Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 161: How many ATP molecules are formed by one turn of palmitic acid oxidation?

A. 2
B. 8
C. 146
D. 129 (Correct Answer)

Explanation: To understand the ATP yield of palmitic acid (a 16-carbon saturated fatty acid), we must look at the energetics of **Beta-oxidation** and the subsequent **TCA cycle**. ### **The Breakdown (The "Why")** 1. **Beta-Oxidation Cycles:** A 16-carbon chain undergoes **7 cycles** of beta-oxidation. Each cycle produces: * 1 FADH₂ (1.5 ATP) * 1 NADH (2.5 ATP) * *Total per cycle = 4 ATP.* For 7 cycles: 7 × 4 = **28 ATP**. 2. **Acetyl-CoA Production:** 7 cycles plus the final cleavage produce **8 Acetyl-CoA** molecules. 3. **TCA Cycle:** Each Acetyl-CoA entering the TCA cycle yields 10 ATP (3 NADH, 1 FADH₂, 1 GTP). * 8 Acetyl-CoA × 10 = **80 ATP**. 4. **Gross Total:** 28 + 80 = **108 ATP**. 5. **Activation Cost:** Fatty acid activation to Acyl-CoA (by Thiokinase) consumes the equivalent of **2 ATP** (ATP → AMP + PPi). 6. **Net Yield:** 108 – 2 = **106 ATP**. ***Note on the Question:*** While the calculated net yield is **106 ATP** (using modern P:O ratios), older textbooks often cited **129 ATP** (using 1 NADH = 3 ATP and 1 FADH₂ = 2 ATP). In NEET-PG, if 106 is not an option, **129** is the traditional "correct" answer. ### **Why other options are incorrect:** * **A (2):** This represents the ATP *consumed* during activation, not the yield. * **B (8):** This is the number of Acetyl-CoA molecules produced, not ATP. * **C (146):** This is an incorrect calculation, sometimes confused with the yield of longer-chain fatty acids or stearic acid. ### **High-Yield Clinical Pearls** * **Rate-limiting step:** Carnitine Palmitoyltransferase-I (CPT-I), inhibited by Malonyl-CoA. * **Location:** Beta-oxidation occurs in the **mitochondrial matrix**. * **Clinical Correlation:** **MCAD Deficiency** (Medium-chain acyl-CoA dehydrogenase deficiency) presents with fasting hypoglycemia and non-ketotic dicarboxylic aciduria.

Question 162: Which is the first intermediate in the conversion of cholesterol to bile acids?

A. Ergosterol
B. Lanosterol
C. 7-hydroxycholesterol (Correct Answer)
D. 7-dehydrocholesterol

Explanation: **Explanation:** The synthesis of bile acids from cholesterol occurs primarily in the liver. The **rate-limiting and first committed step** in this pathway is the hydroxylation of cholesterol at the C7 position. **1. Why 7-hydroxycholesterol is correct:** The enzyme **7-alpha-hydroxylase** (a cytochrome P450 enzyme) catalyzes the conversion of cholesterol to **7-alpha-hydroxycholesterol**. This is the regulatory step of bile acid synthesis. This intermediate eventually undergoes further modifications (reduction and side-chain cleavage) to form primary bile acids: Cholic acid and Chenodeoxycholic acid. **2. Analysis of Incorrect Options:** * **Ergosterol (A):** This is a sterol found in fungal cell membranes; it is a precursor to Vitamin D2 and is not part of human cholesterol metabolism. * **Lanosterol (B):** This is the **first steroid intermediate** formed during the *synthesis* of cholesterol (from squalene), not its breakdown into bile acids. * **7-dehydrocholesterol (D):** This is the immediate precursor of cholesterol. In the skin, it is converted to **Vitamin D3 (Cholecalciferol)** upon exposure to UV light. **3. High-Yield Clinical Pearls for NEET-PG:** * **Regulation:** 7-alpha-hydroxylase is inhibited by bile acids (feedback inhibition) and stimulated by cholesterol. * **Vitamin C Connection:** Vitamin C is a necessary cofactor for 7-alpha-hydroxylase. Deficiency (Scurvy) can lead to cholesterol accumulation and gallstone formation due to impaired bile acid synthesis. * **Bile Acid Sequestrants:** Drugs like Cholestyramine bind bile acids in the gut, preventing their enterohepatic circulation. This relieves feedback inhibition, increasing the conversion of cholesterol to bile acids and thereby lowering LDL levels.

Question 163: What is mainly composed of triacylglycerols synthesized in intestinal epithelial cells?

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

Explanation: **Explanation:** The correct answer is **Chylomicrons**. **1. Why Chylomicrons are correct:** Chylomicrons are the largest and least dense lipoproteins. They are synthesized exclusively in the **intestinal mucosal cells** (enterocytes) following the absorption of dietary fats. Their primary function is the transport of **exogenous (dietary) triacylglycerols (TAGs)** from the intestine to peripheral tissues like adipose tissue and muscle. They consist of approximately 85-90% triacylglycerols, making TAGs their predominant lipid component. **2. Why other options are incorrect:** * **VLDL (Very Low-Density Lipoprotein):** While also rich in TAGs (approx. 60%), VLDL is synthesized in the **liver**, not the intestine. It transports **endogenous** triacylglycerols. * **LDL (Low-Density Lipoprotein):** Formed from VLDL metabolism, LDL is the primary carrier of **cholesterol** to peripheral tissues. It contains very little TAG. * **HDL (High-Density Lipoprotein):** Synthesized in both the liver and intestine, HDL is the smallest lipoprotein. It is primarily composed of **protein** and phospholipids and is involved in **reverse cholesterol transport**. **3. NEET-PG High-Yield Pearls:** * **Apolipoprotein Marker:** **Apo B-48** is the unique structural protein for chylomicrons (synthesized via RNA editing), whereas **Apo B-100** is the marker for VLDL and LDL. * **Rate-Limiting Enzyme:** Lipoprotein Lipase (LPL), activated by **Apo C-II**, clears TAGs from chylomicrons in the capillaries. * **Clinical Correlation:** **Abetalipoproteinemia** is a condition where a deficiency in Microsomal Triglyceride Transfer Protein (MTP) prevents the assembly of chylomicrons, leading to fat malabsorption and steatorrhea. * **Appearance:** Chylomicrons cause the "milky" appearance of plasma (chylous) after a fatty meal.

Question 164: Which molecule serves as a marker for lipid peroxidation?

A. Catalase
B. Glutathione reductase
C. Maltase
D. Malondialdehyde (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Malondialdehyde (MDA)** **Concept:** Lipid peroxidation is a process where free radicals (Reactive Oxygen Species) attack polyunsaturated fatty acids (PUFAs) in cell membranes, leading to cell damage. This oxidative degradation results in the formation of reactive aldehydes. **Malondialdehyde (MDA)** is one of the most well-known secondary end-products of this process. Because MDA levels correlate directly with the degree of lipid damage, it is used clinically and experimentally as a reliable **biomarker for oxidative stress**. **Analysis of Incorrect Options:** * **A. Catalase:** This is an antioxidant enzyme that decomposes hydrogen peroxide ($H_2O_2$) into water and oxygen. It is a protective mechanism *against* oxidative stress, not a marker of the damage itself. * **B. Glutathione Reductase:** This enzyme regenerates reduced glutathione (GSH) from its oxidized form (GSSG) using NADPH. It is part of the cellular antioxidant defense system. * **C. Maltase:** This is a digestive enzyme (disaccharidase) found in the brush border of the small intestine that breaks down maltose into glucose. It has no role in lipid metabolism or oxidative stress. **High-Yield Clinical Pearls for NEET-PG:** * **TBARS Assay:** The Thiobarbituric Acid Reactive Substances (TBARS) test is the standard laboratory method used to measure MDA levels. * **Other Markers:** Apart from MDA, **4-hydroxynonenal (4-HNE)** and **Isoprostanes** (measured in urine) are also potent markers of lipid peroxidation. * **Vitamin E (Tocopherol):** It is the most important lipid-soluble antioxidant that prevents lipid peroxidation by acting as a chain-breaker in cell membranes. * **Pathology Link:** Lipid peroxidation is a key mechanism in atherosclerosis, reperfusion injury, and ionizing radiation damage.

Question 165: Which of the following statements is true about cholesterol?

A. It is made up of 25 carbons.
B. It has a hydroxyl group at the 5th carbon.
C. It contains the cyclic tetra-hydrophenanthrene ring.
D. It is amphipathic in nature. (Correct Answer)

Explanation: **Explanation:** **Why Option D is Correct:** Cholesterol is an **amphipathic** molecule, meaning it possesses both hydrophilic (water-loving) and hydrophobic (water-fearing) properties. The single **hydroxyl (-OH) group** at position C3 provides the polar, hydrophilic head, while the bulky steroid nucleus and the hydrocarbon side chain constitute the non-polar, hydrophobic tail. This property is crucial for its role in cell membranes, where it inserts itself between phospholipids to regulate membrane fluidity. **Analysis of Incorrect Options:** * **Option A:** Cholesterol is a **27-carbon** compound (not 25). It is synthesized from acetyl-CoA via the mevalonate pathway. * **Option B:** The hydroxyl group is located at the **3rd carbon** (C3), not the 5th. There is, however, a double bond between the 5th and 6th carbons. * **Option C:** Cholesterol contains the **cyclopentanoperhydrophenanthrene (CPPP)** ring system (also known as the steroid nucleus). "Tetra-hydrophenanthrene" is an incomplete description of this complex four-ring structure. **High-Yield Facts for NEET-PG:** * **Rate-limiting enzyme:** HMG-CoA Reductase (inhibited by Statins). * **Precursor Role:** It is the parent compound for bile acids, steroid hormones (cortisol, estrogen, testosterone), and Vitamin D3. * **Excretion:** Humans cannot metabolize the cholesterol ring to $CO_2$ and $H_2O$; it is excreted primarily via bile as cholesterol or bile salts. * **Identification:** The **Libermann-Burchard reaction** is a classic chemical test for cholesterol (turns emerald green).

Question 166: What is required for beta-oxidation in the cytosol?

A. Acetyl CoA (Correct Answer)
B. Pyruvate
C. Citrate
D. Alpha-ketoglutarate

Explanation: **Explanation:** The core concept tested here is the **activation of fatty acids**, which is the prerequisite step for beta-oxidation. While the actual breakdown of fatty acids occurs within the mitochondrial matrix, the process begins in the **cytosol**. 1. **Why Acetyl CoA is correct:** Before a fatty acid can undergo beta-oxidation, it must be "activated" into **Fatty Acyl-CoA**. This reaction is catalyzed by the enzyme *Acyl-CoA synthetase* (Thiokinase) located on the outer mitochondrial membrane/cytosol. This process requires **ATP**, **Coenzyme A (CoA)**, and magnesium. Since Acetyl-CoA is essentially a carrier of the acetyl group and a source of Coenzyme A in various metabolic pools, it represents the necessity of the CoA moiety for the formation of the activated thioester bond. 2. **Why the other options are incorrect:** * **Pyruvate:** This is the end-product of glycolysis. While it can be converted to Acetyl-CoA inside the mitochondria via the PDH complex, it is not a direct requirement for the activation of fatty acids in the cytosol. * **Citrate:** Citrate acts as a carrier to move Acetyl-CoA *out* of the mitochondria for fatty acid **synthesis** (not oxidation). It also inhibits Phosphofructokinase-1 (PFK-1). * **Alpha-ketoglutarate:** This is an intermediate of the TCA cycle and is involved in nitrogen metabolism (transamination), having no direct role in the cytosolic activation of fatty acids. **High-Yield NEET-PG Pearls:** * **Carnitine Shuttle:** Once activated to Fatty Acyl-CoA in the cytosol, long-chain fatty acids require the Carnitine shuttle (CPT-I and CPT-II) to cross the inner mitochondrial membrane. * **Rate-Limiting Step:** CPT-I is the rate-limiting enzyme of beta-oxidation and is inhibited by **Malonyl-CoA** (an intermediate of fatty acid synthesis). * **Energy Yield:** The activation step "costs" the equivalent of **2 ATP** (ATP → AMP + PPi).

Question 167: All of the following are bile acids, EXCEPT:

A. Lithocholic acid
B. Taurocholic acid (Correct Answer)
C. Deoxycholic acid
D. Chenodeoxycholic acid

Explanation: **Explanation:** The distinction between **bile acids** and **bile salts** is a high-yield concept in lipid metabolism. Bile acids are synthesized in the liver from cholesterol, while bile salts are the conjugated forms of these acids. **Why Taurocholic acid is the correct answer:** Taurocholic acid is a **bile salt**, not a bile acid. In the liver, primary bile acids are conjugated with amino acids—either **Glycine** or **Taurine**—to form bile salts (e.g., Glycocholic acid or Taurocholic acid). Conjugation lowers the pKa of the molecules, making them more ionized and effective detergents at intestinal pH, which is essential for lipid emulsification. **Analysis of incorrect options:** * **Chenodeoxycholic acid (Option D):** This is a **primary bile acid**, synthesized directly from cholesterol in the liver (along with Cholic acid). * **Deoxycholic acid (Option C) & Lithocholic acid (Option A):** These are **secondary bile acids**. They are formed in the intestine by the action of bacterial enzymes (7-α-dehydroxylase) on primary bile acids. Cholic acid becomes Deoxycholic acid, and Chenodeoxycholic acid becomes Lithocholic acid. **NEET-PG High-Yield Pearls:** * **Rate-limiting enzyme:** Cholesterol 7-α-hydroxylase (inhibited by bile acids via feedback). * **Enterohepatic circulation:** 95% of bile salts are reabsorbed in the **terminal ileum** and returned to the liver. * **Steatorrhea:** Occurs if bile acid synthesis is impaired or if the terminal ileum is resected (Crohn’s disease), leading to fat malabsorption. * **Primary vs. Secondary:** Primary are made in the *Liver*; Secondary are made in the *Gut* (by bacteria).

Question 168: All of the following are Isoprene derivatives (Polyprenoids), except?

A. Ubiquitin (Correct Answer)
B. Dolichol
C. beta-carotene
D. alpha-tocopherol

Explanation: **Explanation:** The question tests your ability to distinguish between **Isoprenoids (Polyprenoids)**—lipids synthesized from 5-carbon isoprene units—and unrelated proteins with similar names. **1. Why Ubiquitin is the Correct Answer:** **Ubiquitin** is a small **regulatory protein** (76 amino acids) found in all eukaryotic cells. Its primary function is to tag misfolded or unneeded proteins for degradation via the Proteasome pathway (Ubiquitin-Proteasome System). Despite the phonetic similarity to *Ubiquinone*, it is **not** a lipid or an isoprene derivative. **2. Analysis of Incorrect Options (Isoprene Derivatives):** Isoprenoids are formed from the condensation of isopentenyl pyrophosphate (IPP) units. * **Dolichol:** A long-chain isoprenoid alcohol (containing up to 20 isoprene units) required for the synthesis of N-linked glycoproteins in the Endoplasmic Reticulum. * **Beta-carotene:** A tetraterpene (8 isoprene units) that serves as a precursor to Vitamin A. * **Alpha-tocopherol (Vitamin E):** A fat-soluble vitamin containing a chromanol ring and an **isoprenoid side chain**. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Ubiquinone (Coenzyme Q10):** Do not confuse this with Ubiquitin. Ubiquinone is a component of the Electron Transport Chain and **is** an isoprene derivative. * **Cholesterol Synthesis:** The precursor for all human isoprenoids is **Mevalonate**, which is formed by the rate-limiting enzyme **HMG-CoA Reductase**. * **Other Isoprenoids:** Include Vitamins A, D, E, K, and bile acids. * **Statin Mechanism:** Statins inhibit HMG-CoA Reductase, which not only lowers cholesterol but also reduces levels of Dolichol and Ubiquinone (explaining some side effects like myopathy).

Question 169: A patient with hyperlipoproteinemia would most likely benefit from a low-carbohydrate diet if the elevated lipoproteins in the blood belong to which class?

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

Explanation: **Explanation:** The correct answer is **VLDL (Very Low-Density Lipoprotein)**. **Why VLDL is the correct answer:** VLDL is synthesized in the liver, primarily from endogenous triglycerides. A high intake of dietary carbohydrates leads to an excess of glucose, which is converted into fatty acids via **de novo lipogenesis**. These fatty acids are then esterified into triglycerides and packaged into VLDL particles for transport. Therefore, a **low-carbohydrate diet** directly reduces the substrate available for hepatic triglyceride synthesis, effectively lowering VLDL levels in the blood. This is particularly relevant in Type IV Hyperlipoproteinemia. **Why the other options are incorrect:** * **Chylomicrons:** These transport **exogenous (dietary) lipids** from the intestines. To lower chylomicrons, a patient would benefit from a **low-fat diet**, not necessarily a low-carbohydrate diet. * **LDL:** LDL is the "bad cholesterol" derived from the catabolism of VLDL. While lowering VLDL can eventually lower LDL, the primary dietary intervention for isolated high LDL is reducing **saturated fats and cholesterol** intake. * **HDL:** HDL is "good cholesterol" involved in reverse cholesterol transport. Low-carbohydrate diets do not typically aim to lower HDL; in fact, increasing HDL is clinically desirable. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme for Lipogenesis:** Acetyl-CoA Carboxylase (stimulated by Insulin, which rises with carb intake). * **Apolipoprotein marker for VLDL:** Apo B-100. * **Friedewald Formula:** LDL = Total Cholesterol – (HDL + VLDL). Note: VLDL is estimated as Triglycerides/5 (if TG <400 mg/dL). * **Type IV Hyperlipoproteinemia:** Characterized by isolated elevation of VLDL; it is highly sensitive to dietary carbohydrates and alcohol.

Question 170: What is the building block for fatty acid biosynthesis?

A. NADH
B. Acetyl-CoA (Correct Answer)
C. Acyl-CoA
D. Acetate

Explanation: **Explanation:** **1. Why Acetyl-CoA is correct:** Fatty acid synthesis (Lipogenesis) occurs primarily in the cytosol. The fundamental building block is **Acetyl-CoA**. However, since Acetyl-CoA is produced in the mitochondria and cannot cross the inner mitochondrial membrane, it is transported to the cytosol via the **Citrate-Malate Shuttle**. Once in the cytosol, Acetyl-CoA is converted to Malonyl-CoA by the rate-limiting enzyme *Acetyl-CoA Carboxylase (ACC)*. The Fatty Acid Synthase (FAS) multienzyme complex then sequentially adds two-carbon units derived from Malonyl-CoA to the growing chain, starting with an initial Acetyl-CoA primer. **2. Why the other options are incorrect:** * **NADH:** This is a coenzyme used in catabolic pathways (like glycolysis). Fatty acid synthesis is a reductive anabolic process that specifically requires **NADPH** (provided by the HMP Shunt) as a reducing agent, not NADH. * **Acyl-CoA:** This is a generic term for a fatty acid chain attached to Coenzyme A. Acyl-CoA molecules are the end products or intermediates of fatty acid metabolism, not the starting building block. * **Acetate:** While Acetyl-CoA is the "active" form of acetate, free acetate cannot be directly utilized by the Fatty Acid Synthase complex. It must first be activated to Acetyl-CoA by *Thiokinase*. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Acetyl-CoA Carboxylase (ACC), which requires **Biotin** as a cofactor. * **Key Activator:** Citrate (signals high energy). * **Key Inhibitor:** Palmitoyl-CoA (feedback inhibition) and Glucagon. * **End product:** The primary end product of the FAS complex is **Palmitate** (a 16-carbon saturated fatty acid). * **Location:** Primarily occurs in the Liver, Lactating Mammary Glands, and Adipose tissue.

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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