Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 651: Which of the following conditions is characterized by a lack of genetically mediated VLDL overproduction?

A. Hypoapobetalipoproteinemia (Correct Answer)
B. Familial dyslipidemic hypertension
C. LDL subclass B
D. Familial combined hyperlipidemia

Explanation: ***Hypoapobetalipoproteinemia*** - This condition is characterized by **reduced production of apolipoprotein B**, leading to abnormally low levels of **VLDL and LDL** in the blood due to genetic mutations affecting apolipoprotein B synthesis or secretion. - Unlike the other options, it explicitly involves a *lack* of VLDL overproduction, making it the correct answer. *Familial combined hyperlipidemia* - This is a common genetic disorder characterized by **overproduction of VLDL** and occasionally decreased clearance of LDL, leading to elevated total cholesterol and triglycerides. - Patients often present with **elevated LDL and VLDL levels**, directly contradicting a lack of VLDL overproduction. *Familial dyslipidemic hypertension* - This condition is associated with a cluster of metabolic abnormalities, including **elevated triglycerides** (often secondary to VLDL overproduction) and hypertension. - The dyslipidemia component involves **increased VLDL production**, contributing to hypertriglyceridemia, rather than a lack of it. *LDL subclass B* - Refers to a predominance of **small, dense LDL particles**, which are more atherogenic than large, buoyant LDL particles. - The presence of small, dense LDL is often associated with conditions like **hypertriglyceridemia** and **insulin resistance**, which can be driven by increased VLDL production, not a lack thereof.

Question 652: Which of the following is NOT a product derived from HMG CoA?

A. Dolichol
B. Ketone bodies
C. Bile pigments (Correct Answer)
D. Ubiquinone

Explanation: ***Bile pigments*** - **Bile pigments** (like bilirubin and biliverdin) are derived from the degradation of **heme** from red blood cells and are completely unrelated to the HMG-CoA pathway. - Bile pigments are NOT products of cholesterol or isoprenoid metabolism. *Ubiquinone* - **Ubiquinone** (Coenzyme Q10) is synthesized via the **mevalonate pathway**, which begins with HMG-CoA being converted to mevalonate by **HMG-CoA reductase**. - The mevalonate pathway produces **isopentenyl pyrophosphate (IPP)**, which is used to synthesize ubiquinone, making it a downstream product of HMG-CoA. *Dolichol* - **Dolichol** is a long-chain isoprenoid alcohol essential for **N-glycosylation** of proteins in the endoplasmic reticulum. - Like ubiquinone, dolichol is synthesized from **isoprenoid units** derived from the mevalonate pathway, making it a product of HMG-CoA metabolism. *Ketone bodies* - **Ketone bodies** (acetoacetate and β-hydroxybutyrate) are directly synthesized from **HMG-CoA** in the mitochondria during fasting or low carbohydrate states. - The enzyme **HMG-CoA lyase** cleaves HMG-CoA to produce acetoacetate, which is then converted to other ketone bodies.

Question 653: What is the parameter that is used to assess lipid peroxidation?

A. Malondialdehyde (Correct Answer)
B. CRP
C. hsCRP
D. Carboxymethyl lysine

Explanation: ***Malondialdehyde*** - **Malondialdehyde (MDA)** is a well-established and commonly used biomarker to quantify the level of **lipid peroxidation** in biological systems. - It is a **reactive aldehyde** formed during the decomposition of polyunsaturated fatty acids, particularly through **free radical attack**. *CRP* - **CRP (C-reactive protein)** is a general **inflammatory marker** that indicates acute phase responses in the body. - While inflammation can be associated with oxidative stress, CRP itself does not directly measure **lipid peroxidation**. *hsCRP* - **hsCRP (high-sensitivity C-reactive protein)** is a more sensitive measure of **inflammation**, often used to assess cardiovascular risk. - Like standard CRP, it is an **indicator of systemic inflammation** and not a direct measure of **lipid peroxidation**. *Carboxymethyl lysine* - **Carboxymethyl lysine (CML)** is an **advanced glycation end-product (AGE)**, formed by non-enzymatic reactions between sugars and proteins or lipids. - It is a marker of **glycation and oxidative stress**, but it does not specifically measure **lipid peroxidation**.

Question 654: Bile acid has a detergent action due to?

A. Amphipathic nature of bile salts (Correct Answer)
B. Formation of medium chain triglycerides
C. Formation of soap
D. Formation of zwitter ion

Explanation: ***Amphipathic nature of bile salts*** - Bile salts are **amphipathic molecules**, meaning they have both **hydrophilic** (water-loving) and **hydrophobic** (fat-loving) regions. - This dual nature allows them to emulsify fats, breaking large fat globules into smaller ones, thereby exhibiting a **detergent action**. *Formation of soap* - While soaps also have a detergent action due to their amphipathic nature, the primary mechanism of bile acid's detergent action in the body is not through the formation of soap as a product. - Soap formation involves a saponification reaction, which is not the main process explaining bile acid's emulsifying role in digestion. *Formation of zwitter ion* - A **zwitterion** is a molecule possessing both positive and negative charges, resulting in an overall neutral charge. - While bile acids can have ionizable groups, their detergent action is primarily attributed to the separation of hydrophilic and hydrophobic domains, not merely the presence of zwitterionic characteristics. *Formation of medium chain triglycerides* - **Medium-chain triglycerides** are a type of fat molecule; their formation is not responsible for the detergent action of bile acids. - Bile acids aid in the digestion and absorption of various dietary fats, including triglycerides, but they do not form them.

Question 655: Lipoprotein involved in reverse cholesterol transport?

A. Very Low-Density Lipoprotein (VLDL)
B. Intermediate-Density Lipoprotein (IDL)
C. Low-Density Lipoprotein (LDL)
D. High-Density Lipoprotein (HDL) (Correct Answer)

Explanation: ***High-Density Lipoprotein (HDL)*** - **HDL** is often referred to as "good cholesterol" because its primary function is **reverse cholesterol transport**, which removes excess cholesterol from peripheral tissues and returns it to the liver for excretion. - It works by picking up **unesterified cholesterol** from cells and esterifying it via **lecithin-cholesterol acyltransferase (LCAT)**, increasing its lipid content. *Very Low-Density Lipoprotein (VLDL)* - **VLDL** is primarily involved in transporting **endogenous triglycerides** synthesized by the liver to peripheral tissues. - While it carries some cholesterol, its main role is not in reverse cholesterol transport but in delivering lipids. *Intermediate-Density Lipoprotein (IDL)* - **IDL** is a transient lipoprotein formed from **VLDL** after it has shed some triglycerides and apoC-II and apoE. - It can be further metabolized to **LDL** or taken up by the liver; it does not directly participate in reverse cholesterol transport. *Low-Density Lipoprotein (LDL)* - **LDL**, often called "bad cholesterol," is responsible for transporting cholesterol from the liver to peripheral tissues. - High levels of **LDL** are associated with increased risk of **atherosclerosis** due to its role in delivering cholesterol to arterial walls.

Question 656: Oxidation of very long chain fatty acids takes place in ?

A. Ribosomes
B. Cytosol
C. Mitochondria
D. Peroxisomes (Correct Answer)

Explanation: ***Peroxisomes (Correct)*** - **Very long chain fatty acids (VLCFAs)**, which have more than 20 carbon atoms, undergo initial **beta-oxidation** in peroxisomes. - This process shortens the VLCFAs before they are transported to mitochondria for further oxidation. - Peroxisomes are essential for breaking down these fatty acids that are too long for direct mitochondrial processing. *Cytosol (Incorrect)* - The cytosol is the site for **fatty acid synthesis**, not oxidation. - It also plays a role in the initial steps of **glycerol phosphorylation** in triglyceride synthesis. *Mitochondria (Incorrect)* - **Mitochondria** primarily handle the beta-oxidation of **medium and short-chain fatty acids** (typically less than 20 carbons). - While VLCFAs are eventually oxidized here after peroxisomal shortening, their initial breakdown must occur in peroxisomes. *Ribosomes (Incorrect)* - **Ribosomes** are responsible for **protein synthesis** (translation) based on mRNA templates. - They have no role in fatty acid metabolism.

Question 657: Which of the following is not a glycerophospholipid?

A. Lecithin
B. Cardiolipin
C. Plasmalogens
D. Sphingomyelin (Correct Answer)

Explanation: ***Sphingomyelin*** - Sphingomyelin is a **sphingolipid**, characterized by a **sphingosine backbone** rather than a glycerol backbone. - It contains a **phosphate group** and **choline head group** attached to the sphingosine. *Lecithin* - **Lecithin** is another name for **phosphatidylcholine**, which is a **glycerophospholipid**. - It has a **glycerol backbone** esterified to two fatty acids and a phosphate group linked to choline. *Cardiolipin* - **Cardiolipin** is a **glycerophospholipid** composed of two phosphatidic acid moieties linked by another glycerol molecule. - It is unique for having **four fatty acyl chains** and is primarily found in the **inner mitochondrial membrane**. *Plasmalogens* - **Plasmalogens** are a class of **glycerophospholipids** characterized by an **ether linkage** at the sn-1 position of the glycerol backbone, instead of an ester linkage. - They also contain an **ester-linked fatty acid** at the sn-2 position and a phosphate group with a head group.

Question 658: A newborn presented with chest retractions, dyspnea, and lethargy. The pediatrician diagnosed the baby with respiratory distress syndrome. This occurs due to the deficiency of:

A. Dipalmitoyl inositol
B. Dipalmitoylphosphatidylethanolamine
C. Lecithin (Correct Answer)
D. Sphingomyelin

Explanation: ***Lecithin*** - **Respiratory distress syndrome (RDS)** in newborns is primarily caused by a deficiency of pulmonary **surfactant**. - **Lecithin (phosphatidylcholine)**, specifically in its dipalmitoyl form (**dipalmitoylphosphatidylcholine or DPPC**), is the main active component of surfactant, constituting ~40-50% of surfactant lipids. - DPPC is crucial for reducing surface tension in the alveoli and preventing their collapse during expiration. - This is the **primary biochemical deficiency** in neonatal RDS. *Dipalmitoyl inositol* - **Inositol** is a sugar alcohol involved in various cellular processes and is present in surfactant as phosphatidylinositol, but it is not a primary functional component. - Deficiency of this compound does not directly lead to **respiratory distress syndrome**. *Dipalmitoylphosphatidylethanolamine* - **Phosphatidylethanolamine (PE)** is a phospholipid found in cell membranes but is not the primary phospholipid responsible for surfactant function. - Note: This is PE, not PC (phosphatidylcholine). While PE is present in surfactant, its deficiency does not specifically cause **neonatal RDS**. *Sphingomyelin* - **Sphingomyelin** is a sphingolipid found in cell membranes and myelin sheaths, but it is not the critical component of pulmonary surfactant. - The **lecithin-to-sphingomyelin (L/S) ratio** is used to assess fetal lung maturity; an L/S ratio >2 indicates mature lungs capable of producing adequate surfactant.

Question 659: Which mineral is required for cholesterol biosynthesis?

A. Fe
B. Mn
C. Mg (Correct Answer)
D. Cu

Explanation: ***Mg*** - **Magnesium (Mg)** is an essential cofactor for multiple enzymes in **cholesterol biosynthesis**, particularly the **ATP-dependent kinases** in the mevalonate pathway. - **Mevalonate kinase** and **phosphomevalonate kinase** require Mg²⁺ as a cofactor for their catalytic activity. - **Squalene synthase**, which catalyzes the formation of squalene from farnesyl pyrophosphate, also requires Mg²⁺. - Deficiency in magnesium can impair these critical steps in **cholesterol synthesis**. *Fe* - **Iron (Fe)** is vital for many enzymatic reactions, particularly in **oxygen transport** (hemoglobin), **electron transport** (cytochromes), and **energy metabolism**. - It does not function as a cofactor in the enzymatic steps of **cholesterol biosynthesis**. *Mn* - **Manganese (Mn)** serves as a cofactor for enzymes involved in **carbohydrate metabolism**, **bone formation**, and **antioxidant defense** (superoxide dismutase). - While important for various metabolic processes, it is not specifically required for **cholesterol synthesis**. *Cu* - **Copper (Cu)** is a component of several enzymes, including **cytochrome c oxidase** and **superoxide dismutase**, involved in electron transport and antioxidant defense. - It does not play a direct role as a cofactor in the key enzymatic steps of **cholesterol synthesis**.

Question 660: Which of the following is the rate limiting step in cholesterol synthesis?

A. HMG CoA reductase (Correct Answer)
B. Thiokinase
C. Mevalonate kinase
D. HMG CoA synthase

Explanation: ***HMG CoA reductase*** - **HMG-CoA reductase** catalyzes the conversion of **HMG-CoA** to **mevalonate**, which is the committed and rate-limiting step in cholesterol synthesis. - This enzyme is a major target for **statins**, a class of drugs used to lower cholesterol levels by inhibiting its activity. *Thiokinase* - **Thiokinase** (or fatty acyl-CoA synthetase) is involved in activating fatty acids for metabolism, not directly in cholesterol synthesis. - It catalyzes the formation of **fatty acyl-CoA** from fatty acids and CoA, a step in fatty acid metabolism. *Mevalonate kinase* - **Mevalonate kinase** catalyzes the phosphorylation of mevalonate to **5-phosphomevalonate**. - While essential for cholesterol synthesis, this step occurs after the rate-limiting step and is not the primary regulatory point. *HMG CoA synthase* - **HMG-CoA synthase** catalyzes the condensation of **acetoacetyl-CoA** with **acetyl-CoA** to form **HMG-CoA**. - This step occurs before the reduction of HMG-CoA by HMG-CoA reductase and is not the rate-limiting enzyme in the pathway.

Practice by Chapter

Lipid Classification and Chemistry

Practice Questions

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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