Lipid Metabolism Practice Questions

Q: All of the following are omega-3 fatty acids, EXCEPT?

gamma-Linolenic acid. **Explanation:** The classification of polyunsaturated fatty acids (PUFAs) into **Omega-3 (n-3)** and **Omega-6 (n-6)** depends on the position of the first double bond from the methyl (omega) end of the carbon chain. **Why gamma-Linolenic acid (GLA) is the correct answer:** **gamma-Linolenic acid (18:3; n-6)** is an **Omega-6 fatty acid**. It is synthesized in the body from Linoleic acid (the parent omega-6) by the enzyme $\Delta^6$-desaturase. Despite having three double bonds, its first double bond is at the 6th carbon from the omega end, placing it in the n-6 family. **Analysis of incorrect options (Omega-3 fatty acids):** * **alpha-Linolenic acid (ALA) (18:3; n-3):** This is the essential parent compound of the Omega-3 series. It is found primarily in plant oils (e.g., flaxseed). * **Eicosapentaenoic acid (EPA) (20:5; n-3):** A long-chain Omega-3 fatty acid found in fish oil. It serves as a precursor for Series-3 prostaglandins and Series-5 leukotrienes, which are anti-inflammatory. * **Docosahexaenoic acid (DHA) (22:6; n-3):** Another long-chain Omega-3 found in fish oil. It is vital for retinal function and brain development in infants. **High-Yield Clinical Pearls for NEET-PG:** * **Essential Fatty Acids (EFA):** Only Linoleic acid and alpha-Linolenic acid are truly essential because humans lack $\Delta^{12}$ and $\Delta^{15}$ desaturases. * **Mnemonic:** "A" comes before "G"—**A**lpha is Omega-**3**, **G**amma is Omega-**6**. * **Clinical Significance:** Omega-3 fatty acids are cardioprotective as they decrease serum triglycerides and inhibit platelet aggregation. * **Deficiency:** EFA deficiency leads to **Phrynoderma** (follicular hyperkeratosis) and poor wound healing.

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

Fatty acid synthesis. **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.

Q: Bile acids are synthesized from which precursor molecule?

Cholesterol. **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.

Q: Which of the following statements is NOT true about Refsum disease?

Lysosomal disorder. **Explanation:** Refsum disease is a rare metabolic disorder characterized by the accumulation of **phytanic acid** in tissues. **Why Option B is the correct (False) statement:** Refsum disease is a **Peroxisomal disorder**, not a lysosomal disorder. It is specifically caused by a deficiency in the peroxisomal enzyme **Phytanoyl-CoA hydroxylase**. This enzyme is essential for **Alpha-oxidation**, the process required to break down phytanic acid (a branched-chain fatty acid derived from chlorophyll in the diet). **Analysis of other options:** * **Option A (Autosomal recessive):** This is true. Like most inborn errors of metabolism, Refsum disease follows an autosomal recessive inheritance pattern. * **Option C (Defective fatty acid oxidation):** This is true. It specifically involves a defect in the **alpha-oxidation** of branched-chain fatty acids. Since the methyl group is present on the beta-carbon of phytanic acid, beta-oxidation is blocked, necessitating initial alpha-oxidation. * **Option D (Neurocutaneous syndrome):** This is true. Refsum disease presents with both neurological and dermatological features, including **Retinitis pigmentosa**, peripheral neuropathy, cerebellar ataxia, and **Ichthyosis** (scaly skin). **High-Yield Clinical Pearls for NEET-PG:** * **Dietary Management:** The primary treatment is the restriction of green leafy vegetables (chlorophyll), ruminant fats, and dairy products. * **Classic Tetrad:** 1. Retinitis pigmentosa (earliest sign), 2. Peripheral neuropathy, 3. Cerebellar ataxia, 4. High CSF protein (without pleocytosis). * **Zellweger Syndrome:** Contrast Refsum with Zellweger; both are peroxisomal, but Zellweger involves a total loss of peroxisome biogenesis (PEX gene mutations) and is much more severe.

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

Leukotriene C4 and D4. **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).

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

Mitochondria. **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**.

Q: Which one of the following polyunsaturated fatty acids (PUFAs) is considered to be the main source of eicosanoids in human tissues?

Arachidonate. **Explanation:** **Arachidonate (Arachidonic Acid)** is the correct answer because it serves as the primary precursor for the synthesis of **eicosanoids**, which include prostaglandins, thromboxanes, and leukotrienes. In human tissues, arachidonic acid (a 20-carbon $\omega$-6 PUFA) is typically esterified in membrane phospholipids. Upon physiological or pathological stimuli, it is released by the enzyme **Phospholipase $A_2$**. Once free, it enters either the **Cyclooxygenase (COX)** pathway to form prostanoids or the **Lipoxygenase (LOX)** pathway to form leukotrienes. **Analysis of Incorrect Options:** * **Linoleate (Linoleic Acid):** This is an essential $\omega$-6 fatty acid. While it is the metabolic precursor to arachidonate, it must first undergo desaturation and elongation. It does not serve as the *direct* substrate for eicosanoid synthesis in tissues. * **Linolenate ($\alpha$-Linolenic Acid):** This is an essential $\omega$-3 fatty acid. It is the precursor for EPA and DHA. While $\omega$-3 eicosanoids exist, they are produced in much smaller quantities compared to the arachidonate-derived $\omega$-6 series in humans. * **Palmitate:** This is a 16-carbon **saturated** fatty acid. It is the first fatty acid produced by the fatty acid synthase complex and is not a PUFA, nor is it involved in eicosanoid production. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The release of arachidonate from phospholipids by **Phospholipase $A_2$** is the rate-limiting step in eicosanoid synthesis. * **Steroids' Mechanism:** Glucocorticoids inhibit Phospholipase $A_2$ (via lipocortin/annexin A1), thereby blocking the production of all eicosanoids. * **NSAIDs:** Aspirin and other NSAIDs specifically inhibit the **COX pathway**, preventing prostaglandin synthesis. * **Essential Fatty Acids:** Humans lack $\Delta^{12}$ and $\Delta^{15}$ desaturases, making Linoleic and $\alpha$-Linolenic acids dietary essentials.

Q: Which of the following is NOT a feature of Refsum disease?

Defect in beta-oxidation. **Explanation:** Refsum disease is a rare autosomal recessive peroxisomal disorder characterized by the inability to degrade **phytanic acid**, a branched-chain fatty acid derived from dietary chlorophyll. **Why Option B is the correct answer:** The fundamental defect in Refsum disease is a deficiency of the enzyme **Phytanoyl-CoA hydroxylase**, which is required for **alpha-oxidation**. Because phytanic acid has a methyl group at the beta-carbon position, it cannot undergo standard beta-oxidation directly. It must first undergo alpha-oxidation to remove one carbon atom. Therefore, the disease is a defect in **alpha-oxidation**, not beta-oxidation. **Analysis of incorrect options:** * **Option A:** Deficiency of alpha-hydroxylase (specifically phytanoyl-CoA hydroxylase) is the primary biochemical cause of the disease. * **Option C:** Due to the enzyme deficiency, **phytanic acid accumulates** in the blood and tissues (especially the nervous system and skin), leading to toxicity. * **Option D:** **Peripheral neuropathy** is a hallmark clinical feature, along with ataxia, retinitis pigmentosa, and sensorineural deafness. **Clinical Pearls for NEET-PG:** * **Dietary Management:** The mainstay of treatment is a diet restricted in phytanic acid (avoiding ruminant fats, dairy, and green leafy vegetables). * **Classic Triad:** Retinitis pigmentosa, peripheral neuropathy, and cerebellar ataxia. * **Ichthyosis:** Patients often present with dry, scaly skin. * **Zellweger Syndrome vs. Refsum:** While both are peroxisomal disorders, Zellweger involves a total failure of peroxisome biogenesis, whereas Refsum is a single enzyme defect.

Q: Which of the following is NOT a phospholipid?

Ceramide. **Explanation:** The core concept in this question is the classification of complex lipids. **Phospholipids** must contain a phosphate group in their structure. **Why Ceramide is the correct answer:** Ceramide is a **sphingosine derivative** consisting of a sphingosine backbone attached to a fatty acid via an amide bond. It is the structural precursor for all complex sphingolipids. Crucially, ceramide **does not contain a phosphate group**; it is a simple lipid intermediate. When a phosphate and choline are added to ceramide, it becomes Sphingomyelin (which *is* a phospholipid). **Analysis of Incorrect Options:** * **Plasmalogens:** These are specialized phospholipids where the fatty acid at the C1 position is attached via an **ether linkage** instead of an ester linkage. They are abundant in cardiac tissue and myelin. * **Dipalmitoyl lecithin (DPPC):** This is a major glycerophospholipid. It contains a glycerol backbone, two palmitic acid chains, a phosphate group, and a choline base. * **Cardiolipin (Diphosphatidylglycerol):** This is a unique phospholipid found exclusively in the **inner mitochondrial membrane**. It consists of two molecules of phosphatidic acid connected by a glycerol bridge, containing two phosphate groups. **High-Yield Clinical Pearls for NEET-PG:** * **DPPC Clinical Correlation:** It is the primary component of **Lung Surfactant**. Deficiency in neonates leads to Respiratory Distress Syndrome (RDS). * **Cardiolipin Clinical Correlation:** It is the antigen used in the **VDRL test** for Syphilis. It is also targeted by antibodies in Antiphospholipid Antibody Syndrome (APS). * **Plasmalogen Fact:** Deficiency of plasmalogen synthesis is seen in **Zellweger Syndrome** (a peroxisomal disorder).

Question 1

All of the following are omega-3 fatty acids, EXCEPT?

Accepted Answer

gamma-Linolenic acid

Answer

alpha-Linolenic acid

Answer

Eicosapentaenoic acid

Answer

Docosahexaenoic acid

Question 2

NADPH is used in which of the following metabolic pathways?

Accepted Answer

Fatty acid synthesis

Answer

Ketone synthesis

Answer

Gluconeogenesis

Answer

Glycolysis

Question 3

Bile acids are synthesized from which precursor molecule?

Accepted Answer

Cholesterol

Answer

Amino acids

Answer

Bilirubin

Answer

Protein

Question 4

Which of the following statements is NOT true about Refsum disease?

Accepted Answer

Lysosomal disorder

Answer

Autosomal recessive inheritance

Answer

Defective fatty acid oxidation

Answer

Neurocutaneous syndrome

Question 5

Carnitine is essential for which of the following processes?

Accepted Answer

Transport of fatty acids from the cytosol to the mitochondria

Answer

Transport of fatty acids from mitochondria to the cytosol

Answer

Transport of pyruvate into the mitochondria

Answer

Transport of malate in the malate shuttle

Question 6

Which of the following are slow-reacting substances of anaphylaxis?

Accepted Answer

Leukotriene C4 and D4

Answer

Leukotriene B4 and C4

Answer

Leukotriene A4 and B4

Answer

Leukotriene A4 and C4

Question 7

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

Accepted Answer

Mitochondria

Answer

Peroxisome

Answer

Endolysosome

Answer

Golgi bodies

Question 8

Which one of the following polyunsaturated fatty acids (PUFAs) is considered to be the main source of eicosanoids in human tissues?

Accepted Answer

Arachidonate

Answer

Linoleate

Answer

Linolenate

Answer

Palmitate

Question 9

Which of the following is NOT a feature of Refsum disease?

Accepted Answer

Defect in beta-oxidation

Answer

Deficiency of alpha-hydroxylase

Answer

Accumulation of phytanic acid

Answer

Peripheral neuropathy

Question 10

Which of the following is NOT a phospholipid?

Accepted Answer

Ceramide

Answer

Plasmalogens

Answer

Dipalmitoyl lecithin

Answer

Cardiolipin

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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