Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 661: Rate limiting step in fatty acid synthesis is?

A. Production of acetyl CoA
B. Production of oxaloacetate
C. Production of malonyl-CoA (Correct Answer)
D. Production of citrate

Explanation: ***Production of malonyl-CoA*** - The conversion of **acetyl-CoA to malonyl-CoA** is catalyzed by **acetyl-CoA carboxylase (ACC)**, which is the **rate-limiting enzyme** in fatty acid synthesis. - This step is highly regulated by **hormones** (e.g., insulin activates, glucagon inactivates) and **nutrient availability**, controlling the overall flux of fatty acid production. *Production of acetyl CoA* - While **acetyl-CoA** is the precursor for fatty acid synthesis, its production (e.g., from glycolysis via pyruvate dehydrogenase) is not the rate-limiting step for the synthesis pathway itself. - The availability of **acetyl-CoA** influences the pathway, but the committed step occurs later. *Production of oxaloacetate* - **Oxaloacetate** is primarily involved in the **citric acid cycle** and gluconeogenesis, and its production is not directly the rate-limiting step in fatty acid synthesis. - It combines with acetyl-CoA to form citrate, allowing acetyl-CoA to be shuttled out of the mitochondria. *Production of citrate* - **Citrate** is formed when **acetyl-CoA and oxaloacetate** combine in the mitochondria and is then transported to the cytoplasm to provide acetyl-CoA for fatty acid synthesis. - Although the availability of **cytoplasmic citrate** is important as a precursor for cytoplasmic acetyl-CoA and an allosteric activator of ACC, its production is not the rate-limiting step of fatty acid synthesis itself.

Question 662: What is the primary defect associated with type II hyperlipidemia?

A. Apo-E
B. Lipoprotein lipase
C. LDL receptor (Correct Answer)
D. HMG-CoA reductase

Explanation: ***LDL receptor*** - A defect in the **LDL receptor** leads to type II hyperlipidemia, characterized by elevated **LDL cholesterol** levels in the blood [1]. - This condition results in increased risk for **atherosclerosis** and cardiovascular diseases due to impaired cellular uptake of cholesterol [1,2]. *Apo-E* - Deficiencies of **Apo-E** typically result in type III hyperlipidemia, associated with **remnant lipoprotein clearance** issues rather than type II. - It affects metabolism of **chylomicron remnants** and intermediate density lipoproteins (IDL), not primarily LDL. *None* - This option incorrectly suggests that there is no defect associated with type II hyperlipidemia; in reality, it is primarily linked to **LDL receptor** dysfunction [1]. - The term "none" implies a lack of specific pathology, which is inaccurate in the context of hyperlipidemia types. *Lipoprotein lipase* - Deficiency in **lipoprotein lipase** leads to type I (or V) hyperlipidemia, characterized by increased **triglyceride** levels rather than just LDL. - It primarily impairs the hydrolysis of triglycerides in chylomicrons and VLDL, which differs from the LDL receptor's function [2]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 157-159. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 156-157.

Question 663: Which of the following is increased in lipoprotein lipase deficiency?

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

Explanation: ***Chylomicrons*** - **Lipoprotein lipase (LPL)** is essential for hydrolyzing triglycerides within **chylomicrons** and VLDL, allowing fatty acids to be taken up by tissues. - Deficiency in LPL leads to a significant accumulation of **chylomicrons** in the plasma, as their degradation is impaired, resulting in **hypertriglyceridemia**. *VLDL* - While LPL also breaks down **VLDL**, the primary and most dramatic accumulation in LPL deficiency is seen with **chylomicrons** due to their larger triglyceride content and direct dependence on LPL for clearance after meals. - **VLDL** levels might also be elevated, but the hallmark is the very high **chylomicron** levels. *LDL* - **LDL** is formed from the catabolism of VLDL, and its levels are generally not directly increased due to a primary LPL deficiency. - LPL's main role is in triglyceride-rich lipoprotein metabolism, not directly in **LDL** catabolism. *HDL* - **HDL** plays a role in reverse cholesterol transport and is not directly metabolized by lipoprotein lipase. - In fact, in severe hypertriglyceridemia due to LPL deficiency, **HDL** levels may sometimes be low rather than increased.

Question 664: Aromatase produces estrogen from -

A. Progesterone
B. Cortisol
C. Aldosterone
D. Androgen (Correct Answer)

Explanation: ***Androgen*** - **Aromatase** is an enzyme complex that converts **androgens** (specifically androstenedione and testosterone) into **estrogens** (estrone and estradiol, respectively). - This conversion is a key step in **estrogen biosynthesis** and occurs in various tissues, including the ovaries, placenta, brain, and adipose tissue. *Progesterone* - **Progesterone** is a precursor to androgens, but it is not directly converted to estrogen by aromatase. - It plays a primary role in the **menstrual cycle** and **pregnancy**. *Cortisol* - **Cortisol** is a **glucocorticoid hormone** produced in the adrenal cortex and is not a substrate for aromatase. - Its primary functions relate to stress response, metabolism, and immune regulation. *Aldosterone* - **Aldosterone** is a **mineralocorticoid hormone** produced in the adrenal cortex and is not involved in estrogen synthesis. - It primarily regulates **blood pressure** and electrolyte balance.

Question 665: In Retinitis pigmentosa, which fatty acid is found to be decreased in the retina?

A. Arachidonic
B. Palmitic acid
C. Linoleic acid
D. Docosahexaenoic acid (Correct Answer)

Explanation: ***Docosahexaenoic acid*** - **Docosahexaenoic acid (DHA)** is a crucial **omega-3 fatty acid** abundantly found in the **photoreceptor outer segments** of the retina. - Its reduced levels are linked to photoreceptor dysfunction and degeneration seen in conditions like **Retinitis Pigmentosa**, influencing membrane fluidity and visual function. - DHA comprises up to **50% of the fatty acids** in photoreceptor membranes and is essential for maintaining retinal structure and function. *Arachidonic acid* - **Arachidonic acid** is an **omega-6 fatty acid** primarily involved in **inflammatory pathways** and cellular signaling. - While present in the retina, its decrease is not typically associated with the pathogenesis of **Retinitis Pigmentosa**. *Linoleic acid* - **Linoleic acid** is an **omega-6 essential fatty acid** and a precursor to **arachidonic acid**, but it is not a direct structural component of photoreceptor membranes. - Its levels are not specifically decreased in **Retinitis Pigmentosa** as a primary factor. *Palmitic acid* - **Palmitic acid** is a **saturated fatty acid** present in retinal membranes but not specifically concentrated in photoreceptors. - Its levels are not characteristically decreased in **Retinitis Pigmentosa**, unlike the omega-3 fatty acid DHA.

Question 666: Which of the following helps in the transport of fatty acids across the inner mitochondrial membrane?

A. Acyl carrier protein
B. Carnitine (Correct Answer)
C. Lecithin-cholesterol acyltransferase
D. Carnitine and albumin

Explanation: ***Carnitine*** - **Carnitine** plays a crucial role in transporting long-chain fatty acids from the **cytosol** into the **mitochondrial matrix** for beta-oxidation. - It acts as a shuttling molecule, forming **acylcarnitine** which can cross the inner mitochondrial membrane via the **carnitine-acylcarnitine translocase**. *Acyl carrier protein* - **Acyl carrier protein (ACP)** is primarily involved in **fatty acid synthesis** in the cytoplasm, not in the transport of fatty acids into mitochondria for degradation. - It carries acyl groups during the elongation reactions of fatty acid synthesis. *Lecithin-cholesterol acyltransferase* - **Lecithin-cholesterol acyltransferase (LCAT)** is an enzyme found in plasma that catalyzes the formation of **cholesterol esters**, which are then transported by lipoproteins. - It is involved in **cholesterol metabolism** and reverse cholesterol transport, not in the mitochondrial transport of fatty acids. *Carnitine and albumin* - While **carnitine** is essential for mitochondrial fatty acid transport, **albumin** transports fatty acids in the blood plasma, from adipose tissue to other tissues. - Albumin does not transport fatty acids across the inner mitochondrial membrane; its role is extra-mitochondrial and related to systemic transport.

Question 667: What is the type of cholesterol primarily found in gallstones?

A. Amorphous cholesterol monohydrate.
B. Amorphous cholesterol dihydrate.
C. Crystalline cholesterol dihydrate.
D. Crystalline cholesterol monohydrate. (Correct Answer)

Explanation: ***Crystalline cholesterol monohydrate*** - The predominant type of cholesterol found in gallstones is **crystalline cholesterol monohydrate** [1], which reflects the solid form of cholesterol precipitating in bile. - It is often associated with **cholesterol gallstones**, occurring when bile contains excessive cholesterol or insufficient bile salts. *Crystalline Cholesterol dihydrate* - Crystalline cholesterol dihydrate is less commonly associated with gallstones and generally forms in different circumstances, not typical of cholesterol stones. - This type does not represent the main component of gallstones, which primarily consist of monohydrate forms. *Amorphous cholesterol dihydrate* - Amorphous cholesterol dihydrate is not a recognized form typically found in gallstones, as gallstone pathology focuses on crystallized forms. - Amorphous substances are less stable than crystalline forms, making this option unlikely in the context of gallstones. *Amorphous cholesterol monohydrate* - Amorphous cholesterol monohydrate is not the major component found in gallstones; gallstones are more likely to be crystalline in structure. - This form lacks the stable crystalline structure needed to precipitate and form gallstones effectively. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Liver and Gallbladder, p. 882.

Question 668: In a patient with lipoprotein lipase deficiency, which of the following is increased following a fatty meal?

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

Explanation: ***Chylomicrons*** - **Lipoprotein lipase (LPL)** is essential for the breakdown of **triglycerides** within chylomicrons, which are absorbed after a fatty meal. - In LPL deficiency, **chylomicron clearance** from the bloodstream is impaired, leading to their accumulation and elevated levels. *High-Density Lipoprotein (HDL)* - HDL particles are primarily involved in **reverse cholesterol transport** and are not directly affected by LPL deficiency in terms of their synthesis or initial concentration post-meal. - While LPL activity can influence HDL metabolism, a deficiency would not directly cause an acute increase in HDL levels following a fatty meal. *Very Low-Density Lipoprotein (VLDL)* - VLDL is synthesized in the liver to transport **endogenous triglycerides**, rather than dietary triglycerides. - While prolonged LPL deficiency can indirectly affect VLDL metabolism, it is not the primary lipoprotein that acutely accumulates in response to a fatty meal in this condition. *Low-Density Lipoprotein (LDL)* - LDL is formed from the catabolism of VLDL and **intermediate-density lipoprotein (IDL)**. - In LPL deficiency, the initial step of **triglyceride hydrolysis** for chylomicrons is impaired, making it unlikely for LDL to increase immediately after a fatty meal.

Question 669: Which apolipoprotein is associated with an increased risk of Alzheimer's disease?

A. APOE4 (Correct Answer)
B. APOE3
C. APOE2
D. APOB

Explanation: ***APOE4*** - The **APOE4 allele** is the strongest genetic risk factor for **late-onset Alzheimer's disease (AD)**, significantly increasing the risk and lowering the age of onset. - Individuals with one copy of APOE4 have a 2-3 times higher risk, while those with two copies have an 8-12 times higher risk of developing AD compared to those with APOE3. *APOE3* - **APOE3** is the most common allele and is considered a **neutral risk factor** for Alzheimer's disease, serving as the reference for risk comparison. - It plays a normal role in **lipid metabolism** and **cholesterol transport** in the brain. *APOE2* - The **APOE2 allele** is associated with a **reduced risk** of Alzheimer's disease. - It is believed to be **protective** against AD, possibly by **improving amyloid-beta clearance** or reducing neuroinflammation. *APOB* - **Apolipoprotein B (APOB)** is primarily involved in the assembly and secretion of **chylomicrons** and **very-low-density lipoproteins (VLDL)** in the liver and intestine. - While important for lipid metabolism, it is **not directly implicated** as a genetic risk factor for Alzheimer's disease in the same way APOE variants are.

Question 670: HDL is called good cholesterol because -

A. Removes cholesterol from peripheral tissues (Correct Answer)
B. Increases cholesterol delivery to peripheral tissues
C. Stimulates cholesterol synthesis in the liver
D. Activates enzymes that break down triglycerides

Explanation: ***Removes cholesterol from peripheral tissues*** - **High-density lipoprotein (HDL)** is known as "good cholesterol" due to its role in **reverse cholesterol transport**, a process where it collects excess cholesterol from peripheral cells and tissues. - This action helps to prevent the accumulation of cholesterol in arteries, thereby reducing the risk of **atherosclerosis** and cardiovascular disease. - HDL then transports this cholesterol to the liver for excretion via bile, completing the protective cycle. *Increases cholesterol delivery to peripheral tissues* - This is actually the opposite of HDL's function and describes the role of **LDL (low-density lipoprotein)**, which is considered "bad cholesterol." - LDL delivers cholesterol to peripheral tissues, and excess LDL can lead to **atherosclerotic plaque formation**. *Stimulates cholesterol synthesis in the liver* - HDL does not directly stimulate cholesterol synthesis in the liver; rather, its role is primarily in **cholesterol efflux** from cells and transport. - The liver's cholesterol synthesis is regulated by various factors, including dietary intake and cellular cholesterol levels via the **SREBP pathway**, but HDL does not upregulate hepatic cholesterol synthesis. *Activates enzymes that break down triglycerides* - While HDL does activate **LCAT (lecithin-cholesterol acyltransferase)** for cholesterol esterification, its primary "good" function is not the breakdown of triglycerides. - **Lipoprotein lipase (LPL)** is the primary enzyme responsible for triglyceride breakdown in lipoproteins like VLDL and chylomicrons.

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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