Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 21: Which of the following is an essential fatty acid?

A. Linoleic acid (Correct Answer)
B. Palmitoleic acid
C. Oleic acid
D. Arachidonic acid

Explanation: **Explanation:** **Essential Fatty Acids (EFAs)** are fatty acids that the human body cannot synthesize de novo because humans lack the enzymes (**$\Delta^{12}$ and $\Delta^{15}$ desaturases**) required to introduce double bonds beyond the $\Delta^9$ position. Therefore, they must be obtained through the diet. 1. **Why Linoleic Acid is Correct:** **Linoleic acid (18:2; $\omega$-6)** is a primary essential fatty acid. It serves as the precursor for the synthesis of $\gamma$-linolenic acid and arachidonic acid. The other strictly essential fatty acid is **$\alpha$-Linolenic acid (18:3; $\omega$-3)**. 2. **Analysis of Incorrect Options:** * **Palmitoleic acid (16:1; $\omega$-7):** A monounsaturated fatty acid that can be synthesized by the body via the $\Delta^9$ desaturase enzyme acting on palmitic acid. * **Oleic acid (18:1; $\omega$-9):** The most common monounsaturated fatty acid in the diet, synthesized by the body from stearic acid. * **Arachidonic acid (20:4; $\omega$-6):** It is considered **semi-essential**. While it is vital for prostaglandin synthesis, the body can synthesize it from linoleic acid. It only becomes "essential" if there is a dietary deficiency of linoleic acid. **High-Yield Clinical Pearls for NEET-PG:** * **EFA Deficiency:** Characterized by scaly dermatitis (phrynoderma/toad skin), alopecia, and poor wound healing. * **Omega Nomenclature:** Count from the methyl ($CH_3$) end to the first double bond. * **Functions:** EFAs are structural components of cell membranes and precursors for **Eicosanoids** (prostaglandins, leukotrienes, and thromboxanes). * **Ratio:** A healthy diet requires a balanced ratio of $\omega$-6 to $\omega$-3 fatty acids.

Question 22: What is the primary effect of Nicotinic acid?

A. Increases HDL (Correct Answer)
B. Increases triglyceride synthesis
C. Treats Type II hyperlipoproteinemia
D. Decreases hydrolysis of VLDL

Explanation: **Explanation:** **Nicotinic Acid (Niacin)** is a potent lipid-lowering agent. Its **primary and most significant effect is increasing HDL levels** (by 15–35%), making it the most effective drug currently available for this purpose. 1. **Why Option A is Correct:** Niacin increases HDL levels by decreasing the fractional fractional clearance of apoA-I (the primary apolipoprotein of HDL) in the liver. This prolongs the half-life of HDL particles, enhancing reverse cholesterol transport. 2. **Why Options B, C, and D are Incorrect:** * **Option B:** Niacin actually **decreases** triglyceride synthesis by inhibiting diacylglycerol acyltransferase-2 (DGAT2) in the liver. * **Option C:** While Niacin can be used as an adjunct, **Statins** are the first-line treatment for Type II hyperlipoproteinemia (Hypercholesterolemia). Niacin is more specifically indicated for Type IV and V (Hypertriglyceridemia). * **Option D:** Niacin **inhibits the lipolysis of triglycerides** in adipose tissue by inhibiting the enzyme hormone-sensitive lipase (HSL). This reduces the flow of free fatty acids to the liver, thereby decreasing VLDL production, rather than decreasing VLDL hydrolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Acts via G protein-coupled receptors (GPR109A) to inhibit hormone-sensitive lipase in adipose tissue. * **Side Effects:** The most common side effect is **cutaneous flushing** (mediated by Prostaglandin $D_2$ and $E_2$; prevented by Aspirin). * **Metabolic Complications:** Can cause **hyperuricemia** (precipitating gout) and **hyperglycemia** (impaired glucose tolerance), so it must be used cautiously in diabetic patients. * **Acanthosis Nigricans:** Niacin therapy is a known pharmacological cause of this skin condition.

Question 23: Sphingomyelinase deficiency is characteristic of which condition?

A. Fabry's disease
B. Krabbe's disease
C. Tay-Sachs disease
D. Niemann-Pick disease (Correct Answer)

Explanation: **Explanation:** **Niemann-Pick Disease (Option D)** is a lysosomal storage disorder characterized by a deficiency of the enzyme **sphingomyelinase**. This enzyme is responsible for the hydrolysis of sphingomyelin into ceramide and phosphorylcholine. Its deficiency leads to the pathological accumulation of sphingomyelin in the reticuloendothelial system (liver, spleen, and bone marrow) and the central nervous system. Clinical hallmarks include hepatosplenomegaly, neurodegeneration, and a characteristic **"cherry-red spot"** on the macula (in Type A). Histologically, it is identified by **"Foam cells"** (lipid-laden macrophages). **Why other options are incorrect:** * **Fabry's Disease (Option A):** Caused by a deficiency of **$\alpha$-galactosidase A**, leading to the accumulation of ceramide trihexoside. It is X-linked recessive and presents with angiokeratomas and renal failure. * **Krabbe's Disease (Option B):** Caused by a deficiency of **galactocerebrosidase**, leading to the accumulation of galactocerebroside and psychosine, which destroys myelin. It is characterized by **Globoid cells**. * **Tay-Sachs Disease (Option C):** Caused by a deficiency of **Hexosaminidase A**, leading to the accumulation of **GM2 ganglioside**. While it also features a cherry-red spot, it is distinguished from Niemann-Pick by the **absence of hepatosplenomegaly**. **NEET-PG High-Yield Pearls:** * **Niemann-Pick:** Sphingomyelinase deficiency + Hepatosplenomegaly + Foam cells. * **Tay-Sachs:** Hexosaminidase A deficiency + NO Hepatosplenomegaly + Onion-skin lysosomes. * **Gaucher's Disease:** Most common lysosomal storage disorder; Glucocerebrosidase deficiency; **Crumpled tissue paper** appearance of macrophages.

Question 24: A 44-year-old man sustains a myocardial infarction and is admitted to the hospital. Serum chemistries reveal a two-fold elevation of his LDL cholesterol. He is prescribed lovastatin. Lovastatin acts by inhibiting which of the following enzymes?

A. Acetyl-CoA carboxylase
B. Carbamoyl phosphate synthetase I
C. Hydroxymethyl glutaryl-CoA reductase (Correct Answer)
D. Pyruvate dehydrogenase

Explanation: **Explanation:** **Correct Answer: C. Hydroxymethyl glutaryl-CoA reductase (HMG-CoA Reductase)** The patient has hypercholesterolemia, a major risk factor for myocardial infarction. Lovastatin belongs to the **Statin** class of drugs, which are competitive inhibitors of **HMG-CoA reductase**. This enzyme catalyzes the rate-limiting step in cholesterol biosynthesis: the conversion of HMG-CoA to mevalonate. By inhibiting this enzyme, statins decrease intracellular cholesterol synthesis, leading to an up-regulation of LDL receptors on hepatocytes, which subsequently increases the clearance of LDL from the plasma. **Incorrect Options:** * **A. Acetyl-CoA carboxylase:** This is the rate-limiting enzyme for **fatty acid synthesis** (converting Acetyl-CoA to Malonyl-CoA). It is inhibited by glucagon and palmitoyl-CoA, not statins. * **B. Carbamoyl phosphate synthetase I (CPS-I):** This is the rate-limiting enzyme of the **Urea cycle**, located in the mitochondria. It is activated by N-acetylglutamate. * **D. Pyruvate dehydrogenase (PDH):** This enzyme complex converts pyruvate to Acetyl-CoA, linking glycolysis to the TCA cycle. Deficiency leads to lactic acidosis. **NEET-PG High-Yield Pearls:** * **Statins** are structural analogs of HMG-CoA. * **Diurnal Variation:** Cholesterol synthesis is maximal at night; hence, short-acting statins (like Lovastatin) are traditionally administered at bedtime. * **Side Effects:** The most high-yield side effects are **myopathy/rhabdomyolysis** (monitored via Creatine Kinase levels) and hepatotoxicity (monitored via LFTs). * **Pleiotropic effects:** Statins also stabilize atherosclerotic plaques and have anti-inflammatory properties.

Question 25: Free fatty acids in the blood are transported by which of the following?

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

Explanation: ### Explanation **Correct Answer: C. Albumin** **Mechanism and Rationale:** Free fatty acids (FFAs), also known as non-esterified fatty acids (NEFA), are hydrophobic molecules released from adipose tissue via lipolysis (mediated by Hormone-Sensitive Lipase). Because they are insoluble in aqueous plasma, they require a carrier protein. **Albumin** serves as this primary carrier. It possesses multiple high-affinity binding sites (hydrophobic pockets) that allow it to transport FFAs to peripheral tissues like the heart, skeletal muscle, and liver for $\beta$-oxidation. **Why the other options are incorrect:** * **VLDL, LDL, and Chylomicrons (A, B, D):** These are lipoproteins responsible for transporting **esterified lipids** (Triacylglycerols and Cholesterol esters). * **Chylomicrons** transport dietary (exogenous) triglycerides. * **VLDL** transports endogenous triglycerides from the liver. * **LDL** primarily transports cholesterol to peripheral tissues. * While these particles contain lipids, they do not carry "free" fatty acids; the fatty acids within them are chemically bonded to glycerol. **High-Yield Clinical Pearls for NEET-PG:** * **Capacity:** One molecule of albumin can bind up to 7–10 fatty acid molecules, though under normal physiological conditions, only 2–3 sites are occupied. * **Brain Exception:** FFAs bound to albumin **cannot cross the blood-brain barrier**; therefore, the brain cannot use long-chain fatty acids for energy and relies on glucose or ketone bodies. * **Drug Interaction:** Many drugs (e.g., sulfonamides, aspirin) compete with fatty acids and bilirubin for binding sites on albumin, which is clinically significant in neonatal jaundice (risk of kernicterus). * **Short/Medium Chain Fatty Acids:** Unlike long-chain FAs, these are more water-soluble and can enter the portal circulation directly without significant reliance on transport proteins.

Question 26: Which of the following can be oxidized by beta-oxidation pathway?

A. Saturated fatty acids
B. Monounsaturated fatty acids
C. Polyunsaturated fatty acids
D. All of these (Correct Answer)

Explanation: **Explanation:** The **$\beta$-oxidation pathway** is the primary metabolic process for the breakdown of fatty acids to generate energy (ATP) in the mitochondrial matrix. While the basic spiral of $\beta$-oxidation (oxidation, hydration, oxidation, thiolysis) is designed for **saturated fatty acids**, the body possesses auxiliary enzymes that allow it to process unsaturated fats as well. * **Saturated Fatty Acids (Option A):** These are the standard substrates for the $\beta$-oxidation cycle. They undergo successive removal of two-carbon units as Acetyl-CoA without requiring additional enzymes. * **Monounsaturated (Option B) and Polyunsaturated Fatty Acids (Option C):** These contain double bonds in the *cis* configuration, which the standard enzyme (Enoyl-CoA hydratase) cannot act upon. However, two auxiliary enzymes—**Isomerase** (converts *cis* to *trans* bonds) and **Reductase** (handles polyunsaturated bonds)—reconfigure these molecules so they can re-enter the standard $\beta$-oxidation pathway. Therefore, **Option D** is correct because all three types of fatty acids are ultimately oxidized via this pathway to produce energy. **High-Yield NEET-PG Pearls:** 1. **Rate-limiting step:** The transport of long-chain fatty acids into the mitochondria via the **Carnitine Shuttle** (inhibited by Malonyl-CoA). 2. **Energy Yield:** The oxidation of Palmitate (16-carbon saturated FA) yields a net of **106 ATP**. 3. **Odd-chain Fatty Acids:** These follow the same pathway but end with **Propionyl-CoA**, which is converted to Succinyl-CoA (a TCA cycle intermediate) via a Vitamin B12-dependent reaction. 4. **Clinical Correlation:** Deficiency of **MCAD** (Medium-chain acyl-CoA dehydrogenase) is the most common inborn error of $\beta$-oxidation, presenting as fasting hypoglycemia.

Question 27: Which of the following is a lipotropic factor?

A. HDL
B. Insulin
C. Choline (Correct Answer)
D. Carnitine

Explanation: ### Explanation **Concept of Lipotropic Factors** Lipotropic factors are substances required for the normal mobilization of fat from the liver. They prevent the excessive accumulation of triacylglycerols (TAGs) in hepatocytes, thereby preventing **Fatty Liver (Steatosis)**. **Why Choline is Correct:** Choline is the most essential lipotropic factor. It is a precursor for **Phosphatidylcholine (Lecithin)**, a major component of the phospholipid shell of **VLDL (Very Low-Density Lipoprotein)**. Since TAGs are exported from the liver primarily as VLDL, a deficiency in choline leads to impaired VLDL assembly and secretion, causing fat to "trap" inside the liver. **Analysis of Incorrect Options:** * **HDL (High-Density Lipoprotein):** Involved in "Reverse Cholesterol Transport" (carrying cholesterol from peripheral tissues back to the liver), not in exporting TAGs from the liver. * **Insulin:** Insulin is actually **anti-lipolytic**. It promotes lipogenesis and inhibits the export of fats, often contributing to fatty liver in states of insulin resistance (NAFLD). * **Carnitine:** While essential for the transport of long-chain fatty acids into the mitochondria for **beta-oxidation**, it is not classified as a lipotropic factor because it does not directly facilitate the export of fat from the liver via VLDL. **High-Yield NEET-PG Pearls:** * **Other Lipotropic Factors:** Methionine (provides methyl groups for choline synthesis), Betaine, Inositol, and Vitamin B12/Folic acid. * **Mechanism:** Lipotropic factors act by increasing the synthesis of phospholipids and ApoB-100, both essential for VLDL formation. * **Clinical Correlation:** Chronic alcoholism leads to fatty liver partly because ethanol metabolism inhibits the oxidation of fatty acids and depletes lipotropic precursors.

Question 28: A 7-year-old boy presents with digestive problems, experiencing severe abdominal cramps after consuming a high-fat meal. Diagnostic workup reveals a genetic defect causing lipoprotein lipase deficiency. Which plasma substance would most likely be elevated following a fatty meal in this patient?

A. Albumin-bound free fatty acids
B. Chylomicrons (Correct Answer)
C. HDL
D. LDL

Explanation: ### Explanation **Correct Option: B. Chylomicrons** The patient is suffering from **Type I Hyperlipoproteinemia (Familial Chylomicronemia Syndrome)**, caused by a genetic deficiency of **Lipoprotein Lipase (LPL)** or its cofactor, **Apo C-II**. LPL is an enzyme located on the capillary endothelium of adipose and muscle tissue. Its primary function is to hydrolyze triglycerides (TGs) found in **chylomicrons** (exogenous lipids) and **VLDL** (endogenous lipids) into free fatty acids and glycerol. Following a fatty meal, dietary lipids are packaged into chylomicrons in the intestine. Without functional LPL, these chylomicrons cannot be cleared from the blood, leading to massive accumulation. This typically presents in childhood with eruptive xanthomas, hepatosplenomegaly, and recurrent **acute pancreatitis** (causing the abdominal cramps described). **Why other options are incorrect:** * **A. Albumin-bound free fatty acids:** These are produced *after* LPL acts on TGs or via lipolysis in adipose tissue. In LPL deficiency, TGs aren't broken down, so free fatty acid levels do not rise significantly. * **C. HDL:** HDL levels are often low in this condition, as the transfer of surface components from chylomicrons (which contributes to HDL formation) is impaired. * **D. LDL:** LDL is the end-product of VLDL metabolism. Since the conversion of VLDL to IDL and then LDL requires LPL, LDL levels are usually normal or low in Type I Hyperlipoproteinemia. **NEET-PG High-Yield Pearls:** * **Appearance of Plasma:** In LPL deficiency, plasma shows a **creamy layer on top** (supranate) with a clear infranate when left standing. * **Key Enzyme:** LPL is activated by **Apo C-II** and inhibited by **Apo C-III**. * **Clinical Triad:** Hypertriglyceridemia, Acute Pancreatitis, and Eruptive Xanthomas. * **Treatment:** Strict fat-restricted diet (LPL deficiency does not respond well to fibrates).

Question 29: Which of the following is a multienzyme complex in humans?

A. Fatty acid synthetase (Correct Answer)
B. Malonyl CoA Carboxylase
C. Carbamoyl phosphate synthetase
D. Adenosine phosphoribosyl transferase

Explanation: **Explanation:** **1. Why Fatty Acid Synthetase (FAS) is correct:** In humans (eukaryotes), Fatty Acid Synthetase is a classic example of a **multienzyme complex**. It exists as a **homodimer**, where each monomer is a single large polypeptide chain containing **seven distinct enzyme activities** and an **Acyl Carrier Protein (ACP)** domain. This structural organization allows for "substrate channeling," where the growing fatty acid chain remains covalently attached to the complex and moves from one active site to another without being released into the cytosol. This increases catalytic efficiency and prevents the loss of intermediates. **2. Why the other options are incorrect:** * **Malonyl CoA Carboxylase:** This is a biotin-dependent enzyme that converts Acetyl-CoA to Malonyl-CoA. While it is a complex enzyme, it is a single-function regulatory enzyme (the rate-limiting step of fatty acid synthesis), not a multienzyme complex with multiple distinct catalytic activities. * **Carbamoyl Phosphate Synthetase (CPS):** CPS-I (Mitochondrial) and CPS-II (Cytosolic) are individual enzymes. While CPS-II is part of a trifunctional protein (CAD) in eukaryotes, FAS is the more classic, high-yield textbook example of a multienzyme complex in lipid metabolism. * **Adenosine Phosphoribosyl Transferase (APRT):** This is a single enzyme involved in the purine salvage pathway. It is not a multienzyme complex. **Clinical Pearls & High-Yield Facts:** * **End product:** The FAS complex in humans primarily produces **Palmitate** (a 16-carbon saturated fatty acid). * **Requirement:** Each cycle of elongation requires **2 NADPH** (derived mainly from the Pentose Phosphate Pathway). * **Prokaryotic Difference:** In *E. coli* (Type II FAS), the enzymes are separate, individual proteins, unlike the integrated Type I system found in humans. * **Linker:** The ACP domain contains **4'-phosphopantetheine** (derived from Vitamin B5), which acts as a flexible "swinging arm" to move the substrate.

Question 30: What are the characteristic findings in familial hypercholesterolemia?

A. HDL
B. LDL
C. total cholesterol (Correct Answer)
D. HDL

Explanation: **Explanation:** **Familial Hypercholesterolemia (FH)** is an autosomal dominant disorder primarily caused by mutations in the **LDL receptor gene**. This defect leads to the inability of the liver and peripheral tissues to clear LDL-cholesterol from the blood. **Why Total Cholesterol is the Correct Answer:** In FH, the hallmark laboratory finding is a massive elevation in **Total Cholesterol** (often >300 mg/dL in heterozygotes and >600 mg/dL in homozygotes). Since LDL is the primary carrier of cholesterol in the blood, the failure of LDL receptors causes a secondary, dramatic rise in total serum cholesterol levels. This leads to premature atherosclerosis and characteristic clinical features like **Xanthomas** (tendon) and **Xanthelasmas**. **Analysis of Incorrect Options:** * **Options A & D (HDL):** High-Density Lipoprotein (HDL) is known as "good cholesterol." In FH, HDL levels are typically normal or slightly decreased; they are never the characteristic elevated marker for this condition. * **Option B (LDL):** While LDL levels are indeed elevated in FH, the question asks for the characteristic finding often used in clinical screening and classification (Fredrickson Type IIa). In the context of this specific question and standard NEET-PG patterns, "Total Cholesterol" is the broader clinical hallmark representing the overall lipid burden. **NEET-PG High-Yield Pearls:** * **Inheritance:** Autosomal Dominant. * **Defect:** Most commonly the **LDL Receptor**; others include ApoB-100 or PCSK9 mutations. * **Clinical Signs:** Tendon xanthomas (especially the **Achilles tendon**) and Corneal arcus at a young age. * **Fredrickson Classification:** FH is classified as **Type IIa** (elevated LDL and Total Cholesterol). * **Treatment:** Statins are the first line; PCSK9 inhibitors (evolocumab) are used for refractory cases.

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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