Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 751: The human plasma lipoprotein containing the highest percentage of triacylglycerol by weight is which one?

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

Explanation: ***Chylomicrons*** - Chylomicrons are primarily responsible for transporting **exogenous (dietary) triglycerides** from the intestines to peripheral tissues. - They are the **largest and least dense** lipoproteins, composed of approximately 85-90% triacylglycerol by weight. *VLDL* - **Very Low-Density Lipoproteins (VLDL)** transport **endogenous (liver-synthesized) triglycerides** to peripheral tissues. - While rich in triglycerides (about 50% by weight), their percentage is significantly lower than that of chylomicrons. *HDL* - **High-Density Lipoproteins (HDL)** are primarily involved in **reverse cholesterol transport**, moving cholesterol from peripheral tissues back to the liver. - They have the **highest percentage of protein** and the lowest percentage of triacylglycerol among the major lipoproteins. *LDL* - **Low-Density Lipoproteins (LDL)** are the primary transporters of **cholesterol** to peripheral tissues. - They are derived from VLDL and contain a high percentage of cholesterol esters, with a relatively low percentage of triacylglycerol.

Question 752: A person is diagnosed with familial type IIa hyperlipoproteinemia. What is the basic defect in this type of hyperlipoproteinemia?

A. Lipoprotein lipase deficiency
B. Defective LDL receptor (Correct Answer)
C. Abnormal activity of Apo E
D. Overproduction of LDL

Explanation: ***Defective LDL receptor*** - **Familial hypercholesterolemia** (Type IIa hyperlipoproteinemia) is characterized by high levels of **LDL cholesterol** due to a genetic defect in the **LDL receptor** gene. - This defective receptor leads to impaired clearance of LDL particles from the bloodstream, resulting in their accumulation. *Lipoprotein lipase deficiency* - This defect is associated with **Type I hyperlipoproteinemia**, which is characterized by elevated **chylomicrons** and **triglycerides**, not primarily LDL cholesterol. - **Lipoprotein lipase (LPL)** is essential for the hydrolysis of triglycerides in chylomicrons and VLDL. *Abnormal activity of Apo E* - Variants of **Apolipoprotein E (Apo E)**, particularly Apo E2, are associated with **Type III hyperlipoproteinemia** (familial dysbetalipoproteinemia). - This condition involves increased levels of **chylomicron remnants** and **VLDL remnants** (IDL), not primarily isolated LDL elevation. *Overproduction of LDL* - While increased **LDL production** can contribute to elevated LDL levels, the primary genetic defect in familial type IIa hyperlipoproteinemia is strictly related to the impaired **clearance** of LDL due to a defective **LDL receptor**, rather than solely overproduction. - Many secondary causes of hypercholesterolemia can involve LDL overproduction, but Type IIa is specifically linked to the receptor defect.

Question 753: NADPH is required in which of the following cellular processes?

A. HMP shunt
B. Glycogenolysis
C. Gluconeogenesis
D. Lipogenesis (Correct Answer)

Explanation: ***Lipogenesis*** - **NADPH** is critically required for anabolic processes such as **fatty acid synthesis** (lipogenesis), where it acts as a **reducing agent**. - It supplies the electrons necessary for the sequential reduction steps in the conversion of acetyl-CoA to fatty acids in the cytoplasm. *HMP shunt* - The **hexose monophosphate (HMP) shunt**, also known as the **pentose phosphate pathway**, is the primary cellular source of **NADPH**. - Therefore, it produces NADPH rather than requiring it as a substrate for its main function. *Gluconeogenesis* - **Gluconeogenesis** is the metabolic pathway that produces **glucose** from non-carbohydrate precursors. - This process primarily uses **ATP** and **GTP** as energy sources, and NADH (not NADPH) is involved in some reduction reactions. *Glycogenolysis* - **Glycogenolysis** is the breakdown of **glycogen** into glucose-6-phosphate and then glucose. - This catabolic process does not directly require **NADPH**; instead, it releases glucose for energy or other metabolic uses.

Question 754: Which of the following enzymes is not required for the formation of estradiol?

A. Aromatase
B. 3β-hydroxysteroid dehydrogenase
C. 11β-hydroxylase (Correct Answer)
D. 17α-hydroxylase

Explanation: ***11β-hydroxylase*** - This enzyme is crucial for the **synthesis of cortisol** and **aldosterone** within the adrenal cortex, converting 11-deoxycortisol to cortisol and 11-deoxycorticosterone to corticosterone. - It plays no direct role in the synthesis pathway of **estrogen**, specifically estradiol, which is synthesized from androgens. *3β-hydroxysteroid dehydrogenase* - This enzyme is required for multiple steps in steroidogenesis, including the conversion of **pregnenolone to progesterone** and **DHEA to androstenedione**, both of which are precursors to estrogens like estradiol. - Its activity is essential for moving from **Δ5 steroids** to **Δ4 steroids**, an early and critical step in androgen and subsequent estrogen synthesis. *Aromatase* - **Aromatase (CYP19A1)** is the enzyme directly responsible for converting androgens (**androstenedione and testosterone**) into estrogens (**estrone and estradiol**, respectively). - It catalyzes the **aromatization of the A-ring** of the steroid structure, a defining step in estrogen synthesis. *17α-hydroxylase* - This enzyme (CYP17A1) is involved in crucial steps leading up to estrogen synthesis, including the conversion of **progesterone to 17α-hydroxyprogesterone** and **pregnenolone to 17α-hydroxypregnenolone**. - Its activity is necessary for the formation of **androgens** (like DHEA and androstenedione), which are direct precursors for estrogen synthesis.

Question 755: Which of the following is not a ketone body produced by the liver?

A. Acetone
B. β-hydroxybutyrate
C. Acetoacetate
D. Glycerol 3-phosphate (Correct Answer)

Explanation: ***Glycerol 3-phosphate*** - **Glycerol 3-phosphate** is a molecule involved in **triglyceride synthesis** and glycolysis, not a ketone body produced by the liver. - It is formed from **dihydroxyacetone phosphate** (a glycolysis intermediate) or by phosphorylation of **glycerol**. *β-hydroxybutyrate* - **β-hydroxybutyrate** is one of the primary **ketone bodies** produced by the liver. - It is formed from **acetoacetate** and is a major energy source during prolonged fasting or ketogenic states. *Acetoacetate* - **Acetoacetate** is a principal **ketone body** synthesized by the liver. - It is an intermediate formed during the breakdown of **fatty acids** and supplies energy to peripheral tissues. *Acetone* - **Acetone** is a ketone body that arises from the **spontaneous decarboxylation of acetoacetate**. - While produced by the liver, it is primarily **excreted through respiration** and is not used as an energy source by peripheral tissues.

Question 756: Which apolipoprotein is the primary structural component of LpA-I particles?

A. Apo B-48
B. Apo A-I (Correct Answer)
C. Apo A-II
D. Apo B-100

Explanation: ***Apo A-I*** - **Apolipoprotein A-I (Apo A-I)** is the main structural and functional protein of **high-density lipoprotein (HDL)**. - It plays a crucial role in **reverse cholesterol transport**, facilitating the removal of excess cholesterol from peripheral tissues back to the liver. *Apo B-48* - **Apo B-48** is found exclusively in **chylomicrons**, which are responsible for transporting dietary lipids from the intestines. - It is synthesized in the **intestine** and is critical for the assembly and secretion of chylomicrons. *Apo A-II* - **Apo A-II** is another apolipoprotein found in HDL particles, but it is not the primary structural component. - While present, it is less abundant than Apo A-I and its precise role is still being researched, though it may influence **HDL metabolism**. *Apo B-100* - **Apo B-100** is the primary structural protein of **low-density lipoprotein (LDL)** and very-low-density lipoprotein (VLDL). - It is essential for the binding of LDL to the **LDL receptor**, mediating the uptake of cholesterol into cells.

Question 757: What is the primary role of cholesterol in low-density lipoprotein (LDL) metabolism?

A. Cholesterol binds to receptors on cell membranes.
B. Excess cholesterol in cells reduces the number of LDL receptors. (Correct Answer)
C. Cholesterol regulates the activity of enzymes involved in cholesterol metabolism.
D. Cholesterol in LDL is primarily involved in transporting cholesterol to tissues.

Explanation: ***Excess cholesterol in cells reduces the number of LDL receptors.*** - High intracellular **cholesterol levels** signal the cell to *downregulate* the production of **LDL receptors** via the **SREBP-2 pathway**. - This negative feedback mechanism prevents excessive accumulation of cholesterol within cells and maintains cellular **cholesterol homeostasis**. - This is the primary regulatory mechanism specifically related to **LDL receptor-mediated metabolism**. *Cholesterol binds to receptors on cell membranes.* - It is actually the **LDL particle**, specifically its **apolipoprotein B-100 (apoB-100)** component, that binds to the **LDL receptors** on cell membranes. - While cholesterol is the cargo within LDL, it does not directly bind to the receptors itself. *Cholesterol regulates the activity of enzymes involved in cholesterol metabolism.* - While **intracellular cholesterol levels** do regulate various enzymes (e.g., **HMG-CoA reductase** via SREBP-2, and **ACAT**), this describes cholesterol's broader role in **cholesterol synthesis regulation** rather than specifically in **LDL metabolism**. - The question asks specifically about cholesterol's role in **LDL metabolism**, which refers to the receptor-mediated pathway and its regulation. *Cholesterol in LDL is primarily involved in transporting cholesterol to tissues.* - This statement describes the *function of LDL itself*, which is to transport cholesterol to peripheral tissues. - However, the question asks for the **primary role of cholesterol *in* LDL metabolism**, referring to its regulatory effects on the LDL receptor pathway rather than its transport function.

Question 758: What is the effect of progesterone on lipids?

A. Lowers HDL and lowers LDL
B. Lowers HDL and increases LDL (Correct Answer)
C. Lowers LDL, increases HDL
D. Increases LDL and HDL

Explanation: ***Lowers HDL and increases LDL*** - This describes the effect of **synthetic progestins** (particularly older generation ones like levonorgestrel) on lipid profiles. - Synthetic progestins have been shown to **decrease HDL cholesterol** and **increase LDL cholesterol**, contributing to an unfavorable cardiovascular risk profile. - **Natural progesterone** has minimal or neutral effects on lipids, but this question refers to the progestin effects commonly discussed in contraceptive and hormone replacement therapy contexts. - This is the **classical teaching** for progesterone effects on lipids in most medical textbooks. *Lowers LDL, increases HDL* - This effect is characteristic of **estrogen**, not progesterone. - Estrogen improves lipid profiles by increasing HDL and lowering LDL cholesterol. - Progestins generally have opposite or antagonistic effects compared to estrogen on lipid metabolism. *Lowers HDL and lowers LDL* - While synthetic progestins do lower HDL, they typically **increase LDL**, not lower it. - A simultaneous decrease in both HDL and LDL is not a characteristic effect of progesterone or progestins. *Increases LDL and HDL* - Synthetic progestins tend to increase LDL, but they typically **lower HDL**, not increase it. - An increase in both LDL and HDL simultaneously is not a typical effect of progesterone on lipid metabolism.

Question 759: Lipoprotein lipase is activated by:

A. Apo C3
B. Apo C2 (Correct Answer)
C. Apo C1
D. Apo A1

Explanation: ***Apo C2*** - **Apo C2** (apolipoprotein C-II) acts as a **cofactor** for **lipoprotein lipase (LPL)**. - Its presence is essential for LPL to efficiently **hydrolyze triglycerides** within chylomicrons and VLDL, releasing fatty acids for tissue uptake. *Apo C3* - **Apo C3** is known to **inhibit** lipoprotein lipase activity, which is the opposite of activation. - It plays a role in slowing down the clearance of triglyceride-rich lipoproteins by interfering with LPL function. *Apo C1* - **Apo C1** (apolipoprotein C-I) plays a role in **activating lecithin-cholesterol acyltransferase (LCAT)** and may inhibit cholesteryl ester transfer protein (CETP), but it does not directly activate LPL. - Its primary functions are related to **cholesterol metabolism** and reverse cholesterol transport. *Apo A1* - **Apo A1** (apolipoprotein A-I) is the major protein component of **high-density lipoprotein (HDL)**. - It is a potent **activator of LCAT**, which is crucial for cholesterol esterification in HDL, but it does not activate LPL.

Question 760: Which of the following statements about trans fatty acids is false?

A. Refining reduces the level of TFA (Correct Answer)
B. Hydrogenation increases the level of TFA
C. Increases LDL cholesterol levels
D. Fried rice has a high content of TFA

Explanation: ***Refining reduces the level of TFA*** - This statement is **false** because refining processes, particularly high-temperature deodorization during oil refining, can actually *increase* the formation of **trans fatty acids (TFAs)** through thermal isomerization of cis unsaturated fatty acids. - While refining removes impurities and improves oil stability, temperatures above 200°C during deodorization can convert some cis bonds to trans configuration, typically resulting in 1-3% TFA formation. *Hydrogenation increases the level of TFA* - **Partial hydrogenation** is the primary industrial source of **trans fatty acids (TFAs)**, converting liquid vegetable oils into semi-solid fats. - During this process, hydrogen is added to unsaturated fatty acids, and some double bonds shift from the natural *cis* configuration to the *trans* configuration, potentially creating 25-45% TFA content. - This statement is **true**. *Increases LDL cholesterol levels* - **Trans fatty acids (TFAs)** have a well-established dual negative effect: they raise **LDL cholesterol** ("bad" cholesterol) and lower **HDL cholesterol** ("good" cholesterol). - Even small amounts of TFA intake (1-3% of total energy) significantly increase **cardiovascular disease** risk. - This statement is **true**. *Fried rice has a high content of TFA* - This statement is **generally true** when referring to commercially prepared fried rice in settings where **partially hydrogenated oils** or repeatedly used/degraded cooking oils are employed. - However, home-cooked fried rice using fresh vegetable oils contains minimal TFAs, as stir-frying at typical cooking temperatures (150-200°C) produces negligible trans fat formation. - The high TFA content is primarily associated with commercial/restaurant preparation using poor quality or hydrogenated fats, not the dish itself.

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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