Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 231: Lingual lipase is secreted by which of the following glands?

A. Parotid glands
B. Ebner's glands (Correct Answer)
C. Sublingual glands
D. None of the above

Explanation: **Explanation:** The correct answer is **Ebner's glands** (Option B). **1. Why Ebner's glands are correct:** Lingual lipase is an enzyme responsible for the initial digestion of dietary fats (triacylglycerols). It is secreted by the **Ebner’s glands** (also known as von Ebner's glands), which are minor serous salivary glands located on the dorsum of the tongue, specifically surrounding the circumvallate and foliate papillae. **2. Why other options are incorrect:** * **Parotid glands:** These are the largest salivary glands and primarily secrete serous fluid rich in **salivary amylase** (ptyalin) for carbohydrate digestion, not lingual lipase. * **Sublingual glands:** These are mixed glands (predominantly mucous) located under the tongue. While they contribute to saliva production, they are not the source of lingual lipase. **3. High-Yield Clinical Pearls for NEET-PG:** * **Acid Stability:** Lingual lipase is highly acid-stable. Unlike pancreatic lipase, it functions optimally at a low pH (3.0–6.0), allowing it to remain active in the acidic environment of the stomach. * **Role in Neonates:** Lingual lipase (along with gastric lipase) is critically important in neonates because their pancreatic function is not yet fully developed. It helps in the digestion of milk fats. * **Short/Medium-Chain Fatty Acids:** It specifically targets the primary ester linkages of triglycerides, releasing short and medium-chain fatty acids, which are then absorbed directly into the portal circulation. * **Acid Lipases:** Together, lingual and gastric lipases are referred to as "Acid Lipases." They do not require bile salts for their activation.

Question 232: PUFA consumption is associated with which of the following changes in serum lipid profile?

A. Lowering of serum cholesterol and a rise in LDL cholesterol
B. Lowering of serum cholesterol and lowering of LDL cholesterol (Correct Answer)
C. A rise in serum cholesterol and a rise in LDL cholesterol
D. A rise in serum cholesterol and lowering of LDL cholesterol

Explanation: **Explanation:** Polyunsaturated Fatty Acids (PUFAs), such as Omega-3 and Omega-6, are known for their cardioprotective effects. The primary mechanism by which PUFAs lower serum cholesterol levels is by **increasing the expression of LDL receptors** on the surface of hepatocytes. This leads to an enhanced clearance of Low-Density Lipoprotein (LDL) from the circulation, thereby reducing both total serum cholesterol and LDL cholesterol levels. Additionally, PUFAs decrease the synthesis of VLDL (the precursor to LDL) in the liver. **Analysis of Options:** * **Option B (Correct):** PUFAs effectively lower total cholesterol and LDL cholesterol. They also tend to reduce triglyceride levels, particularly Omega-3 fatty acids. * **Option A & D:** These are incorrect because PUFAs do not cause a rise in LDL cholesterol. In fact, replacing Saturated Fatty Acids (SFAs) with PUFAs is a standard dietary recommendation to lower LDL. * **Option C:** This is incorrect as it describes the effect typically associated with high intake of Saturated Fatty Acids or Trans-fats, which downregulate LDL receptors and increase endogenous cholesterol synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **P:S Ratio:** A healthy diet should maintain a Polyunsaturated to Saturated fat ratio of approximately **0.8 to 1.0**. * **Essential Fatty Acids:** Linoleic acid (Omega-6) and Linolenic acid (Omega-3) are PUFAs that cannot be synthesized by the body. * **HDL Effect:** While PUFAs lower LDL, excessive intake of certain Omega-6 PUFAs may also slightly lower "good" HDL cholesterol; however, the net cardiovascular benefit remains positive. * **Hypolipidemic Mechanism:** PUFAs act as ligands for **PPAR-alpha**, promoting fatty acid oxidation and reducing VLDL secretion.

Question 233: Lipogenesis occurs in which cellular compartment?

A. Cytosol (Correct Answer)
B. Endoplasmic reticulum
C. Golgi body
D. Mitochondria

Explanation: **Explanation:** **1. Why Cytosol is Correct:** Lipogenesis (De novo synthesis of fatty acids) primarily occurs in the **cytosol**. This is because the key enzymes required for the process, specifically **Acetyl-CoA Carboxylase (ACC)**—the rate-limiting enzyme—and the **Fatty Acid Synthase (FAS) complex**, are located within the cytosolic compartment. While Acetyl-CoA is generated in the mitochondria, it must be transported to the cytosol via the **Citrate-Malate Shuttle** to initiate lipogenesis. **2. Why Other Options are Incorrect:** * **Mitochondria:** This is the primary site for **Beta-oxidation** (fatty acid breakdown) and the TCA cycle. While some fatty acid elongation occurs here, it is not the site of de novo synthesis. * **Endoplasmic Reticulum (ER):** The smooth ER is responsible for **fatty acid elongation** (beyond 16 carbons) and **desaturation** (adding double bonds), as well as the synthesis of complex lipids like phospholipids and cholesterol. * **Golgi Body:** This organelle is involved in the modification, sorting, and packaging of proteins and lipids (e.g., VLDL assembly) rather than the primary synthesis of fatty acids. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Rate-limiting enzyme:** Acetyl-CoA Carboxylase (requires **Biotin** as a cofactor). * **Reducing equivalent:** **NADPH** is essential for lipogenesis, primarily supplied by the Hexose Monophosphate (HMP) Shunt. * **End product:** The primary product of the FAS complex is **Palmitate** (a 16-carbon saturated fatty acid). * **Hormonal Regulation:** Lipogenesis is stimulated by **Insulin** (well-fed state) and inhibited by Glucagon and Epinephrine. * **Mnemonic:** "S"ynthesis occurs in the "S"ol (Cytosol); "B"reakdown occurs in the "B"ody (Mitochondrial body).

Question 234: Which of the following is NOT a product of the oxygenase pathway?

A. PGE2
B. PG D2
C. PGF2
D. LT6 (Correct Answer)

Explanation: **Explanation:** The question focuses on the metabolism of **Arachidonic Acid**, a 20-carbon polyunsaturated fatty acid. Arachidonic acid is metabolized via two primary pathways: the **Cyclooxygenase (COX) pathway** and the **Lipoxygenase (LOX) pathway**. **1. Why LT6 is the correct answer:** Leukotrienes (LTs) are products of the **Lipoxygenase (LOX)** pathway, not the oxygenase (COX) pathway. Furthermore, the number in the subscript of a leukotriene (e.g., LTA4, LTB4) denotes the number of double bonds in the molecule. Since arachidonic acid has only 4 double bonds, the standard leukotrienes produced from it belong to the **"4-series"** (LT4). There is no naturally occurring **LT6** derived from the standard human oxygenase pathways, making it the correct "incorrect" option. **2. Why the other options are incorrect:** * **PGE2, PGD2, and PGF2:** These are all **Prostaglandins**. Prostaglandins, along with Thromboxanes and Prostacyclins, are collectively known as prostanoids. They are synthesized via the **Cyclooxygenase (COX) pathway** (also known as the Prostaglandin Endoperoxide Synthase pathway). Since the question asks for products of the oxygenase (COX) pathway, these are all valid products and thus incorrect choices for the "NOT" criteria. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The release of arachidonic acid from membrane phospholipids by **Phospholipase A2**. * **Inhibition:** Corticosteroids inhibit Phospholipase A2, while NSAIDs (like Aspirin) specifically inhibit the COX pathway. * **LOX Pathway:** Leads to Leukotrienes (involved in asthma and anaphylaxis). Zileuton inhibits LOX, while Montelukast blocks leukotriene receptors. * **Prostacyclin (PGI2):** Produced by vascular endothelium; causes vasodilation and inhibits platelet aggregation. * **Thromboxane (TXA2):** Produced by platelets; causes vasoconstriction and promotes platelet aggregation.

Question 235: All of the following inhibit hormone-dependent lipase, EXCEPT:

A. Adrenaline (Correct Answer)
B. Insulin
C. Prostaglandin E1
D. Nicotinic acid

Explanation: **Explanation:** The enzyme **Hormone-Sensitive Lipase (HSL)** is the rate-limiting enzyme for lipolysis in adipose tissue. It catalyzes the breakdown of stored triacylglycerols into free fatty acids and glycerol. Its activity is regulated via **cAMP-dependent phosphorylation**. **1. Why Adrenaline is the Correct Answer:** Adrenaline (and Noradrenaline) acts via **$\beta_3$-adrenergic receptors** to activate Adenylate Cyclase. This increases intracellular cAMP levels, which activates Protein Kinase A (PKA). PKA then phosphorylates and **activates** HSL. Therefore, Adrenaline is a potent **stimulator**, not an inhibitor, of HSL. **2. Why the other options are incorrect (Inhibitors of HSL):** * **Insulin:** The most significant inhibitor of HSL. It activates a phosphatase that dephosphorylates (inactivates) HSL and promotes the degradation of cAMP via phosphodiesterase. * **Prostaglandin E1 (PGE1):** Inhibits Adenylate Cyclase, leading to decreased cAMP levels and reduced HSL activity. * **Nicotinic Acid (Niacin):** Potently inhibits lipolysis in adipose tissue by inhibiting Adenylate Cyclase, which is why it is used clinically to lower plasma free fatty acids. **Clinical Pearls for NEET-PG:** * **HSL vs. LPL:** Do not confuse Hormone-Sensitive Lipase (adipose tissue; mobilizes fat) with **Lipoprotein Lipase** (capillary endothelium; clears chylomicrons/VLDL). * **Glucagon & ACTH:** Like Adrenaline, these hormones also stimulate HSL. * **Caffeine/Theophylline:** These inhibit phosphodiesterase, maintaining high cAMP levels and thus **stimulating** HSL activity (promoting lipolysis).

Question 236: In which of the following enzyme deficiencies is steatorrhea associated with increased triglycerides?

A. Pancreatic lipase
B. Serum lipase
C. Lipoprotein lipase (Correct Answer)
D. Acetyl CoA carboxylase

Explanation: **Explanation:** **1. Why Lipoprotein Lipase (LPL) is the Correct Answer:** Lipoprotein Lipase is the key enzyme responsible for the hydrolysis of triglycerides (TAGs) within chylomicrons and VLDL into free fatty acids and glycerol. A deficiency in LPL (or its cofactor Apo C-II) leads to **Type I Hyperlipoproteinemia (Familial Chylomicronemia Syndrome)**. * **Increased Triglycerides:** Since chylomicrons cannot be cleared, plasma TAG levels rise significantly (often >1000 mg/dL). * **Steatorrhea:** While steatorrhea is primarily associated with malabsorption, in the context of severe LPL deficiency, the massive accumulation of chylomicrons can interfere with normal lipid processing, and some patients may present with fat malabsorption symptoms or "creamy" stools. *Note: In NEET-PG, LPL deficiency is the classic association for massive hypertriglyceridemia and eruptive xanthomas.* **2. Analysis of Incorrect Options:** * **A. Pancreatic Lipase:** Deficiency leads to severe steatorrhea because dietary fats cannot be digested in the gut. However, this results in **decreased** absorption of lipids, leading to low or normal serum triglycerides, not increased. * **B. Serum Lipase:** This is a diagnostic marker for pancreatitis rather than a metabolic enzyme involved in systemic lipid clearance. Its deficiency is not a recognized clinical entity causing steatorrhea. * **C. Acetyl CoA Carboxylase:** This is the rate-limiting enzyme for **fatty acid synthesis**. Deficiency would impair the body's ability to create fat, not cause steatorrhea or elevated serum TAGs. **3. Clinical Pearls for NEET-PG:** * **Type I Hyperlipoproteinemia Triad:** Eruptive xanthomas, Hepatosplenomegaly, and Recurrent Pancreatitis. * **Appearance:** On standing, the plasma shows a "creamy layer" on top with a clear infranatant. * **Cofactor:** Remember that **Apo C-II** is required to activate LPL; its deficiency mimics LPL deficiency. * **Diagnosis:** Measurement of LPL activity after an intravenous injection of **heparin** (which releases LPL into the blood).

Question 237: Conversion of stearic acid to oleic acid is catalyzed by which enzyme?

A. D3 desaturase
B. D6 desaturase
C. D9 desaturase (Correct Answer)
D. D12 desaturase

Explanation: **Explanation:** The conversion of **stearic acid** (a 18-carbon saturated fatty acid, 18:0) to **oleic acid** (a 18-carbon monounsaturated fatty acid, 18:1; Δ9) is a process of desaturation. **1. Why Option C is Correct:** In humans, the synthesis of monounsaturated fatty acids is catalyzed by the **Fatty Acid Desaturase system** (specifically **Δ9-desaturase**), located in the endoplasmic reticulum. This enzyme introduces a double bond between **carbons 9 and 10** of the fatty acid chain. Stearic acid is the primary substrate for this enzyme, resulting in the formation of oleic acid. This reaction requires molecular oxygen, NADH, and cytochrome b5. **2. Why Other Options are Incorrect:** * **Δ3 and Δ6 desaturases (Options A & B):** These enzymes are involved in the further desaturation of polyunsaturated fatty acids (PUFAs) like linoleic and α-linolenic acid to form longer-chain PUFAs (e.g., arachidonic acid). They do not act directly on saturated stearic acid to form oleic acid. * **Δ12 desaturase (Option D):** This enzyme introduces a double bond at the 12th position. **Humans lack Δ12 and Δ15 desaturases**, which is why linoleic (18:2; Δ9,12) and linolenic (18:3; Δ9,12,15) acids cannot be synthesized endogenously and are classified as **Essential Fatty Acids (EFA)**. **Clinical Pearls & High-Yield Facts:** * **Essential Fatty Acids:** Humans cannot introduce double bonds beyond the Δ9 position. Therefore, any fatty acid with a double bond at Δ12 or Δ15 must be obtained from the diet. * **Non-essentiality of Oleic Acid:** Since we possess Δ9-desaturase, oleic acid (Omega-9) is considered a non-essential fatty acid. * **Enzyme Components:** The desaturase system is a multienzyme complex consisting of **Desaturase, Cytochrome b5, and Cytochrome b5 reductase.**

Question 238: Delayed clotting in abetalipoproteinemia is due to:

A. A decreased VLDL
B. Reduced clotting factor synthesis (Correct Answer)
C. Decreased chylomicrons
D. Decreased fatty acid synthesis

Explanation: **Explanation:** **Abetalipoproteinemia** is an autosomal recessive disorder caused by a mutation in the **Microsomal Triglyceride Transfer Protein (MTP)**. This defect prevents the assembly and secretion of ApoB-containing lipoproteins (Chylomicrons, VLDL, and LDL). **Why the correct answer is right:** The hallmark of abetalipoproteinemia is severe **fat malabsorption**. Because dietary fats cannot be packaged into chylomicrons, fat-soluble vitamins (A, D, E, and K) are not absorbed from the intestine. **Vitamin K** is a vital cofactor for the γ-carboxylation of clotting factors **II, VII, IX, and X**. A deficiency in Vitamin K leads to the production of non-functional clotting factors, resulting in a bleeding diathesis and delayed clotting. **Why incorrect options are wrong:** * **A & C (Decreased VLDL/Chylomicrons):** While these are characteristic biochemical findings in abetalipoproteinemia, they are the *cause* of malabsorption, not the direct mechanism for delayed clotting. Clotting is a protein-mediated process, not a lipid-mediated one. * **D (Decreased fatty acid synthesis):** De novo fatty acid synthesis occurs primarily in the liver and is not the primary defect in this condition. **Clinical Pearls for NEET-PG:** * **Blood Smear:** Look for **Acanthocytes** (spur cells) due to altered membrane lipid composition. * **Neurological symptoms:** Ataxia and retinitis pigmentosa occur due to severe **Vitamin E** deficiency. * **Biopsy:** Intestinal biopsy shows **lipid-laden enterocytes** (vacuoles) because triglycerides are trapped and cannot be exported. * **Labs:** Extremely low levels of Cholesterol and Triglycerides; absent ApoB-48 and ApoB-100.

Question 239: What is produced with the help of NADP in extramitochondrial sites?

A. Glycogen
B. Steroids (Correct Answer)
C. Ketone bodies
D. None of the above

Explanation: **Explanation:** The correct answer is **Steroids**. The synthesis of fatty acids and steroids (cholesterol) occurs in the **cytosol** (extramitochondrial site) and requires **NADPH** as a crucial reducing equivalent. **Why Steroids?** Steroidogenesis involves the conversion of Acetyl-CoA to cholesterol and subsequently to various steroid hormones. This process requires NADPH, which is primarily generated via the **Hexose Monophosphate (HMP) Shunt** (Pentose Phosphate Pathway). NADPH provides the electrons necessary for the reductive biosynthetic reactions and the action of the cytochrome P450 monooxygenase system involved in steroid hydroxylation. **Analysis of Incorrect Options:** * **A. Glycogen:** Glycogen synthesis (Glycogenesis) occurs in the cytosol but requires **UTP** (Uridine triphosphate) and ATP, not NADPH. * **C. Ketone bodies:** Ketogenesis occurs exclusively within the **mitochondrial matrix** of hepatocytes. It does not require NADPH; rather, it is a pathway used to export energy when glucose is low. **High-Yield Clinical Pearls for NEET-PG:** * **Sources of NADPH:** The HMP shunt (via G6PD enzyme) is the major source. Another important source is the **Malic enzyme**, which converts malate to pyruvate in the cytosol. * **Key NADPH-dependent processes:** 1. Reductive biosynthesis (Fatty acids, Cholesterol, Steroids). 2. Maintenance of reduced **Glutathione** (protecting RBCs from oxidative stress). 3. Phagocytosis by WBCs (Respiratory burst via NADPH oxidase). 4. Nitric Oxide (NO) synthesis. * **Location Tip:** Remember "Fatty Acid synthesis and Steroid synthesis stay together in the Cytosol and both need NADPH."

Question 240: Which of the following is not synthesized from mevalonate?

A. Farnesyl pyrophosphate
B. Geranyl pyrophosphate
C. Dolichol
D. Ubiquitin (Correct Answer)

Explanation: **Explanation:** The synthesis of cholesterol involves the **Mevalonate pathway**, which produces several vital isoprenoid intermediates. The question asks which substance is *not* derived from this pathway. **Why Ubiquitin is the correct answer:** **Ubiquitin** is a small, highly conserved **regulatory protein** found in almost all eukaryotic tissues. Its primary function is to mark proteins for degradation via the proteasome (the Ubiquitin-Proteasome Pathway). It is synthesized through protein translation (amino acids), not the lipid-based mevalonate pathway. **Note:** Do not confuse **Ubiquitin** with **Ubiquinone** (Coenzyme Q). Ubiquinone *is* synthesized from mevalonate. **Why the other options are incorrect:** The mevalonate pathway follows this sequence: Mevalonate → Isopentenyl pyrophosphate (IPP) → **Geranyl pyrophosphate (GPP)** → **Farnesyl pyrophosphate (FPP)** → Squalene → Cholesterol. * **Geranyl pyrophosphate (Option B):** A 10-carbon intermediate in the pathway. * **Farnesyl pyrophosphate (Option A):** A 15-carbon intermediate used for cholesterol synthesis and the prenylation of proteins (like Ras). * **Dolichol (Option C):** A specialized lipid synthesized from farnesyl pyrophosphate. It is essential for the N-linked glycosylation of proteins in the endoplasmic reticulum. **High-Yield Clinical Pearls for NEET-PG:** * **HMG-CoA Reductase:** The rate-limiting enzyme of this pathway, inhibited by **Statins**. * **Statin-induced Myopathy:** May be caused by the depletion of **Ubiquinone (CoQ10)**, a side product of the mevalonate pathway required for the mitochondrial electron transport chain. * **Dolichol deficiency:** Leads to Congenital Disorders of Glycosylation (CDG). * **Protein Prenylation:** FPP and GPP are used to anchor proteins (e.g., Ras G-protein) to cell membranes.

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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