Lipid Metabolism Practice Questions

Q: One of the following has the least density:

Chylomicron. **Chylomicron** - Chylomicrons are the **largest** and **least dense** of all lipoproteins due to their very high triglyceride content. - Their primary function is to transport **dietary lipids** from the intestines to other parts of the body. *HDL* - **High-density lipoprotein (HDL)** is the **most dense** lipoprotein due to its high protein-to-lipid ratio. - It is responsible for **reverse cholesterol transport**, removing excess cholesterol from tissues. *VLDL* - **Very low-density lipoprotein (VLDL)** is less dense than LDL and HDL, but **denser than chylomicrons**. - Its main role is to transport **endogenously synthesized triglycerides** from the liver to peripheral tissues. *LDL* - **Low-density lipoprotein (LDL)** is denser than VLDL and chylomicrons but less dense than HDL. - It primarily transports **cholesterol** from the liver to peripheral cells, and elevated levels are associated with increased cardiovascular risk.

Q: Active metabolite in the synthesis of fatty acids is:

Malonyl CoA. ***Malonyl CoA*** - **Malonyl CoA** is the immediate **two-carbon donor** in fatty acid synthesis, formed from acetyl CoA and bicarbonate. - It adds **two-carbon units** to the growing fatty acid chain during each cycle of synthesis, making it the primary active metabolic form in this process. *Stearate (an end product of fatty acid synthesis)* - **Stearate** is a **saturated fatty acid end product** (C18:0) of fatty acid synthesis, not an active metabolite that directly participates in the elongation process. - While it is a result of fatty acid synthesis, it does not serve as a building block for further elongation in the manner of malonyl CoA. *Acetyl CoA (a precursor in fatty acid synthesis)* - **Acetyl CoA** is the **initial precursor** for fatty acid synthesis, which is then carboxylated to form malonyl CoA. - It is not the *active* two-carbon donor during the elongation steps of fatty acid synthesis itself, but rather the substrate for malonyl CoA synthesis. *Palmitate (an end product of fatty acid synthesis)* - **Palmitate** is the **primary 16-carbon saturated fatty acid** and is the usual end product of *de novo* fatty acid synthesis in humans. - Like stearate, it is an end product and does not serve as an active metabolic intermediate for chain elongation during the synthesis process itself.

Q: In a preterm baby with respiratory distress syndrome, which of the following lipids would be deficient?

Lecithin. ***Lecithin*** - **Lecithin** (also known as **phosphatidylcholine**) is the primary component of **surfactant** in the lungs, which reduces surface tension and prevents alveolar collapse. - In **preterm babies**, insufficient production of lecithin due to immature lung development leads to **respiratory distress syndrome (RDS)**. *Cardiolipin* - **Cardiolipin** is a major phospholipid found in the **inner mitochondrial membrane**, crucial for oxidative phosphorylation. - Deficiency is associated with mitochondrial disorders like **Barth syndrome**, not primary respiratory distress. *Sphingomyelin* - **Sphingomyelin** is a significant component of **cell membranes** and **myelin sheaths**, important for nerve insulation. - While present in the lungs, its primary role is not in surface tension reduction, and its deficiency is not directly linked to RDS. *Phosphatidylinositol* - **Phosphatidylinositol** is a precursor for various **signaling molecules** and plays a role in cell membrane structure. - While involved in cellular processes, it is not the critical surfactant component whose deficiency causes RDS.

Q: Type-I hyperlipoproteinemia is caused by deficiency of:-

Lipoprotein lipase. ***Lipoprotein lipase*** - **Type I hyperlipoproteinemia**, also known as **familial lipoprotein lipase deficiency**, is caused by a genetic defect leading to **deficiency or defect in lipoprotein lipase (LPL)** or its cofactor **apolipoprotein C-II**. - LPL is crucial for the **hydrolysis of triglycerides** in chylomicrons and VLDL at the capillary endothelium. - This enzymatic deficiency leads to **massive accumulation of chylomicrons** and severe hypertriglyceridemia (often >1000 mg/dL). - Clinical features include **eruptive xanthomas, lipemia retinalis, hepatosplenomegaly**, and **recurrent pancreatitis**. *Elevated triglycerides in plasma* - This is indeed the **most prominent laboratory finding** in Type I hyperlipoproteinemia, with triglyceride levels often exceeding 1000-2000 mg/dL. - However, this is the **consequence/manifestation** of the LPL deficiency, not the underlying cause. - The question asks what causes Type I hyperlipoproteinemia, which is the enzyme deficiency itself. *Elevated LDL* - Type I hyperlipoproteinemia typically has **normal or even reduced LDL levels**. - **Elevated LDL** is characteristic of **Type IIa hyperlipoproteinemia (familial hypercholesterolemia)**, which involves defects in LDL receptor or ApoB-100. - Type I primarily affects **chylomicron metabolism**, not LDL. *Elevated cholesterol* - Cholesterol levels are typically **normal or only mildly elevated** in Type I hyperlipoproteinemia. - The triglyceride elevation is disproportionately massive compared to any cholesterol elevation. - Significant isolated cholesterol elevation points to Type IIa or IIb dyslipidemias.

Q: Retinitis pigmentosa is associated with deficiency of:

DHA. ***DHA*** - **Docosahexaenoic acid (DHA)** is the major polyunsaturated fatty acid in the **retinal photoreceptor outer segments** and is crucial for their function. - Deficiency in DHA has been linked to several retinal degeneration disorders, including **retinitis pigmentosa**, suggesting its importance in maintaining retinal health. *Timnodonic acid* - This is an older term for **eicosapentaenoic acid (EPA)**, which is an omega-3 fatty acid. - While EPA is beneficial for overall health, it is **not the primary fatty acid** associated with the direct structural and functional health of retinal photoreceptors as DHA is. *Eicosa pentaenoic acid* - **Eicosapentaenoic acid (EPA)** is an omega-3 fatty acid found in fish oil, known for its anti-inflammatory properties. - While important for general health, EPA is **not as abundant or critical for retinal structure and function** as DHA. *Arachidonic acid* - **Arachidonic acid (AA)** is an omega-6 fatty acid found in cell membranes and is a precursor to pro-inflammatory mediators. - While present in the retina, AA is generally **not associated with a protective or causative role in retinitis pigmentosa** in the same way DHA is.

Q: In case of LPL deficiency, which of the following will increase after a fat rich diet?

Chylomicron. ***Chylomicron*** - **LPL (lipoprotein lipase)** is crucial for the breakdown of **chylomicrons** and VLDL. A deficiency leads to an accumulation of undigested chylomicrons in the bloodstream after a fat-rich meal. - **Chylomicrons** transport dietary triglycerides from the intestines to tissues. Without LPL, these triglycerides remain packaged in chylomicrons. *LDL* - **LDL (low-density lipoprotein)** levels are not directly increased by a short-term fat-rich diet in the context of LPL deficiency. LDL primarily carries cholesterol and is formed from VLDL remnants, a process that is also impaired by LPL deficiency indirectly. - While chronic LPL deficiency can affect overall lipid metabolism, the immediate post-meal increase is not in LDL but in triglyceride-rich lipoproteins. *HDL* - **HDL (high-density lipoprotein)** is involved in reverse cholesterol transport and is generally not directly increased after a fat-rich diet, especially in LPL deficiency. - In fact, severe hypertriglyceridemia, often seen in LPL deficiency, can sometimes lead to lower HDL levels due to altered lipid exchange. *Lipoprotein (a)* - **Lipoprotein (a)**, or Lp(a), is a genetically determined lipoprotein similar to LDL but with an added apolipoprotein (a) and its levels are not acutely affected by dietary fat intake or LPL deficiency. - Lp(a) levels are determined primarily by genetic factors and do not participate in the post-prandial handling of dietary fats.

Q: A child presented with hypotonia and seizures. It was confirmed to be Zellweger syndrome. Which of the following accumulates in brain?

Very long chain fatty acid. ***Long chain fatty acid*** - Zellweger syndrome is a **peroxisomal biogenesis disorder**, leading to non-functional peroxisomes. - Peroxisomes are crucial for the **beta-oxidation of very long-chain fatty acids (VLCFAs)**; their dysfunction causes VLCFA accumulation in tissues, including the brain. *Lactic acid* - Accumulation of **lactic acid** is typically associated with **mitochondrial disorders** or conditions leading to anaerobic metabolism, which are not the primary pathology in Zellweger syndrome. - While lactate levels might be altered in metabolic stress, it is not the hallmark accumulating substance for this condition. *Glucose* - **Glucose** accumulation or dysregulation is primarily seen in disorders like **diabetes mellitus** or specific **glycogen storage diseases**, which involve carbohydrate metabolism, not peroxisomal function. - High glucose levels do not directly result from the peroxisomal defect in Zellweger syndrome. *Triglycerides* - **Triglyceride** accumulation is often linked to disorders of **lipid synthesis, transport, or degradation** in adipocytes or hepatocytes, or conditions like obesity and metabolic syndrome. - While peroxisomes participate in lipid metabolism, the primary accumulation in Zellweger syndrome due to impaired beta-oxidation is **very long-chain fatty acids**, not bulk triglycerides.

Q: In a cerebrohepatorenal syndrome, which of the following accumulate in brain?

Very long-chain fatty acid. **Very long-chain fatty acid** - **Cerebrohepatorenal syndrome**, also known as **Zellweger syndrome**, is a **peroxisomal biogenesis disorder**. - In this syndrome, peroxisomes are deficient or non-functional, leading to the accumulation of **very long-chain fatty acids (VLCFAs)** in various tissues, including the brain. *Short-chain fatty acid* - **Short-chain fatty acids** are primarily produced by gut bacteria and are readily metabolized in the **mitochondria**. - They do not accumulate in Zellweger syndrome, as their metabolism is not dependent on peroxisomal function. *Pyruvate* - **Pyruvate** is a product of **glycolysis** and a key intermediate in cellular metabolism, typically converted to lactate or acetyl-CoA. - Its accumulation is usually indicative of defects in **pyruvate dehydrogenase complex** or conditions leading to anaerobic metabolism, not peroxisomal disorders. *Acetyl CoA* - **Acetyl CoA** is a central molecule in metabolism, involved in the citric acid cycle and fatty acid synthesis and oxidation. - While VLCFAs are broken down to acetyl-CoA in peroxisomes, the primary defect in Zellweger syndrome is the packaging and breakdown of VLCFAs, not a direct accumulation of acetyl-CoA.

Q: Which gene defect causes familial hypercholesterolemia?

LDL Receptor. ***LDL Receptor*** - Familial hypercholesterolemia (FH) is primarily caused by mutations in the **LDL receptor (LDLR) gene**, which leads to impaired clearance of **low-density lipoprotein (LDL)** from the blood. - This defect results in significantly elevated levels of **LDL cholesterol** and an increased risk of premature cardiovascular disease. *Apo E* - Mutations in the **Apo E gene** are associated with **Type III hyperlipoproteinemia (dysbetalipoproteinemia)**, characterized by elevated **chylomicron remnants** and **VLDL remnants**. - This condition presents with xanthomas and premature atherosclerosis, but is distinct from the primary defect in FH. *Apo CII* - **Apo CII** is a cofactor for **lipoprotein lipase (LPL)**, an enzyme essential for the breakdown of **triglycerides** in chylomicrons and VLDL. - Deficiency in Apo CII or LPL causes **Type I hyperlipoproteinemia (familial chylomicronemia syndrome)**, leading to marked **hypertriglyceridemia**, not hypercholesterolemia. *Apo B48* - **Apo B48** is a structural component of **chylomicrons**, which are responsible for transporting dietary fats from the intestines. - It is not directly involved in the primary defect of **LDL clearing** that characterizes familial hypercholesterolemia.

Q: In fatty acid oxidation, what 2-carbon unit is released after each cycle of beta-oxidation?

Acetyl-CoA. ***Acetyl-CoA*** - Each cycle of **beta-oxidation** cleaves a **2-carbon unit** from the fatty acid chain, releasing one molecule of **acetyl-CoA**. - This **acetyl-CoA** then enters the **citric acid cycle** (Krebs cycle) for further oxidation and energy production. - Acetyl-CoA is the primary product representing the carbon units removed from the fatty acid. *FADH2* - **FADH2** is indeed produced during each cycle of beta-oxidation by the enzyme **acyl-CoA dehydrogenase** in the first step. - However, it is an **electron carrier**, not the 2-carbon unit being released from the fatty acid chain. - It feeds electrons into the **electron transport chain** to generate ATP. *NADH* - **NADH** is also produced during each cycle of beta-oxidation by **β-hydroxyacyl-CoA dehydrogenase** in the third step. - Like FADH2, it is an **electron carrier** rather than the carbon product being cleaved off. - It functions in the **electron transport chain** for ATP synthesis. *ATP* - **ATP** is not directly produced during the beta-oxidation cycle itself. - ATP is generated later through **oxidative phosphorylation** when FADH2 and NADH donate electrons to the electron transport chain, and when acetyl-CoA is oxidized in the citric acid cycle. - One complete cycle of beta-oxidation actually requires one ATP equivalent (consumed in the activation step) before producing energy through downstream processes.

Question 1

One of the following has the least density:

Accepted Answer

Chylomicron

Answer

HDL

Answer

VLDL

Answer

LDL

Question 2

Active metabolite in the synthesis of fatty acids is:

Accepted Answer

Malonyl CoA

Answer

Stearate

Answer

Acetyl CoA

Answer

Palmitate

Question 3

In a preterm baby with respiratory distress syndrome, which of the following lipids would be deficient?

Accepted Answer

Lecithin

Answer

Cardiolipin

Answer

Sphingomyelin

Answer

Phosphatidylinositol

Question 4

Type-I hyperlipoproteinemia is caused by deficiency of:-

Accepted Answer

Lipoprotein lipase

Answer

Elevated triglycerides in plasma

Answer

Elevated LDL

Answer

Elevated cholesterol

Question 5

Retinitis pigmentosa is associated with deficiency of:

Accepted Answer

DHA

Answer

Timnodonic acid

Answer

Eicosa pentaenoic acid

Answer

Arachidonic acid

Question 6

In case of LPL deficiency, which of the following will increase after a fat rich diet?

Accepted Answer

Chylomicron

Answer

LDL

Answer

HDL

Answer

Lipoprotein (a)

Question 7

A child presented with hypotonia and seizures. It was confirmed to be Zellweger syndrome. Which of the following accumulates in brain?

Accepted Answer

Very long chain fatty acid

Answer

Lactic acid

Answer

Glucose

Answer

Triglycerides

Question 8

In a cerebrohepatorenal syndrome, which of the following accumulate in brain?

Accepted Answer

Very long-chain fatty acid

Answer

Short-chain fatty acid

Answer

Pyruvate

Answer

Acetyl CoA

Question 9

Which gene defect causes familial hypercholesterolemia?

Accepted Answer

LDL Receptor

Answer

Apo E

Answer

Apo CII

Answer

Apo B48

Question 10

In fatty acid oxidation, what 2-carbon unit is released after each cycle of beta-oxidation?

Accepted Answer

Acetyl-CoA

Answer

FADH2

Answer

NADH

Answer

ATP

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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