Lipid Metabolism Practice Questions

Q: Which of the following lipoproteins is most atherogenic?

LDL. **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).

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

HDL (High-density lipoprotein). **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).

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

Glucagon. **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).

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

Acetyl CoA. ### 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).

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

Phytanic acid. **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.

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

Lipid metabolism. **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.

Q: Which of the following organs cannot utilize ketone bodies?

Liver. **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.

Question 1

Which of the following lipoproteins is most atherogenic?

Accepted Answer

LDL

Answer

Cholesterol

Answer

VLDL

Answer

Triglycerides

Question 2

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?

Accepted Answer

An inability of the lung to expand when taking in air

Answer

An inability of the lung to contract to exhale

Answer

An inability of the lung to respond to insulin

Answer

An inability of the lung to respond to glucagon

Question 3

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

Accepted Answer

HDL (High-density lipoprotein)

Answer

LDL (Low-density lipoprotein)

Answer

VLDL (Very-low-density lipoprotein)

Answer

Chylomicrons

Question 4

Which enzyme levels are decreased during starvation?

Accepted Answer

Fatty acid synthase and Lipoprotein lipase

Answer

CoA carnitine transferase

Answer

Fatty acid synthase and Citrate lyase

Answer

Fatty acid synthase and CoA carnitine transferase

Question 5

Which of the following inhibits the activity of acetyl-CoA carboxylase?

Accepted Answer

Glucagon

Answer

Citrate

Answer

High-carbohydrate, low-fat diet

Answer

Insulin

Question 6

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

Accepted Answer

Acetyl CoA

Answer

Palmitoyl CoA

Answer

Malonyl CoA

Answer

Oleate

Question 7

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

Accepted Answer

Phytanic acid

Answer

Ascorbic acid

Answer

Acetic acid

Answer

None of the above

Question 8

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

Accepted Answer

Lipid metabolism

Answer

Protein metabolism

Answer

Carbohydrate metabolism

Answer

Mineral metabolism

Question 9

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

Accepted Answer

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

Answer

NADH is Required - no; Major Product is - Palmitic acid; Occurs at a Glucagon-to-insulin Ratio Best Described as - low; Required Cofactors - Biotin

Answer

NADH is Required - no; Major Product is - Stearic acid; Occurs at a Glucagon-to-insulin Ratio Best Described as - high; Required Cofactors - Coenzyme A

Answer

NADH is Required - yes; Major Product is - Stearic acid; Occurs at a Glucagon-to-insulin Ratio Best Described as - high; Required Cofactors - Biotin

Question 10

Which of the following organs cannot utilize ketone bodies?

Accepted Answer

Liver

Answer

Muscle

Answer

Heart

Answer

Brain

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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