Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 531: Fatty liver caused due to excess alcohol consumption is primarily due to an altered ratio of which of the following?

A. NAD/NADH
B. NADP/NADPH
C. NADH/NAD (Correct Answer)
D. NADPH/NADP

Explanation: **Explanation:** The primary metabolic consequence of ethanol metabolism is an **increased NADH/NAD+ ratio** in the liver. Alcohol is metabolized in the cytosol by *Alcohol Dehydrogenase* and in the mitochondria by *Acetaldehyde Dehydrogenase*. Both reactions reduce NAD+ to NADH, leading to an overabundance of NADH. **Why NADH/NAD+ is the correct answer:** The high NADH/NAD+ ratio signals a "pseudo-fed" state, shifting the liver's metabolic equilibrium: 1. **Inhibition of Fatty Acid Oxidation:** High NADH inhibits $\beta$-oxidation (which requires NAD+), leading to the accumulation of fatty acids. 2. **Increased Lipogenesis:** Excess NADH promotes the conversion of Dihydroxyacetone phosphate (DHAP) to Glycerol-3-phosphate, providing the backbone for Triglyceride synthesis. 3. **Shift in Redox State:** Pyruvate is diverted to Lactate to regenerate NAD+, leading to lactic acidosis and inhibiting gluconeogenesis. **Why other options are incorrect:** * **NAD/NADH (Option A):** This is the inverse. In chronic alcoholism, NAD+ is depleted, not increased. * **NADP/NADPH & NADPH/NADP (Options B & D):** While NADPH is involved in fatty acid synthesis, the acute metabolic derangement caused by alcohol is driven by the NAD-dependent dehydrogenase enzymes, not the pentose phosphate pathway or NADPH-dependent reactions. **High-Yield Clinical Pearls for NEET-PG:** * **Key Enzyme:** Alcohol Dehydrogenase is the rate-limiting step (follows zero-order kinetics). * **Histology:** Alcohol-induced fatty liver (Steatosis) typically shows **macrovesicular steatosis**. * **Associated Findings:** The high NADH/NAD+ ratio also causes **hyperuricemia** (due to lactate competing with urate for excretion) and **fasting hypoglycemia**. * **Biochemical Marker:** Serum **Gamma-Glutamyl Transferase (GGT)** is a sensitive marker for chronic alcohol ingestion.

Question 532: Platelet-activating factor is biochemically a?

A. Prostaglandin
B. Glycolipid
C. Glycerol ether phospholipid (Correct Answer)
D. Leukotrienes

Explanation: ### Explanation **Platelet-activating factor (PAF)** is a potent lipid mediator involved in inflammation, platelet aggregation, and anaphylaxis. **Why Option C is correct:** Biochemically, PAF is a **glycerol ether phospholipid** (specifically, 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine). Its structure is unique due to two key features: 1. **Ether Linkage:** It contains an alkyl group attached to the carbon-1 (C1) of glycerol via an **ether bond**, rather than the typical ester bond found in most phospholipids. 2. **Acetyl Group:** It has an **acetyl residue** at the C2 position instead of a long-chain fatty acid. This makes it more water-soluble than other membrane lipids, allowing it to act as a signaling molecule. **Why other options are incorrect:** * **Options A & D (Prostaglandins & Leukotrienes):** These are **Eicosanoids**, derived from 20-carbon polyunsaturated fatty acids (like arachidonic acid). While they are also inflammatory mediators, they do not contain a glycerol backbone or phosphate group. * **Option B (Glycolipid):** Glycolipids (like cerebrosides or gangliosides) contain a carbohydrate component and a sphingosine or glycerol base, but they lack the phosphate group and the specific ether-linked alkyl chain characteristic of PAF. **High-Yield Clinical Pearls for NEET-PG:** * **Source:** PAF is synthesized by platelets, neutrophils, monocytes, and endothelial cells. * **Potency:** It is one of the most potent known biological molecules, active at concentrations as low as $10^{-11}$ mol/L. * **Functions:** It triggers platelet aggregation, induces systemic vasodilation (hypotension), and causes bronchoconstriction. * **Enzyme:** It is inactivated by the enzyme **PAF acetylhydrolase**, which removes the acetyl group at the C2 position.

Question 533: After a meal rich in dietary lipids, lipids are absorbed by the small intestine and transported in the lymph primarily as which of the following?

A. Very-low-density lipoproteins (VLDLs)
B. Free fatty acids bound to albumin
C. Chylomicrons (Correct Answer)
D. Low-density lipoproteins (LDLs)

Explanation: **Explanation:** **1. Why Chylomicrons are the Correct Answer:** Dietary lipids (exogenous lipids), primarily triacylglycerols, are emulsified by bile salts and digested by pancreatic lipase in the small intestine. Once absorbed by the enterocytes, they are re-esterified and packaged into **Chylomicrons**. These are the largest and least dense lipoproteins, characterized by the presence of **Apolipoprotein B-48**. Because they are too large to enter the blood capillaries directly, they are secreted into the **lacteals (lymphatic vessels)** of the intestinal villi and enter the systemic circulation via the thoracic duct. **2. Why the Other Options are Incorrect:** * **VLDLs (Option A):** These transport **endogenous** lipids (synthesized in the liver), not dietary lipids. They contain Apo B-100. * **Free Fatty Acids (Option B):** While FFAs are transported bound to albumin, this occurs primarily during **fasting** (lipolysis from adipose tissue), not during the post-prandial absorption phase. * **LDLs (Option D):** Known as "bad cholesterol," LDLs are metabolic products of VLDLs (via IDL) and function to transport cholesterol to peripheral tissues. **3. High-Yield NEET-PG Clinical Pearls:** * **Apo B-48 vs. Apo B-100:** Both are products of the same gene. Apo B-48 is produced in the intestine via **RNA editing** (C to U conversion creating a premature stop codon). * **Milky Plasma:** After a fatty meal, plasma appears milky (lipemia) due to the high concentration of chylomicrons. * **LPL Activation:** Chylomicrons acquire **Apo C-II** and **Apo E** from HDL in the blood. Apo C-II is essential for activating **Lipoprotein Lipase (LPL)** to release fatty acids to tissues. * **Abetalipoproteinemia:** A deficiency in Microsomal Triglyceride Transfer Protein (MTP) leads to an inability to form Chylomicrons and VLDLs, resulting in fat malabsorption and steatorrhea.

Question 534: Which enzyme catalyzes the rate-limiting step in the synthesis of bile acids?

A. 12a-Hydroxylase
B. 7a-Hydroxylase (Correct Answer)
C. HMG CoA reductase inhibitors and bile acid sequestrants
D. 25-hydroxycholecalciferol 1-hydroxylase

Explanation: **Explanation:** The synthesis of bile acids occurs in the liver from cholesterol. The conversion of cholesterol into **7α-hydroxycholesterol** is the first and **rate-limiting step** in this pathway. This reaction is catalyzed by the enzyme **7α-Hydroxylase** (a cytochrome P450 enzyme). * **Regulation:** This enzyme is inhibited by bile acids (feedback inhibition) and stimulated by cholesterol. Vitamin C and NADPH are essential cofactors for this reaction. **Analysis of Incorrect Options:** * **12α-Hydroxylase:** This enzyme is involved later in the pathway to determine the ratio of cholic acid to chenodeoxycholic acid, but it is not the rate-limiting step. * **HMG CoA Reductase:** This is the rate-limiting enzyme for **cholesterol synthesis**, not bile acid synthesis. (Note: Option C mentions inhibitors/sequestrants, which are pharmacological classes, not enzymes). * **25-hydroxycholecalciferol 1-hydroxylase:** This enzyme is located in the kidneys and is responsible for the activation of Vitamin D (converting 25-(OH)D₃ to 1,25-(OH)₂D₃). **High-Yield Clinical Pearls for NEET-PG:** * **Bile Acid Sequestrants (e.g., Cholestyramine):** These drugs bind bile acids in the gut, preventing their reabsorption. This relieves the feedback inhibition on 7α-Hydroxylase, leading to increased diversion of cholesterol into bile acid synthesis, thereby lowering serum LDL levels. * **Vitamin C Deficiency:** Scurvy can lead to cholesterol accumulation in the liver because 7α-Hydroxylase requires Vitamin C as a cofactor. * **Primary vs. Secondary Bile Acids:** Primary bile acids (Cholic/Chenodeoxycholic acid) are made in the liver; secondary bile acids (Deoxycholic/Lithocholic acid) are formed by bacterial action in the colon.

Question 535: Which of the following is not a step of beta-oxidation?

A. NADP dependent oxidation (Correct Answer)
B. FAD dependent oxidation
C. Thiolysis
D. Hydration

Explanation: **Explanation:** Beta-oxidation is the primary catabolic pathway for fatty acids, occurring within the mitochondrial matrix. It involves a repeating sequence of four reactions that shorten the fatty acid chain by two carbons (as Acetyl-CoA) in each cycle. **Why Option A is correct:** The oxidation steps in beta-oxidation utilize **NAD⁺** and **FAD** as electron acceptors, not NADP⁺. * **NAD⁺** is used in the third step (3-hydroxyacyl-CoA dehydrogenase). * **NADPH** is typically utilized in **reductive biosynthesis** (e.g., Fatty Acid Synthesis in the cytoplasm), not in catabolic oxidation. Therefore, NADP-dependent oxidation is not a part of beta-oxidation. **Analysis of Incorrect Options:** * **B. FAD dependent oxidation:** This is the **first step** of the cycle, catalyzed by *Acyl-CoA dehydrogenase*, which creates a double bond between C2 and C3 (trans-Δ²-enoyl-CoA). * **D. Hydration:** This is the **second step**, where *Enoyl-CoA hydratase* adds water across the double bond to form 3-L-hydroxyacyl-CoA. * **C. Thiolysis:** This is the **fourth and final step**, catalyzed by *Thiolase* (β-ketothiolase). It uses a molecule of CoA-SH to cleave the bond, releasing Acetyl-CoA and a fatty acyl-CoA shortened by two carbons. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** Remember the sequence as **OHOT** (Oxidation by FAD → Hydration → Oxidation by NAD → Thiolysis). * **Location:** Beta-oxidation occurs in the **Mitochondria**, whereas Fatty Acid Synthesis occurs in the **Cytosol**. * **Rate-limiting step:** The transport of fatty acids into the mitochondria via the **Carnitine Shuttle** (inhibited by Malonyl-CoA). * **Energy Yield:** Oxidation of one Palmitate (16C) molecule yields a net of **106 ATP**.

Question 536: Which of the following types of hypertriglyceridemia is associated with an increase in chylomicron and VLDL remnants?

A. Type I
B. Type IIa
C. Type III (Correct Answer)
D. Type IV

Explanation: **Explanation:** **Type III Hyperlipoproteinemia** (also known as **Dysbetalipoproteinemia** or Broad Beta Disease) is characterized by a deficiency in **Apolipoprotein E (Apo E)**. Apo E is the essential ligand required for the liver to recognize and clear **chylomicron remnants** and **VLDL remnants (IDL)** via the LDL-receptor-related protein (LRP). When Apo E is defective (specifically the E2/E2 phenotype), these remnants accumulate in the plasma, leading to elevated cholesterol and triglycerides. **Analysis of Incorrect Options:** * **Type I (Familial Chylomicronemia):** Caused by a deficiency in Lipoprotein Lipase (LPL) or Apo C-II. It results in a massive increase in **chylomicrons** only, not remnants. * **Type IIa (Familial Hypercholesterolemia):** Caused by a defect in LDL receptors. It leads to isolated elevation of **LDL** (cholesterol), with normal triglyceride levels. * **Type IV (Familial Hypertriglyceridemia):** Characterized by the overproduction or decreased clearance of **VLDL**. Remnant levels are typically not the primary finding. **High-Yield Clinical Pearls for NEET-PG:** * **Pathognomonic Sign:** **Palmar xanthomas** (xanthoma striatum palmare) are highly specific for Type III. * **Electrophoresis:** Shows a characteristic **"Broad Beta Band"** due to the presence of IDL. * **Genetics:** Associated with the **Apo E2 homozygote** genotype. * **Risk:** Significant increase in the risk of premature coronary artery disease and peripheral vascular disease.

Question 537: Phospholipase A2 acts on which of the following?

A. Phosphoglyceric acid
B. Phosphate
C. Calcium ion
D. Phosphatidyl-inositol (Correct Answer)

Explanation: **Explanation:** **Phospholipase A2 (PLA2)** is a specific esterase enzyme that hydrolyzes the ester bond at the **second carbon (C2)** of a phospholipid, releasing a free fatty acid (usually arachidonic acid) and a lysophospholipid. 1. **Why Phosphatidyl-inositol is correct:** Phosphatidyl-inositol is a classic **glycerophospholipid** found in cell membranes. PLA2 acts specifically on the glycerol backbone of such phospholipids. When PLA2 acts on phosphatidyl-inositol, it releases **Arachidonic acid**, which serves as the precursor for the synthesis of eicosanoids (prostaglandins, leukotrienes, and thromboxanes). This is a critical step in the inflammatory cascade. 2. **Why other options are incorrect:** * **Phosphoglyceric acid:** This is an intermediate in glycolysis (e.g., 3-phosphoglycerate), not a phospholipid substrate for phospholipases. * **Phosphate:** This is an inorganic ion or a functional group, not a complex lipid molecule. * **Calcium ion:** While PLA2 is a **calcium-dependent enzyme** (it requires $Ca^{2+}$ for its catalytic activity), calcium is a cofactor, not the substrate itself. **Clinical Pearls for NEET-PG:** * **Snake Venom:** The venom of cobras and vipers contains high concentrations of PLA2, which causes hemolysis by producing lysolecithin (a potent detergent that dissolves RBC membranes). * **Steroid Mechanism:** Glucocorticoids induce the synthesis of **Lipocortin (Annexin A1)**, which inhibits PLA2. This is the primary mechanism by which steroids exert their anti-inflammatory effect (by preventing the release of arachidonic acid). * **Pancreatitis:** Serum PLA2 levels are often elevated in acute pancreatitis and are associated with the development of pulmonary complications (ARDS).

Question 538: What are the products of complete hydrolysis of cardiolipin?

A. 3 glycerol, 4 fatty acids, 2 phosphates (Correct Answer)
B. 3 glycerol, 4 fatty acids, 1 phosphate
C. 3 glycerol, 3 fatty acids, 2 phosphates
D. 5 glycerol, 4 fatty acids, 2 phosphates

Explanation: **Explanation:** **Cardiolipin** (Diphosphatidylglycerol) is a unique phospholipid found exclusively in the inner mitochondrial membrane. To understand its hydrolysis products, one must look at its chemical structure: it consists of **two molecules of phosphatidic acid** joined together by a **central glycerol bridge**. 1. **Why Option A is correct:** * **Glycerol:** Each phosphatidic acid unit contains one glycerol backbone (2 total), plus the central linking glycerol. Total = **3 Glycerols**. * **Fatty Acids:** Each phosphatidic acid unit carries two fatty acid chains. Total = **4 Fatty Acids**. * **Phosphate:** Each phosphatidic acid unit contains one phosphate group. Total = **2 Phosphates**. Therefore, complete hydrolysis yields 3 glycerol, 4 fatty acids, and 2 phosphates. 2. **Why other options are incorrect:** * **Option B:** Incorrect because it suggests only 1 phosphate; cardiolipin is a *di*phosphatidyl compound. * **Option C:** Incorrect because it suggests 3 fatty acids; a "double" phospholipid structure must have 4 acyl chains. * **Option D:** Incorrect because 5 glycerols would imply a much larger, non-existent polymer. **High-Yield Clinical Pearls for NEET-PG:** * **Mitochondrial Marker:** Cardiolipin is essential for the optimal function of enzymes in the **Electron Transport Chain** (especially Complex IV). * **Barth Syndrome:** An X-linked genetic disorder caused by a defect in cardiolipin metabolism (tafazzin mutation), leading to cardiomyopathy and muscle weakness. * **Syphilis Testing:** Cardiolipin is the antigen used in the **VDRL/RPR tests** to detect non-specific antibodies (reagin) in syphilis. * **Antiphospholipid Antibody Syndrome (APS):** Anti-cardiolipin antibodies are a hallmark finding, associated with arterial/venous thrombosis and recurrent miscarriages.

Question 539: Which of the following is the rate-limiting enzyme of bile acid synthesis?

A. Mitochondrial 17 α-hydroxylase
B. Cytoplasmic 7 α-hydroxylase
C. Microsomal 7 α-hydroxylase (Correct Answer)
D. Mitochondrial 7 α-hydroxylase

Explanation: **Explanation:** The synthesis of bile acids from cholesterol is the primary pathway for cholesterol excretion in the body. This process occurs in the liver and is regulated by a critical rate-limiting step. **Why Option C is correct:** The enzyme **7 α-hydroxylase** catalyzes the first and most important regulatory step in bile acid synthesis, converting cholesterol into 7 α-hydroxycholesterol. This enzyme is a **microsomal** enzyme (located in the endoplasmic reticulum) and belongs to the cytochrome P450 superfamily (CYP7A1). It requires NADPH, molecular oxygen, and Vitamin C as cofactors. **Analysis of Incorrect Options:** * **Option A:** 17 α-hydroxylase is involved in the synthesis of steroid hormones (adrenal and gonadal steroids), not bile acids. * **Option B:** While the enzyme name is correct, its localization is wrong. 7 α-hydroxylase is membrane-bound in the microsomes, not free in the cytoplasm. * **Option C vs D:** The distinction between microsomal and mitochondrial is a common "trap" in NEET-PG. While some later steps in the "alternative pathway" of bile acid synthesis occur in mitochondria (via 27-hydroxylase), the **primary rate-limiting step** (CYP7A1) is strictly **microsomal**. **High-Yield Clinical Pearls for NEET-PG:** * **Feedback Inhibition:** Bile acids (the end product) inhibit 7 α-hydroxylase via the Farnesoid X Receptor (FXR). * **Vitamin C Deficiency:** Scurvy can lead to cholesterol accumulation and gallstone formation because Vitamin C is a necessary cofactor for 7 α-hydroxylase. * **Thyroid Connection:** Thyroid hormones (T3) upregulate this enzyme; thus, hypothyroidism often presents with hypercholesterolemia due to decreased bile acid conversion. * **Bile Acid Sequestrants:** Drugs like Cholestyramine increase the activity of this enzyme by removing the feedback inhibition of bile acids.

Question 540: Apolipoproteins E and C are synthesized by which organ?

A. Liver (Correct Answer)
B. Kidney
C. Intestine
D. RBCs

Explanation: ### Explanation **1. Why the Correct Answer is Right:** The **liver** is the primary metabolic hub for lipid synthesis and transport. It is the major site for the synthesis of most apolipoproteins, including **Apo C (C-I, C-II, C-III)** and **Apo E**. * **Apo C-II** is a vital cofactor that activates Lipoprotein Lipase (LPL) for triglyceride hydrolysis. * **Apo E** serves as the essential ligand for the hepatic uptake of chylomicron remnants and IDL via the LDL receptor and LRP (LDL Receptor-related Protein). While the intestine synthesizes specific apolipoproteins (like Apo B-48 and Apo A-I), the bulk of the circulating "exchangeable" apolipoproteins (C and E) are produced and secreted by hepatocytes into the plasma, where they are primarily associated with VLDL and HDL. **2. Why the Incorrect Options are Wrong:** * **Kidney:** The kidneys are involved in the filtration and reabsorption of small molecules but do not possess the biosynthetic machinery for lipoprotein assembly or apolipoprotein synthesis. * **Intestine:** The enterocytes are responsible for synthesizing **Apo B-48** (unique to chylomicrons) and some **Apo A-I**. While they contribute small amounts of Apo A-IV, they are not the source of Apo C or Apo E. * **RBCs:** Red blood cells lack a nucleus and organelles (ribosomes/ER); therefore, they cannot synthesize proteins. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Apo B-100 vs. B-48:** Remember that Apo B-100 is made in the **Liver**, while Apo B-48 is made in the **Intestine** (due to RNA editing by the enzyme *cytidine deaminase*). * **Apo C-II Deficiency:** Leads to Type I Hyperlipoproteinemia (marked elevation of Chylomicrons). * **Apo E Deficiency:** Leads to Type III Hyperlipoproteinemia (Dysbetalipoproteinemia), characterized by the accumulation of IDL and Chylomicron remnants. * **Alzheimer’s Link:** The **Apo E4** isoform is a significant genetic risk factor for late-onset Alzheimer’s disease.

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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