Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 401: Which of the following lipoproteins is most atherogenic?

A. Cholesterol
B. VLDL
C. Triglycerides
D. LDL (Correct Answer)

Explanation: **Explanation:** **Low-Density Lipoprotein (LDL)** is considered the most atherogenic lipoprotein because it is the primary carrier of cholesterol from the liver to the peripheral tissues. Due to its small size, LDL can easily penetrate the arterial endothelium. Once in the sub-endothelial space, it undergoes **oxidation**. Oxidized LDL is engulfed by macrophages via scavenger receptors, leading to the formation of **foam cells**, which are the hallmark of early atherosclerotic plaques. **Analysis of Incorrect Options:** * **Cholesterol (A) & Triglycerides (C):** These are lipids, not lipoproteins. While high levels are associated with cardiovascular risk, they do not circulate freely in the blood; they must be packaged into lipoproteins. Therefore, they are components of the atherogenic process rather than the transport vehicle itself. * **VLDL (B):** Very Low-Density Lipoprotein primarily transports endogenous triglycerides. While VLDL remnants (IDL) are pro-atherogenic, VLDL itself is less directly involved in plaque formation compared to LDL. **High-Yield Facts for NEET-PG:** * **Small Dense LDL (Pattern B):** This specific subtype of LDL is even more atherogenic than standard LDL because it penetrates the arterial wall more easily and is more susceptible to oxidation. * **Lp(a):** An independent risk factor for atherosclerosis; it is a modified LDL particle that also inhibits fibrinolysis. * **HDL (High-Density Lipoprotein):** Known as "Good Cholesterol" because it mediates **Reverse Cholesterol Transport**, moving cholesterol from tissues back to the liver, thus acting as an anti-atherogenic agent. * **Friedewald Formula:** LDL = Total Cholesterol – (HDL + VLDL). Note: VLDL is estimated as TG/5 (valid only if TG <400 mg/dL).

Question 402: A 2-day-old infant born at 32 weeks' gestation has had breathing difficulties since birth and is currently on a respirator and 100% oxygen. These difficulties occur because of which one of the following?

A. An inability of the lung to contract to exhale
B. An inability of the lung to expand when taking in air (Correct Answer)
C. An inability of the lung to respond to insulin
D. An inability of the lung to respond to glucagon

Explanation: This question describes **Respiratory Distress Syndrome (RDS)**, formerly known as Hyaline Membrane Disease, which is common in premature infants (born before 34 weeks). ### **Explanation of the Correct Answer** The fundamental defect in RDS is a deficiency of **pulmonary surfactant**. Surfactant is a lipoprotein complex primarily composed of **Dipalmitoylphosphatidylcholine (DPPC/Lecithin)**. Its physiological role is to reduce surface tension at the alveolar air-liquid interface. * According to the **Law of Laplace** ($P = 2T/r$), smaller alveoli have a higher tendency to collapse. * Surfactant prevents this collapse during expiration. * Without surfactant, the surface tension remains high, causing alveoli to collapse (atelectasis). This makes the lungs "stiff" (low compliance), resulting in an **inability of the lung to expand** during inspiration, leading to severe respiratory distress. ### **Analysis of Incorrect Options** * **Option A:** Exhalation is a passive process driven by the elastic recoil of the lungs. In RDS, the lungs recoil too much (collapse), making inhalation—not exhalation—the primary difficulty. * **Options C & D:** While hormones like **Glucocorticoids** (which stimulate surfactant production) and **Insulin** (which can inhibit it in infants of diabetic mothers) play a role in lung maturation, the clinical pathology is a mechanical failure of expansion due to surfactant deficiency, not a direct signaling failure of insulin or glucagon in the lung tissue. ### **High-Yield NEET-PG Pearls** * **Synthesis:** Surfactant is produced by **Type II Pneumocytes**. * **Composition:** 90% lipids, 10% proteins. The most important component is **Dipalmitoylphosphatidylcholine (DPPC)**. * **Maturity Marker:** Fetal lung maturity is assessed via the **Lecithin/Sphingomyelin (L/S) ratio** in amniotic fluid. A ratio **> 2.0** indicates mature lungs. * **Clinical Correlation:** Maternal diabetes increases fetal insulin, which antagonizes the effects of cortisol, delaying surfactant production and increasing RDS risk.

Question 403: Triglyceride content is lowest in which of the following lipoprotein classes?

A. HDL (High-density lipoprotein) (Correct Answer)
B. LDL (Low-density lipoprotein)
C. VLDL (Very-low-density lipoprotein)
D. Chylomicrons

Explanation: **Explanation:** Lipoproteins are classified based on their density, which is inversely proportional to their lipid content. The core of a lipoprotein consists primarily of **Triglycerides (TG)** and **Cholesterol esters**. **1. Why HDL is the correct answer:** HDL (High-density lipoprotein) is the smallest and densest of all lipoproteins. It is composed of approximately **50% protein** and only **5% triglycerides**. Its primary function is "Reverse Cholesterol Transport," carrying cholesterol from peripheral tissues back to the liver. Because it has the highest protein-to-lipid ratio, it contains the lowest percentage of triglycerides among the options provided. **2. Analysis of incorrect options:** * **Chylomicrons:** These have the **highest triglyceride content (~90-95%)** and the lowest density. they transport dietary (exogenous) lipids from the intestine. * **VLDL:** Produced by the liver to transport endogenous lipids, VLDL contains approximately **60% triglycerides**. * **LDL:** Often called "bad cholesterol," LDL is the primary carrier of cholesterol in the blood. While its TG content is low (~5-10%), it is still typically higher than or equal to the minimal TG levels found in mature HDL. **High-Yield Clinical Pearls for NEET-PG:** * **Density Gradient:** HDL > LDL > IDL > VLDL > Chylomicrons. * **Size Gradient:** Chylomicrons > VLDL > IDL > LDL > HDL. * **Apolipoprotein Markers:** * HDL: **Apo A-I** (activates LCAT). * Chylomicrons: **Apo B-48**. * VLDL/LDL: **Apo B-100**. * **Electrophoretic Mobility:** HDL moves fastest toward the anode (alpha-lipoprotein), followed by VLDL (pre-beta), LDL (beta), and Chylomicrons (remain at origin).

Question 404: Which enzyme levels are decreased during starvation?

A. Fatty acid synthase and Lipoprotein lipase (Correct Answer)
B. CoA carnitine transferase
C. Fatty acid synthase and Citrate lyase
D. Fatty acid synthase and CoA carnitine transferase

Explanation: ### Explanation In the starvation state, the body shifts from **anabolic (synthetic)** processes to **catabolic (breakdown)** processes to maintain blood glucose and provide energy. **1. Why Option A is Correct:** * **Fatty Acid Synthase (FAS):** This is a key multienzyme complex involved in *de novo* lipogenesis. During starvation, insulin levels are low and glucagon is high. This inhibits lipogenesis to conserve energy and carbon skeletons. Consequently, the synthesis and activity of FAS decrease significantly. * **Lipoprotein Lipase (LPL):** LPL is responsible for clearing triglycerides from chylomicrons and VLDL for storage in adipose tissue. In starvation, the body aims to mobilize fat, not store it. Insulin (which normally induces LPL in adipose tissue) is low, leading to decreased LPL levels in the fed-state storage sites. **2. Why Other Options are Incorrect:** * **Carnitine Acyltransferase (CAT/CPT):** (Referred to as CoA carnitine transferase in options). This enzyme is the rate-limiting step for **Beta-oxidation** (fatty acid breakdown). During starvation, its activity **increases** to allow fatty acids to enter the mitochondria for energy production. * **Citrate Lyase:** While Citrate Lyase levels do decrease during starvation (as it provides Acetyl-CoA for lipogenesis), Option A is a more complete answer regarding the primary regulatory enzymes affected in both synthesis and storage pathways. **3. Clinical Pearls for NEET-PG:** * **Rate-Limiting Step of Lipogenesis:** Acetyl-CoA Carboxylase (ACC). It is inactivated by phosphorylation (via AMP-activated protein kinase) during starvation. * **Hormone Sensitive Lipase (HSL):** This enzyme’s activity **increases** during starvation (stimulated by Epinephrine/Glucagon) to mobilize stored triglycerides from adipose tissue. Do not confuse LPL (decreased) with HSL (increased). * **Ketogenesis:** Starvation leads to an increase in HMG-CoA Synthase (mitochondrial) to produce ketone bodies for the brain.

Question 405: Which of the following inhibits the activity of acetyl-CoA carboxylase?

A. Citrate
B. Glucagon (Correct Answer)
C. High-carbohydrate, low-fat diet
D. Insulin

Explanation: **Explanation:** **Acetyl-CoA Carboxylase (ACC)** is the rate-limiting enzyme in fatty acid synthesis, responsible for converting Acetyl-CoA to Malonyl-CoA. Its regulation is a high-yield topic for NEET-PG, involving both allosteric and hormonal control. **Why Glucagon is correct:** Glucagon (and Epinephrine) inhibits ACC through **cAMP-dependent phosphorylation**. When glucagon levels are high (fasting state), it activates Protein Kinase A, which phosphorylates and inactivates ACC. This prevents the synthesis of new fatty acids when the body needs to mobilize energy stores instead. **Analysis of Incorrect Options:** * **Citrate (A):** This is the primary **allosteric activator** of ACC. High citrate levels signal an abundance of energy in the mitochondria, causing ACC dimers to polymerize into their active filamentous form. * **High-carbohydrate, low-fat diet (C):** This diet **induces** the synthesis of ACC. Excess glucose leads to increased insulin and citrate, promoting fatty acid synthesis (lipogenesis). * **Insulin (D):** Insulin **activates** ACC. It triggers a phosphatase that dephosphorylates the enzyme, shifting it into its active state. It also increases ACC gene expression. **High-Yield Clinical Pearls for NEET-PG:** * **Malonyl-CoA** (the product of ACC) inhibits **Carnitine Palmitoyltransferase-I (CPT-I)**. This prevents a "futile cycle" by ensuring fatty acid synthesis and beta-oxidation do not occur simultaneously. * **Metformin** also inhibits ACC via the activation of AMP-activated protein kinase (AMPK), which is one mechanism for its lipid-lowering effects. * **Cofactor Requirement:** ACC requires **Biotin (B7)**, CO₂, and ATP (ABC enzyme).

Question 406: What is the primary building block utilized in fatty acid synthesis?

A. Acetyl CoA (Correct Answer)
B. Palmitoyl CoA
C. Malonyl CoA
D. Oleate

Explanation: ### Explanation **Correct Answer: A. Acetyl CoA** **Why it is correct:** Fatty acid synthesis (Lipogenesis) occurs primarily in the cytosol. The process begins with **Acetyl CoA**, which serves as the fundamental two-carbon building block. Since Acetyl CoA is produced in the mitochondria (via pyruvate decarboxylation) and cannot cross the inner mitochondrial membrane, it is transported into the cytosol as **Citrate** (the "Citrate Shuttle"). Once in the cytosol, Citrate is cleaved back into Acetyl CoA and Oxaloacetate by the enzyme *ATP Citrate Lyase*. **Analysis of Incorrect Options:** * **B. Palmitoyl CoA:** This is the activated form of Palmitate, which is the **end product** of the fatty acid synthase complex, not the building block. * **C. Malonyl CoA:** While Malonyl CoA is the immediate donor of two-carbon units during elongation, it is itself synthesized from Acetyl CoA via the enzyme *Acetyl CoA Carboxylase (ACC)*. Acetyl CoA remains the primary precursor. * **D. Oleate:** This is an 18-carbon monounsaturated fatty acid (18:1). It is a product of further modification (desaturation) of palmitate, not a building block. **NEET-PG High-Yield Pearls:** * **Rate-limiting enzyme:** Acetyl CoA Carboxylase (ACC), which requires **Biotin** as a cofactor. * **Key Activator:** Citrate (signals high energy status). * **Key Inhibitor:** Palmitoyl CoA (feedback inhibition) and Glucagon/Epinephrine. * **Reducing Power:** **NADPH** is essential for synthesis, primarily supplied by the Hexose Monophosphate (HMP) Shunt. * **Multienzyme Complex:** Fatty Acid Synthase (FAS) is a dimer with seven catalytic activities, including the **Acyl Carrier Protein (ACP)** which contains Vitamin B5 (Pantothenic acid).

Question 407: Refsum's disease is characterized by increased levels of which substance?

A. Phytanic acid (Correct Answer)
B. Ascorbic acid
C. Acetic acid
D. None of the above

Explanation: **Explanation:** **Refsum’s disease** is a rare autosomal recessive peroxisomal disorder caused by a deficiency in the enzyme **Phytanoyl-CoA hydroxylase**. This enzyme is essential for **$\alpha$-oxidation**, a process required to break down branched-chain fatty acids. 1. **Why Phytanic acid is correct:** Phytanic acid is a 20-carbon branched-chain fatty acid derived from chlorophyll in the diet (found in dairy and ruminant fats). Because it has a methyl group at the beta-carbon, it cannot undergo normal $\beta$-oxidation. It must first undergo $\alpha$-oxidation to remove one carbon atom. In Refsum’s disease, this pathway is blocked, leading to the toxic accumulation of **phytanic acid** in the blood and tissues (especially the nervous system and retina). 2. **Why other options are incorrect:** * **Ascorbic acid:** This is Vitamin C. Its levels are unrelated to peroxisomal fatty acid metabolism. * **Acetic acid:** This is a 2-carbon short-chain fatty acid and a common metabolic intermediate (as Acetyl-CoA). It does not accumulate in $\alpha$-oxidation defects. **Clinical Pearls for NEET-PG:** * **Classic Tetrad:** Retinitis pigmentosa (earliest sign), Peripheral neuropathy, Cerebellar ataxia, and Sensorineural hearing loss. * **Ichthyosis:** Patients often present with dry, scaly skin. * **Management:** Treatment involves a **dietary restriction** of chlorophyll-containing foods (green leafy vegetables) and ruminant fats (beef, lamb, dairy). * **Zellweger Syndrome vs. Refsum’s:** While both are peroxisomal disorders, Zellweger involves a total failure of peroxisome biogenesis, whereas Refsum’s is a specific enzyme defect.

Question 408: Niemann-Pick disease is due to a disturbance in which type of metabolism?

A. Lipid metabolism (Correct Answer)
B. Protein metabolism
C. Carbohydrate metabolism
D. Mineral metabolism

Explanation: **Explanation:** **Niemann-Pick Disease** is a classic example of a **Lysosomal Storage Disorder (LSD)**, specifically categorized under **Sphingolipidoses**. The correct answer is **Lipid metabolism** because the disease is caused by a deficiency of the enzyme **Acid Sphingomyelinase (ASM)**. This deficiency leads to the pathological accumulation of **sphingomyelin** (a major structural lipid of cell membranes) within the lysosomes of various tissues, particularly the liver, spleen, and brain. **Why other options are incorrect:** * **Protein metabolism:** Disorders in this category typically involve amino acidopathies (e.g., Phenylketonuria) or urea cycle defects, not the accumulation of complex lipids. * **Carbohydrate metabolism:** These include Glycogen Storage Diseases (e.g., Von Gierke’s) or Mucopolysaccharidoses. While some LSDs involve carbohydrates, Niemann-Pick specifically involves sphingolipids. * **Mineral metabolism:** These involve disturbances in elements like Copper (Wilson’s disease) or Iron (Hemochromatosis). **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Deficiency:** Acid Sphingomyelinase (Types A and B). Type C is due to defects in cholesterol transport proteins (NPC1/NPC2). * **Pathognomonic Feature:** Presence of **"Foam Cells"** (lipid-laden macrophages with a vacuolated appearance) in the bone marrow. * **Clinical Triad:** Hepatosplenomegaly, progressive neurodegeneration, and a **Cherry-red spot** on the macula (also seen in Tay-Sachs, but Tay-Sachs lacks organomegaly). * **Inheritance:** Autosomal Recessive.

Question 409: Which one of the following best represents de novo fatty acid biosynthesis starting with cytosolic citrate?

A. NADH is Required - no; Major Product is - Palmitic acid; Occurs at a Glucagon-to-insulin Ratio Best Described as - low; Required Cofactors - Biotin
B. NADH is Required - no; Major Product is - Stearic acid; Occurs at a Glucagon-to-insulin Ratio Best Described as - high; Required Cofactors - Coenzyme A
C. NADH is Required - no; Major Product is - Palmitic acid; Occurs at a Glucagon-to-insulin Ratio Best Described as - low; Required Cofactors - Biotin and Coenzyme A (Correct Answer)
D. NADH is Required - yes; Major Product is - Stearic acid; Occurs at a Glucagon-to-insulin Ratio Best Described as - high; Required Cofactors - Biotin

Explanation: ### Explanation **De novo fatty acid synthesis (Lipogenesis)** primarily occurs in the liver and lactating mammary glands. The process involves the conversion of excess dietary carbohydrates into fat. #### 1. Why Option C is Correct: * **Reducing Equivalents:** Lipogenesis requires **NADPH** (derived from the HMP shunt), **not NADH**. Therefore, "NADH is required - no" is correct. * **Major Product:** The primary end product of the fatty acid synthase complex is **Palmitic acid** (a 16-carbon saturated fatty acid). * **Hormonal Regulation:** Lipogenesis is an anabolic process stimulated by **Insulin** (fed state). Thus, it occurs when the **Glucagon-to-Insulin ratio is low**. * **Cofactors:** * **Biotin:** Required by *Acetyl-CoA Carboxylase (ACC)*, the rate-limiting enzyme, for carboxylation. * **Coenzyme A:** Required to activate acetate to Acetyl-CoA and Malonyl-CoA. #### 2. Why Other Options are Wrong: * **Option A:** Incorrect because it misses **Coenzyme A** as a vital cofactor for the substrate precursors. * **Option B:** Incorrect because the major product is Palmitate (not Stearic acid), and it occurs during a **low** glucagon-to-insulin ratio (not high). * **Option D:** Incorrect because **NADPH** is the required reductant, not NADH. High glucagon levels actually inhibit lipogenesis via phosphorylation of ACC. #### 3. High-Yield NEET-PG Pearls: * **Rate-Limiting Enzyme:** Acetyl-CoA Carboxylase (ACC). It is activated by **Citrate** and inhibited by **Palmitoyl-CoA**. * **Citrate Shuttle:** Since Acetyl-CoA cannot cross the mitochondrial membrane, it enters the cytosol as **Citrate** (the "Citrate-Malate Shuttle"). * **Multienzyme Complex:** Fatty Acid Synthase (FAS) is a dimer with 7 catalytic activities; its "swinging arm" is **Acyl Carrier Protein (ACP)**, which contains Vitamin B5 (Pantothenic acid). * **Location:** Occurs in the **Cytosol**, whereas Beta-oxidation occurs in the Mitochondria.

Question 410: Which of the following organs cannot utilize ketone bodies?

A. Muscle
B. Heart
C. Brain
D. Liver (Correct Answer)

Explanation: **Explanation:** The **Liver** is the primary site of ketogenesis (the production of ketone bodies), yet it is the only organ that **cannot utilize** them for energy. **Why the Liver cannot utilize ketone bodies:** The utilization of ketone bodies (ketolysis) requires the enzyme **Thiophorase** (also known as Succinyl-CoA:3-ketoacid CoA transferase). This enzyme converts acetoacetate into acetoacetyl-CoA, which then enters the TCA cycle. The liver lacks Thiophorase; this deficiency is a physiological protective mechanism that ensures ketone bodies produced in the liver are exported to peripheral tissues rather than being consumed by the liver itself. **Analysis of Incorrect Options:** * **A & B (Muscle and Heart):** These tissues are the primary consumers of ketone bodies during brief periods of fasting. They have high Thiophorase activity, allowing them to spare glucose for the brain. * **C (Brain):** While the brain normally relies on glucose, it can adapt to utilize ketone bodies as its major fuel source during prolonged starvation (usually after 3–4 days) to reduce the need for gluconeogenesis. **High-Yield NEET-PG Pearls:** 1. **Rate-limiting enzyme of Ketogenesis:** HMG-CoA Synthase (Mitochondrial). 2. **Rate-limiting enzyme of Ketolysis:** Thiophorase (absent in Liver). 3. **Ketone Bodies:** Acetoacetate, 3-Hydroxybutyrate, and Acetone (Acetone is a non-metabolizable waste product excreted via lungs, causing "fruity breath"). 4. **Detection:** Rothera’s test detects Acetoacetate and Acetone, but **not** Beta-hydroxybutyrate.

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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