.

Question 442: In which condition does hemolysis occur on oxidation?

A. G6PD deficiency (Correct Answer)
B. Hereditary spherocytosis
C. Sickle cell anemia
D. Hemophilia

Explanation: **Explanation:** **G6PD Deficiency (Correct Answer):** Glucose-6-Phosphate Dehydrogenase (G6PD) is the rate-limiting enzyme of the **Hexose Monophosphate (HMP) Shunt**. This pathway is the sole source of **NADPH** in mature erythrocytes. NADPH is essential for maintaining a pool of **reduced glutathione**, which acts as a cellular antioxidant by neutralizing reactive oxygen species (ROS) like hydrogen peroxide. In G6PD deficiency, oxidative stress (triggered by infections, fava beans, or drugs like Primaquine and Sulfonamides) leads to the oxidation of hemoglobin. This results in the formation of **Heinz bodies** (denatured hemoglobin), which damage the RBC membrane, leading to acute hemolysis. **Why other options are incorrect:** * **Hereditary Spherocytosis:** This is a **cytoskeletal defect** (deficiency of Spectrin or Ankyrin) causing RBCs to become spherical and trapped in the spleen. It is not primarily driven by oxidative stress. * **Sickle Cell Anemia:** This is a **qualitative hemoglobinopathy** (Glu → Val at 6th position of β-chain). Hemolysis occurs due to polymerization of HbS under hypoxic conditions, not oxidative stress. * **Hemophilia:** This is a **coagulation disorder** (deficiency of Factor VIII or IX) leading to bleeding tendencies; it does not involve intrinsic RBC defects or hemolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** G6PD deficiency is **X-linked Recessive**. * **Morphology:** Look for **"Bite cells"** (Degmacytes) and **"Blister cells"** on a peripheral smear. * **Staining:** Heinz bodies are visualized using supravital stains like **Crystal Violet**. * **Protection:** G6PD deficiency provides a selective advantage against *Plasmodium falciparum* malaria.

Question 443: Bile acids are synthesized from which precursor molecule?

A. Cholesterol (Correct Answer)
B. Amino acids
C. Bilirubin
D. Protein

Explanation: **Explanation:** **1. Why Cholesterol is Correct:** Bile acids are the primary end-products of cholesterol metabolism. This synthesis occurs exclusively in the **liver**. The process begins with the conversion of cholesterol into 7-alpha-hydroxycholesterol, catalyzed by the enzyme **7-alpha-hydroxylase** (the rate-limiting step). These primary bile acids (Cholic acid and Chenodeoxycholic acid) are essential for the emulsification and absorption of dietary lipids and fat-soluble vitamins (A, D, E, K). **2. Why Other Options are Incorrect:** * **Amino acids:** While amino acids like **Glycine** and **Taurine** are conjugated with bile acids to form bile salts, they are not the precursor molecules themselves. * **Bilirubin:** Bilirubin is the breakdown product of **Heme** (from hemoglobin), not cholesterol. While both bile acids and bilirubin are components of bile, their metabolic pathways are distinct. * **Protein:** Proteins are polymers of amino acids and do not serve as structural precursors for steroid-based molecules like bile acids. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** 7-alpha-hydroxylase (inhibited by bile acids via feedback inhibition). * **Primary vs. Secondary:** Primary bile acids (Cholic/Chenodeoxycholic acid) are made in the liver. Secondary bile acids (Deoxycholic/Lithocholic acid) are formed by **bacterial action** in the gut. * **Enterohepatic Circulation:** Approximately 95% of bile acids are reabsorbed in the **terminal ileum** and returned to the liver. * **Clinical Link:** Bile acid sequestrants (e.g., Cholestyramine) lower LDL cholesterol by preventing bile acid reabsorption, forcing the liver to use more cholesterol to synthesize new bile acids.

Question 444: Which of the following lipoproteins will be elevated in the bloodstream approximately 4 hours after a fat-rich meal?

A. Chylomicrons
B. Ketone bodies
C. VLDL (Correct Answer)
D. LDL

Explanation: **Explanation:** The correct answer is **VLDL (Very Low-Density Lipoprotein)**. **Why VLDL is the correct answer:** While Chylomicrons are the primary transporters of dietary lipids immediately after a meal, their half-life is very short (approx. 5–15 minutes). By **4 hours post-prandial**, Chylomicrons have largely been cleared by lipoprotein lipase. At this stage, the liver processes the incoming fatty acids and dietary carbohydrates, re-packaging them into **VLDL** for endogenous transport. Therefore, VLDL levels remain elevated or peak several hours after a meal as the liver redistributes these lipids to peripheral tissues. **Analysis of Incorrect Options:** * **A. Chylomicrons:** These transport exogenous (dietary) triglycerides. Due to their rapid clearance, they are typically absent from the blood after an overnight fast and peak much earlier than 4 hours. * **B. Ketone bodies:** These are produced by the liver during states of carbohydrate deprivation (fasting, starvation, or ketoacidosis). In a "fat-rich meal" (fed state), insulin levels are high, which inhibits ketogenesis. * **D. LDL:** LDL is the end-product of VLDL metabolism (VLDL → IDL → LDL). While it carries cholesterol, its levels do not show a significant acute spike 4 hours after a single meal compared to the primary triglyceride-rich transporters. **High-Yield Clinical Pearls for NEET-PG:** * **Apo-B48** is the structural protein for Chylomicrons (synthesized in the intestine). * **Apo-B100** is the structural protein for VLDL, IDL, and LDL (synthesized in the liver). * **Lipoprotein Lipase (LPL)** is the enzyme responsible for clearing triglycerides from both Chylomicrons and VLDL; it is activated by **Apo-CII**. * **Type IV Hyperlipoproteinemia** is characterized by isolated elevation of VLDL.

Question 445: Carnitine is essential for which of the following processes?

A. Transport of fatty acids from mitochondria to the cytosol
B. Transport of fatty acids from the cytosol to the mitochondria (Correct Answer)
C. Transport of pyruvate into the mitochondria
D. Transport of malate in the malate shuttle

Explanation: **Explanation:** **Why the correct answer is right:** Carnitine is the central component of the **Carnitine Shuttle**, which is the rate-limiting step for **Beta-oxidation of long-chain fatty acids (LCFA)**. While short and medium-chain fatty acids can cross the mitochondrial membrane freely, LCFAs cannot. They must first be activated to Fatty Acyl-CoA in the cytosol. The enzyme **Carnitine Palmitoyltransferase-I (CPT-I)** then converts Acyl-CoA to Acyl-carnitine, allowing it to cross the inner mitochondrial membrane via a translocase. Once inside the matrix, CPT-II reconverts it back to Acyl-CoA for oxidation. **Why the incorrect options are wrong:** * **Option A:** Transport from mitochondria to cytosol is primarily handled by the **Citrate Shuttle** (for fatty acid synthesis). Carnitine works in the opposite direction. * **Option C:** Pyruvate enters the mitochondria via a specific **Pyruvate Carrier** (MPC), not carnitine. This is a precursor step for the TCA cycle or gluconeogenesis. * **Option D:** The **Malate-Aspartate Shuttle** is used to transport reducing equivalents (NADH) from the cytosol into the mitochondria; it involves malate dehydrogenase and aminotransferase, not carnitine. **High-Yield Clinical Pearls for NEET-PG:** * **Inhibitor:** CPT-I is inhibited by **Malonyl-CoA** (an intermediate of fatty acid synthesis), preventing a futile cycle where fatty acids are synthesized and degraded simultaneously. * **Systemic Carnitine Deficiency:** Presents with non-ketotic hypoglycemia, cardiomyopathy, and muscle weakness. * **Location:** CPT-I is located on the outer mitochondrial membrane, while CPT-II is on the inner membrane.

Question 446: Lithogenic bile contains an increased amount of which substance?

A. Bile acid
B. Bile salt
C. Cholesterol (Correct Answer)
D. Bilirubin

Explanation: **Explanation:** The formation of **lithogenic bile** (stone-forming bile) is primarily driven by an imbalance in the solubility of cholesterol. Cholesterol is insoluble in water and is kept in a liquid state within bile by being incorporated into micelles, which are formed by the detergent action of **bile salts** and **phospholipids** (specifically lecithin). **Why Cholesterol is Correct:** Bile becomes lithogenic when it is supersaturated with cholesterol. This occurs due to either an absolute increase in cholesterol secretion or a relative decrease in bile salts/phospholipids. When the concentration of cholesterol exceeds the solubilizing capacity of the bile salts, it precipitates as microcrystals, leading to the formation of **cholesterol gallstones** (the most common type of gallstone). **Analysis of Incorrect Options:** * **A & B (Bile Acid/Bile Salt):** These act as solubilizing agents. An *increase* in bile salts actually prevents stone formation by increasing the capacity to hold cholesterol in the micellar phase. A *decrease* in bile salts (e.g., due to ileal resection) leads to lithogenic bile. * **D (Bilirubin):** While bilirubin is a component of "pigment stones" (seen in chronic hemolysis), the term "lithogenic bile" in a general metabolic context typically refers to the supersaturation of cholesterol, which is the primary metabolic defect in the majority of cholelithiasis cases. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme for Bile Acid synthesis:** Cholesterol 7-alpha-hydroxylase. * **The "4 F’s" for Gallstones:** Female, Fat, Fertile, Forty. * **Lecithin:** The major phospholipid in bile that aids cholesterol solubility. * **Clofibrate/Fibrates:** These drugs increase the risk of gallstones because they inhibit cholesterol 7-alpha-hydroxylase, reducing bile acid synthesis and increasing biliary cholesterol excretion.

Question 447: Which of the following are slow-reacting substances of anaphylaxis?

A. Leukotriene B4 and C4
B. Leukotriene A4 and B4
C. Leukotriene A4 and C4
D. Leukotriene C4 and D4 (Correct Answer)

Explanation: **Explanation** The term **Slow-Reacting Substance of Anaphylaxis (SRS-A)** refers to a mixture of specific cysteinyl leukotrienes that cause prolonged bronchoconstriction and increased vascular permeability during allergic reactions. **Why Option D is Correct:** SRS-A is composed of **Leukotriene C4 (LTC4), D4 (LTD4), and E4 (LTE4)**. These are synthesized from Arachidonic acid via the 5-Lipoxygenase pathway. They are significantly more potent than histamine in inducing smooth muscle contraction in the airways, and their effects are slower in onset but much longer-lasting, hence the name "slow-reacting." **Analysis of Incorrect Options:** * **Leukotriene A4 (LTA4):** This is the unstable intermediate precursor for all other leukotrienes. It does not possess the biological activity associated with SRS-A. * **Leukotriene B4 (LTB4):** While a potent inflammatory mediator, LTB4 is primarily involved in **chemotaxis**. It recruits and activates neutrophils and does not cause the smooth muscle contraction characteristic of SRS-A. **High-Yield Clinical Pearls for NEET-PG:** * **Synthesis:** Leukotrienes are produced from Arachidonic acid by the enzyme **5-Lipoxygenase (5-LOX)**. * **Zileuton:** A drug that inhibits 5-LOX, preventing the production of all leukotrienes. * **Montelukast/Zafirlukast:** These are **CysLT1 receptor antagonists** that specifically block the action of LTC4, LTD4, and LTE4, making them effective in aspirin-induced asthma and maintenance therapy for bronchial asthma. * **LTB4 Mnemonic:** Remember **B** for **B**e-line (Chemotaxis) and **C, D, E** for **C**onstriction (SRS-A).

Question 448: Ketosis is caused by?

A. Excessive utilization of glucose
B. Overproduction of acetyl-CoA (Correct Answer)
C. Excessive secretion of insulin
D. Excessive intake of carbohydrates

Explanation: **Explanation:** Ketosis occurs when the rate of ketone body formation (ketogenesis) in the liver exceeds the rate of peripheral utilization. The fundamental biochemical trigger is an **overproduction of acetyl-CoA**, primarily derived from the rapid β-oxidation of free fatty acids. **1. Why "Overproduction of acetyl-CoA" is correct:** In states of low glucose availability (starvation) or inability to use glucose (Diabetes Mellitus), the body shifts to fat mobilization. This floods the liver with fatty acids, which are converted into acetyl-CoA. Simultaneously, **oxaloacetate (OAA)** is diverted toward gluconeogenesis to maintain blood glucose. Since OAA is depleted, acetyl-CoA cannot enter the Citric Acid Cycle (TCA cycle). This excess acetyl-CoA is then shunted into the ketogenic pathway to form acetoacetate, β-hydroxybutyrate, and acetone. **2. Why other options are incorrect:** * **A & D:** Excessive utilization or intake of carbohydrates/glucose leads to high insulin levels, which inhibits lipolysis and promotes fatty acid synthesis, thereby **preventing** ketosis. * **C:** Insulin is a potent anti-ketogenic hormone. It inhibits the enzyme **Hormone Sensitive Lipase (HSL)**, reducing the supply of fatty acids to the liver. Ketosis is typically caused by a *deficiency* of insulin. **High-Yield Facts for NEET-PG:** * **Rate-limiting enzyme of ketogenesis:** HMG-CoA Synthase (Mitochondrial). * **Site of Ketogenesis:** Liver mitochondria (Note: The liver *produces* but cannot *utilize* ketones because it lacks the enzyme **Thiophorase/β-ketoacyl-CoA transferase**). * **Ketone bodies:** Acetoacetate and β-hydroxybutyrate are acidic; their accumulation leads to **Anion Gap Metabolic Acidosis**. * **Detection:** Rothera’s test detects acetoacetate and acetone, but *not* β-hydroxybutyrate.

Question 449: In which cellular organelle does the catabolism of long-chain fatty acids occur?

A. Mitochondria (Correct Answer)
B. Peroxisome
C. Endolysosome
D. Golgi bodies

Explanation: **Explanation:** The catabolism of **long-chain fatty acids (LCFA)**, ranging from 12 to 20 carbons, occurs primarily via **$\beta$-oxidation** within the **Mitochondrial Matrix**. This process involves the sequential removal of two-carbon units (as Acetyl-CoA) to generate energy. For LCFAs to enter the mitochondria, they must be activated to fatty acyl-CoA and transported across the inner mitochondrial membrane via the **Carnitine Shuttle** (the rate-limiting step). **Analysis of Options:** * **A. Mitochondria (Correct):** The primary site for $\beta$-oxidation of short, medium, and long-chain fatty acids. * **B. Peroxisomes:** These organelles are responsible for the initial oxidation of **Very Long Chain Fatty Acids (VLCFA)** (≥22 carbons) and branched-chain fatty acids (via $\alpha$-oxidation). They shorten these chains before transferring them to mitochondria for final oxidation. * **C. Endolysosome:** Involved in the degradation of macromolecules (proteins, complex lipids like sphingolipids) but not the metabolic oxidation of fatty acids for energy. * **D. Golgi bodies:** Primarily involved in the post-translational modification, sorting, and packaging of proteins and lipids, not their catabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Carnitine Deficiency:** Presents with non-ketotic hypoglycemia during fasting, as LCFAs cannot enter the mitochondria for energy production. * **Zellweger Syndrome:** A peroxisomal biogenesis disorder leading to the accumulation of VLCFAs (Option B context). * **MCAD Deficiency:** The most common inborn error of $\beta$-oxidation, affecting the breakdown of medium-chain fatty acids. * **Energy Yield:** The complete oxidation of one molecule of **Palmitate (16C)** yields a net of **106 ATP**.

Question 450: Tangier's disease is characterized by:

A. Low or absence of HDL (Correct Answer)
B. Low LDL concentration
C. Raised chylomicrons
D. Deficiency of LPL

Explanation: ### Explanation **Tangier’s Disease** (Familial Alpha-lipoprotein Deficiency) is an autosomal recessive disorder caused by a mutation in the **ABCA1 gene** (ATP-Binding Cassette transporter A1). **1. Why Option A is Correct:** The ABCA1 transporter is essential for the "cholesterol efflux" process, where cellular free cholesterol is transferred to lipid-poor **Apolipoprotein A-I (Apo A-I)** to form nascent HDL. In Tangier’s disease, this transport is defective. Consequently, Apo A-I cannot be "loaded" with lipids and is rapidly cleared by the kidneys. This leads to a **near-total absence of HDL** in the plasma. **2. Why Other Options are Incorrect:** * **Option B (Low LDL):** While LDL levels are often moderately reduced in Tangier’s disease due to increased catabolism, the *hallmark* and diagnostic feature is the absence of HDL. * **Option C (Raised Chylomicrons):** This is characteristic of Type I Hyperlipoproteinemia (Familial Chylomicronemia Syndrome), not Tangier’s. * **Option D (Deficiency of LPL):** Lipoprotein Lipase (LPL) deficiency leads to Type I Hyperlipoproteinemia, resulting in severe hypertriglyceridemia and eruptive xanthomas. **3. Clinical Pearls for NEET-PG:** * **Pathognomonic Sign:** Large, **orange-colored tonsils** (due to accumulation of cholesteryl esters in reticuloendothelial cells). * **Clinical Features:** Hepatosplenomegaly, lymphadenopathy, and peripheral neuropathy. * **Biochemical Profile:** Extremely low HDL (<5 mg/dL), low total cholesterol, and mild-to-moderate hypertriglyceridemia. * **Contrast:** Do not confuse with **LCAT deficiency** (Fish-eye disease), which also has low HDL but presents with corneal opacities and renal failure.

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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