Lipid Metabolism Practice Questions

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

Carnitine. ***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.

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

Docosahexaenoic acid. ***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.

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

APOE4. ***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.

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

Chylomicrons. ***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.

Q: Which of the following statements about adiponectin is incorrect?

Positive Correlation with BMI. ***Positive Correlation with BMI (INCORRECT STATEMENT)*** - Adiponectin levels are **inversely correlated with BMI**, NOT positively correlated; as BMI increases, adiponectin levels generally decrease. - This inverse relationship is significant because lower adiponectin levels are associated with increased insulin resistance and **metabolic syndrome**. - This statement is **false**, making it the correct answer to this question. *Secreted by adipose tissue (Correct statement)* - Adiponectin is a **hormone primarily secreted by adipocytes** (fat cells). - It plays a crucial role in regulating glucose and lipid metabolism, and its secretion is altered in conditions like obesity. - This statement is **true**. *Lowers glucose (Correct statement)* - Adiponectin **enhances insulin sensitivity** in peripheral tissues like skeletal muscle and liver, leading to increased glucose uptake and utilization. - This action helps to **lower blood glucose levels** and improve glycemic control. - This statement is **true**. *Increases FFA oxidation (Correct statement)* - Adiponectin **promotes fatty acid oxidation** in muscle and liver, reducing intracellular lipid accumulation. - By increasing fatty acid burning, it helps to **decrease circulating free fatty acid (FFA) levels**, which can contribute to insulin resistance if elevated. - This statement is **true**.

Q: Which of the following stimulates Acetyl CoA Carboxylase?

Citrate. ***Citrate*** - **Citrate** is an allosteric activator of **Acetyl-CoA Carboxylase (ACC)**, indicating abundant energy and precursor availability for fatty acid synthesis. - This activation promotes the conversion of **Acetyl-CoA** to **Malonyl-CoA**, the committed step in **fatty acid synthesis**. *Starvation* - **Starvation** leads to energy deficit, which generally **inhibits** anabolic processes like fatty acid synthesis. - In this state, enzymes involved in anabolic pathways are often downregulated or inhibited to conserve energy. *Glucagon* - **Glucagon** is a hormone that signals low blood glucose and promotes catabolic processes such as **glycogenolysis** and **gluconeogenesis**. - It **inhibits** fatty acid synthesis by phosphorylating and inactivating **Acetyl-CoA Carboxylase**, thus opposing citrate's activating effect. *None of the options* - **Citrate** is a known stimulator of Acetyl CoA Carboxylase. - This option is incorrect because there is a correct answer among the choices.

Q: Apo B48 is synthesized in -

Intestine. ***Intestine*** - **Apo B48** is a truncated form of apolipoprotein B-100, uniquely synthesized in the **intestine** through RNA editing. - It is a crucial structural component of **chylomicrons**, which are lipoprotein particles responsible for transporting exogenous dietary lipids from the intestine to other tissues. *Liver* - The liver primarily synthesizes **Apo B100**, which is a full-length apolipoprotein B and a major component of VLDL, IDL, and LDL. - It does not produce Apo B48. *Kidney* - The kidneys are involved in filtering waste products and regulating fluid balance, but they do not play a role in the synthesis of apolipoproteins like Apo B48. - Kidney cells are not equipped with the specific machinery for Apo B mRNA editing. *RBCs* - Red blood cells (RBCs) are primarily responsible for oxygen transport and lack a nucleus and most organelles, including those required for protein synthesis. - Therefore, RBCs cannot synthesize proteins such as Apo B48.

Q: Cholesterol is not a precursor for the synthesis of which of the following?

Lipocortin. ***Lipocortin*** - **Lipocortin** (also known as annexin A1) is a **protein**, not a steroid compound - It is synthesized via **protein translation** from mRNA, not from cholesterol - Mediates anti-inflammatory effects of glucocorticoids and is involved in cell growth regulation - **Key point:** Only steroids and bile acids are derived from cholesterol, not proteins *Testosterone* - **Testosterone** is an androgen (male sex hormone) **synthesized from cholesterol** - Cholesterol → Pregnenolone → DHEA → Androstenedione → Testosterone - Produced in the gonads (Leydig cells) and adrenal glands *Cortisol* - **Cortisol** is a glucocorticoid hormone **derived from cholesterol** - Cholesterol → Pregnenolone → 17-hydroxypregnenolone → Cortisol - Synthesized in the zona fasciculata of the adrenal cortex *Aldosterone* - **Aldosterone** is a mineralocorticoid hormone **synthesized from cholesterol** - Cholesterol → Pregnenolone → Progesterone → Aldosterone - Produced in the zona glomerulosa of the adrenal cortex

Q: HDL is called good cholesterol because -

Removes cholesterol from peripheral tissues. ***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.

Q: Epinephrine increases free fatty acid levels by causing which of the following?

Increasing lipolysis. ***Increasing lipolysis*** - Epinephrine activates **hormone-sensitive lipase** in adipose tissue through a **cAMP-dependent mechanism**, leading to the breakdown of stored triglycerides into free fatty acids and glycerol. - This process, known as **lipolysis**, directly increases the release of free fatty acids into the bloodstream. *Increased fatty acid synthesis* - **Fatty acid synthesis** is a process that builds fatty acids, which would decrease, not increase, free fatty acid levels in the blood. - Epinephrine's primary action is to mobilize energy reserves, which involves breaking down stored fats rather than synthesizing new ones. *Increasing cholesterol catabolism* - While cholesterol metabolism is important, epinephrine does not directly or significantly increase **cholesterol catabolism** as a primary mechanism for raising free fatty acid levels. - The catabolism of cholesterol primarily involves its conversion to bile acids and steroid hormones, which is distinct from fatty acid release. *None of the options* - This option is incorrect because increasing lipolysis is a direct and well-established mechanism by which epinephrine raises free fatty acid levels.

Question 1

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

Accepted Answer

Carnitine

Answer

Acyl carrier protein

Answer

Lecithin-cholesterol acyltransferase

Answer

Carnitine and albumin

Question 2

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

Accepted Answer

Docosahexaenoic acid

Answer

Arachidonic

Answer

Palmitic acid

Answer

Linoleic acid

Question 3

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

Accepted Answer

APOE4

Answer

APOE3

Answer

APOE2

Answer

APOB

Question 4

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

Accepted Answer

Chylomicrons

Answer

High-Density Lipoprotein (HDL)

Answer

Very Low-Density Lipoprotein (VLDL)

Answer

Low-Density Lipoprotein (LDL)

Question 5

Which of the following statements about adiponectin is incorrect?

Accepted Answer

Positive Correlation with BMI

Answer

Secreted by adipose tissue

Answer

Increases FFA oxidation

Answer

Lowers glucose

Question 6

Which of the following stimulates Acetyl CoA Carboxylase?

Accepted Answer

Citrate

Answer

Starvation

Answer

Glucagon

Answer

None of the options

Question 7

Apo B48 is synthesized in -

Accepted Answer

Intestine

Answer

Liver

Answer

Kidney

Answer

RBCs

Question 8

Cholesterol is not a precursor for the synthesis of which of the following?

Accepted Answer

Lipocortin

Answer

Testosterone

Answer

Cortisol

Answer

Aldosterone

Question 9

HDL is called good cholesterol because -

Accepted Answer

Removes cholesterol from peripheral tissues

Answer

Increases cholesterol delivery to peripheral tissues

Answer

Stimulates cholesterol synthesis in the liver

Answer

Activates enzymes that break down triglycerides

Question 10

Epinephrine increases free fatty acid levels by causing which of the following?

Accepted Answer

Increasing lipolysis

Answer

Increasing fatty acid synthesis

Answer

Increasing cholesterol catabolism

Answer

None of the options

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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