Lipid Metabolism Practice Questions

Q: Which of the following is NOT a feature of Refsum disease?

Defect in beta-oxidation. **Explanation:** Refsum disease is a rare autosomal recessive peroxisomal disorder characterized by the inability to degrade **phytanic acid**, a branched-chain fatty acid derived from dietary chlorophyll. **Why Option B is the correct answer:** The fundamental defect in Refsum disease is a deficiency of the enzyme **Phytanoyl-CoA hydroxylase**, which is required for **alpha-oxidation**. Because phytanic acid has a methyl group at the beta-carbon position, it cannot undergo standard beta-oxidation directly. It must first undergo alpha-oxidation to remove one carbon atom. Therefore, the disease is a defect in **alpha-oxidation**, not beta-oxidation. **Analysis of incorrect options:** * **Option A:** Deficiency of alpha-hydroxylase (specifically phytanoyl-CoA hydroxylase) is the primary biochemical cause of the disease. * **Option C:** Due to the enzyme deficiency, **phytanic acid accumulates** in the blood and tissues (especially the nervous system and skin), leading to toxicity. * **Option D:** **Peripheral neuropathy** is a hallmark clinical feature, along with ataxia, retinitis pigmentosa, and sensorineural deafness. **Clinical Pearls for NEET-PG:** * **Dietary Management:** The mainstay of treatment is a diet restricted in phytanic acid (avoiding ruminant fats, dairy, and green leafy vegetables). * **Classic Triad:** Retinitis pigmentosa, peripheral neuropathy, and cerebellar ataxia. * **Ichthyosis:** Patients often present with dry, scaly skin. * **Zellweger Syndrome vs. Refsum:** While both are peroxisomal disorders, Zellweger involves a total failure of peroxisome biogenesis, whereas Refsum is a single enzyme defect.

Q: Which of the following is NOT a phospholipid?

Ceramide. **Explanation:** The core concept in this question is the classification of complex lipids. **Phospholipids** must contain a phosphate group in their structure. **Why Ceramide is the correct answer:** Ceramide is a **sphingosine derivative** consisting of a sphingosine backbone attached to a fatty acid via an amide bond. It is the structural precursor for all complex sphingolipids. Crucially, ceramide **does not contain a phosphate group**; it is a simple lipid intermediate. When a phosphate and choline are added to ceramide, it becomes Sphingomyelin (which *is* a phospholipid). **Analysis of Incorrect Options:** * **Plasmalogens:** These are specialized phospholipids where the fatty acid at the C1 position is attached via an **ether linkage** instead of an ester linkage. They are abundant in cardiac tissue and myelin. * **Dipalmitoyl lecithin (DPPC):** This is a major glycerophospholipid. It contains a glycerol backbone, two palmitic acid chains, a phosphate group, and a choline base. * **Cardiolipin (Diphosphatidylglycerol):** This is a unique phospholipid found exclusively in the **inner mitochondrial membrane**. It consists of two molecules of phosphatidic acid connected by a glycerol bridge, containing two phosphate groups. **High-Yield Clinical Pearls for NEET-PG:** * **DPPC Clinical Correlation:** It is the primary component of **Lung Surfactant**. Deficiency in neonates leads to Respiratory Distress Syndrome (RDS). * **Cardiolipin Clinical Correlation:** It is the antigen used in the **VDRL test** for Syphilis. It is also targeted by antibodies in Antiphospholipid Antibody Syndrome (APS). * **Plasmalogen Fact:** Deficiency of plasmalogen synthesis is seen in **Zellweger Syndrome** (a peroxisomal disorder).

Q: Which molecule is involved in the transport of long-chain acyl-CoA into the mitochondria?

Carnitine. **Explanation:** The transport of long-chain fatty acids into the mitochondria is the rate-limiting step of **beta-oxidation**. While short and medium-chain fatty acids can diffuse freely, long-chain acyl-CoA molecules cannot cross the inner mitochondrial membrane. **1. Why Carnitine is correct:** The **Carnitine Shuttle** facilitates this transport. The enzyme **Carnitine Palmitoyltransferase-I (CPT-I)**, located on the outer mitochondrial membrane, converts acyl-CoA to **acyl-carnitine**. This molecule is then transported across the inner membrane by a translocase. Once inside the matrix, **CPT-II** reconverts it back into acyl-CoA and free carnitine, allowing beta-oxidation to proceed. **2. Why the other options are incorrect:** * **Ornithine:** An amino acid involved in the **Urea Cycle**, acting as a carrier that combines with carbamoyl phosphate to form citrulline. * **Xanthine:** An intermediate in the **Purine degradation pathway**, converted into uric acid by the enzyme xanthine oxidase. * **Albumin:** The primary transport protein in the blood. It carries **free fatty acids (FFAs)** from adipose tissue to peripheral tissues but does not facilitate transport across mitochondrial membranes. **3. High-Yield Clinical Pearls for NEET-PG:** * **Inhibitor:** CPT-I is inhibited by **Malonyl-CoA** (an intermediate of fatty acid synthesis), preventing a futile cycle where synthesis and degradation occur simultaneously. * **Systemic Carnitine Deficiency:** Presents with non-ketotic hypoglycemia, muscle weakness, and liver dysfunction because the body cannot utilize fat for energy during fasting. * **Myopathic CPT-II Deficiency:** The most common disorder of lipid metabolism, characterized by muscle pain and myoglobinuria triggered by prolonged exercise.

Q: Congenital 17-hydroxylase deficiency leads to hypertension due to the accumulation of which substance?

Deoxycorticosterone. In **17α-hydroxylase deficiency**, there is a complete block in the conversion of Pregnenolone to 17-OH Pregnenolone and Progesterone to 17-OH Progesterone. This shunts the steroid biosynthetic pathway exclusively toward the mineralocorticoid pathway (the "outer" layer of the adrenal cortex). [1] **Why Deoxycorticosterone (DOC) is the correct answer:** Because the 17-alpha-hydroxylase enzyme is missing, precursors are diverted to produce excessive amounts of **11-Deoxycorticosterone (DOC)** and Corticosterone. DOC is a potent mineralocorticoid. Its accumulation leads to sodium and water retention, volume expansion, and subsequent **hypertension** and hypokalemia. [1] This volume expansion also causes a feedback suppression of Renin and Aldosterone (resulting in "Low-Renin Hypertension"). **Analysis of Incorrect Options:** * **B. Corticosterone:** While corticosterone levels are also elevated, it has much weaker mineralocorticoid activity compared to DOC. DOC is the primary driver of the hypertensive phenotype. * **C & D. 17-hydroxy pregnanediol / 17-hydroxyprogesterone:** These are products/intermediates *downstream* of the 17-hydroxylase enzyme. In this deficiency, these levels are characteristically **low or absent**. (Note: 17-OHP is elevated in 21-hydroxylase deficiency, not 17-hydroxylase deficiency). **NEET-PG High-Yield Pearls:** * **Clinical Triad:** Hypertension + Hypokalemia + Primary Amenorrhea/Delayed Puberty (due to lack of sex steroids). * **Genotype/Phenotype:** Females (46,XX) present with primary amenorrhea; Males (46,XY) present with female external genitalia or ambiguous genitalia (Pseudohermaphroditism) because testosterone cannot be synthesized. * **Key Lab Finding:** Low Renin, Low Aldosterone, Low Cortisol, and Low Sex Steroids.

Q: What is the best predictor for coronary artery disease?

LDL. **Explanation:** **Why LDL is the correct answer:** Low-Density Lipoprotein (LDL) is the primary carrier of cholesterol from the liver to peripheral tissues. It is highly susceptible to oxidation; **oxidized LDL** is taken up by macrophages via scavenger receptors, leading to the formation of **foam cells**, which are the hallmark of atherosclerotic plaques. Because LDL levels correlate most directly with the initiation and progression of atherosclerosis, it is clinically regarded as the "Bad Cholesterol" and the most significant predictor for Coronary Artery Disease (CAD) among the given options. **Why the other options are incorrect:** * **HDL (High-Density Lipoprotein):** Known as "Good Cholesterol," it mediates **reverse cholesterol transport** (carrying cholesterol from tissues back to the liver). High levels are cardioprotective, while low levels are a risk factor, but it is not a primary driver of plaque formation like LDL. * **VLDL (Very Low-Density Lipoprotein):** Primarily transports endogenous triglycerides. While elevated VLDL contributes to metabolic syndrome, it is a precursor to LDL and not as direct a predictor of CAD as LDL itself. * **Chylomicrons:** These transport exogenous (dietary) triglycerides. They are rapidly cleared from the blood postprandially and are not typically associated with the chronic process of atherosclerosis. **NEET-PG High-Yield Pearls:** * **Friedewald Equation:** $LDL = \text{Total Cholesterol} - [HDL + (TG/5)]$. Note: This is invalid if Triglycerides (TG) are $>400\text{ mg/dL}$. * **Apolipoproteins:** **Apo B-100** is the primary protein component of LDL and VLDL (atherogenic), while **Apo A-1** is found in HDL (protective). * **Lp(a):** Lipoprotein (a) is an independent, genetically determined risk factor for CAD that competes with plasminogen, potentially inhibiting fibrinolysis.

Q: Which of the following is NOT a ketone body?

Acetyl-CoA. **Explanation:** Ketone bodies are water-soluble molecules produced by the liver from fatty acids during periods of low glucose availability (starvation, fasting, or untreated diabetes mellitus). **Why Acetyl-CoA is the correct answer:** Acetyl-CoA is the **precursor** for ketone body synthesis (ketogenesis), but it is not a ketone body itself. It is a central metabolic intermediate that enters the TCA cycle for energy production. In the liver, when Acetyl-CoA levels exceed the capacity of the TCA cycle, it is diverted into the HMG-CoA pathway to form ketone bodies. **Why the other options are incorrect:** * **Acetoacetate:** This is the primary ketone body formed in the liver. It can be converted into the other two forms. * **Beta-hydroxybutyrate:** Formed by the reduction of acetoacetate. Although technically a carboxylic acid and not a "ketone" by chemical structure, it is biologically classified as a ketone body and is the predominant form found in the blood during ketosis. * **Acetone:** Produced by the spontaneous non-enzymatic decarboxylation of acetoacetate. It is highly volatile and excreted via the lungs, giving the characteristic "fruity odor" to the breath in ketoacidosis. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Ketogenesis:** Liver mitochondria (but the liver **cannot** utilize ketone bodies because it lacks the enzyme **Thiophorase** / Succinyl-CoA:3-ketoacid CoA transferase). * **Rate-limiting enzyme:** HMG-CoA Synthase. * **Detection:** The **Rothera’s test** detects Acetoacetate and Acetone, but **not** beta-hydroxybutyrate. * **Utilization:** Ketone bodies are the preferred fuel for the heart and renal cortex; the brain uses them only during prolonged starvation.

Q: Which apolipoprotein is considered the primary apoprotein of cholesterol?

Apolipoprotein E. **Explanation:** The correct answer is **Apolipoprotein E (Apo E)**. In the context of lipid metabolism, Apo E is recognized as the "primary apoprotein of cholesterol" because of its critical role in the hepatic uptake of cholesterol-rich particles. It serves as the high-affinity ligand for the **LDL receptor (Apo B100/E receptor)** and the **LRP (LDL Receptor-Related Protein)**. This interaction allows the liver to clear chylomicron remnants and VLDL remnants (IDL), which are highly enriched in cholesterol esters, from the circulation. **Analysis of Options:** * **Apolipoprotein A1 (A):** The primary structural protein of **HDL**. Its main role is the activation of LCAT (Lecithin-Cholesterol Acyltransferase) for reverse cholesterol transport. * **Apolipoprotein A2 (B):** A secondary protein found in HDL; its physiological function is less defined but it is not the primary mediator of cholesterol particle clearance. * **Apolipoprotein C1 (C):** Primarily involved in the activation of LCAT and inhibition of CETP; it does not serve as the primary cholesterol-clearing ligand. **High-Yield Clinical Pearls for NEET-PG:** * **Type III Hyperlipoproteinemia (Dysbetalipoproteinemia):** Caused by a deficiency or polymorphism of **Apo E (specifically E2/E2 isoform)**, leading to the accumulation of cholesterol-rich remnants and xanthomas. * **Alzheimer’s Disease:** The **Apo E4** isoform is a significant genetic risk factor for late-onset Alzheimer’s. * **Apo B-100** is the structural protein for VLDL/LDL, while **Apo B-48** is unique to chylomicrons (lacks the LDL receptor-binding domain).

Q: All of the following about prostaglandins are TRUE, EXCEPT:

PGE1 and PGE2 increase in bronchial asthma. **Explanation:** **Why Option D is the Correct Answer (The "Except"):** In bronchial asthma, the primary lipid mediators involved are **Leukotrienes (LTC4, LTD4, and LTE4)**, which are potent bronchoconstrictors. Regarding prostaglandins, **PGE2** actually acts as a **bronchodilator** and has anti-inflammatory properties in the airway. While PGF2$\alpha$ and PGD2 are bronchoconstrictors, the statement that PGE1 and PGE2 increase to cause asthma is physiologically incorrect. In fact, aspirin-induced asthma occurs because the inhibition of the COX pathway shunts arachidonic acid toward the lipoxygenase (LOX) pathway, increasing leukotriene production. **Analysis of Other Options:** * **Option A:** Prostaglandins are eicosanoids (20-carbon compounds) derived primarily from **arachidonic acid** (an omega-6 fatty acid) via the Cyclooxygenase (COX) pathway. * **Option B:** They were first isolated from **human semen** by Ulf von Euler in the 1930s. He mistakenly believed they originated from the prostate gland (hence the name "prostaglandins"), though they are actually produced by the seminal vesicles. * **Option C:** Structurally, all prostaglandins are characterized by a **20-carbon carboxylic acid** chain containing a **5-carbon (cyclopentane) ring**. **NEET-PG High-Yield Pearls:** * **Rate-limiting step:** The release of arachidonic acid from membrane phospholipids by **Phospholipase A2** (inhibited by Corticosteroids). * **PGE2 Clinical Use:** Used for cervical ripening and induction of labor (Dinoprostone). * **PGI2 (Prostacyclin):** Produced by vascular endothelium; causes vasodilation and inhibits platelet aggregation. * **TXA2 (Thromboxane):** Produced by platelets; causes vasoconstriction and promotes platelet aggregation (antagonistic to PGI2).

Q: The main component of hyperlipidemia constituting a major risk factor for atherosclerosis is:

Low-density lipoprotein (LDL) cholesterol. **Explanation:** **1. Why LDL Cholesterol is the Correct Answer:** Low-density lipoprotein (LDL) is the primary carrier of cholesterol from the liver to peripheral tissues. It is considered the most significant risk factor for atherosclerosis because of its small size and high cholesterol content. LDL particles can easily penetrate the arterial endothelium, where they become oxidized. These **oxidized LDL particles** are engulfed by macrophages to form **foam cells**, which are the hallmark of early atherosclerotic plaques (fatty streaks). High levels of LDL are directly correlated with an increased risk of Coronary Artery Disease (CAD). **2. Why the Other Options are Incorrect:** * **HDL (Option A):** Known as "Good Cholesterol," HDL mediates **reverse cholesterol transport**, moving cholesterol from peripheral tissues back to the liver. High levels are cardioprotective, not a risk factor. * **IDL (Option B):** IDL is a transient intermediate formed during the conversion of VLDL to LDL. While it is pro-atherogenic, it is not the "main" component or the primary marker used for risk stratification in clinical practice. * **VLDL (Option C):** VLDL primarily transports endogenous triglycerides. While elevated VLDL (hypertriglyceridemia) is a risk factor, it is less directly linked to the initiation of the atherosclerotic plaque compared to LDL. **3. NEET-PG High-Yield Pearls:** * **Friedewald Formula:** LDL = Total Cholesterol – [HDL + (Triglycerides/5)]. (Note: This is invalid if TG >400 mg/dL). * **Apolipoproteins:** LDL contains **Apo B-100**, which acts as a ligand for the LDL receptor. * **Lp(a):** Lipoprotein (a) is an independent genetic risk factor for atherosclerosis; it is essentially an LDL particle with an added Apo(a) protein. * **Statins:** The first-line treatment for hyperlipidemia, they work by inhibiting HMG-CoA reductase, leading to an up-regulation of LDL receptors on hepatocytes.

Q: Which reaction requires HMG–COA reductase activity?

β-hydroxy–β-methyl glutaryl COA → Mevalonic acid. **Explanation:** **1. Why Option A is Correct:** The conversion of **$\beta$-hydroxy-$\beta$-methylglutaryl CoA (HMG-CoA)** to **Mevalonic acid** is the committed and **rate-limiting step** in cholesterol biosynthesis. This reaction is catalyzed by the enzyme **HMG-CoA Reductase**, which utilizes 2 molecules of NADPH as a reducing agent. This step occurs in the cytosol/endoplasmic reticulum of cells, primarily in the liver. **2. Analysis of Incorrect Options:** * **Option B:** This describes the conversion of Acetyl CoA to Malonyl CoA, catalyzed by **Acetyl-CoA Carboxylase (ACC)**. This is the rate-limiting step of **Fatty Acid Synthesis** and requires Biotin (Vitamin B7) as a cofactor. * **Option C:** This is the isomerization of L-methylmalonyl-CoA to Succinyl CoA, catalyzed by **Methylmalonyl-CoA Mutase**. This reaction is critical for the metabolism of odd-chain fatty acids and requires **Vitamin B12 (Cobalamin)**. * **Option D:** This is the hydrolysis of lactose into its constituent monosaccharides, catalyzed by the enzyme **Lactase** (a $\beta$-galactosidase) found in the intestinal brush border. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Pharmacology Link:** **Statins** (e.g., Atorvastatin, Rosuvastatin) are competitive inhibitors of HMG-CoA Reductase, used to treat hypercholesterolemia. * **Regulation:** HMG-CoA Reductase is inhibited by high levels of cholesterol (feedback inhibition) and glucagon (via phosphorylation), while it is stimulated by **Insulin**. * **Location:** Do not confuse this with HMG-CoA **Lyase**, which is involved in **Ketogenesis** (occurring in the mitochondria). HMG-CoA Reductase is for cholesterol synthesis (cytosol).

Question 1

Which of the following is NOT a feature of Refsum disease?

Accepted Answer

Defect in beta-oxidation

Answer

Deficiency of alpha-hydroxylase

Answer

Accumulation of phytanic acid

Answer

Peripheral neuropathy

Question 2

Which of the following is NOT a phospholipid?

Accepted Answer

Ceramide

Answer

Plasmalogens

Answer

Dipalmitoyl lecithin

Answer

Cardiolipin

Question 3

Which molecule is involved in the transport of long-chain acyl-CoA into the mitochondria?

Accepted Answer

Carnitine

Answer

Ornithine

Answer

Xanthine

Answer

Albumin

Question 4

Congenital 17-hydroxylase deficiency leads to hypertension due to the accumulation of which substance?

Accepted Answer

Deoxycorticosterone

Answer

Corticosterone

Answer

17-hydroxy pregnanediol

Answer

17-hydroxyprogesterone

Question 5

What is the best predictor for coronary artery disease?

Accepted Answer

LDL

Answer

HDL

Answer

VLDL

Answer

Chylomicron

Question 6

Which of the following is NOT a ketone body?

Accepted Answer

Acetyl-CoA

Answer

Acetoacetate

Answer

Acetone

Answer

beta-hydroxybutyrate

Question 7

Which apolipoprotein is considered the primary apoprotein of cholesterol?

Accepted Answer

Apolipoprotein E

Answer

Apolipoprotein A1

Answer

Apolipoprotein A2

Answer

Apolipoprotein C1

Question 8

All of the following about prostaglandins are TRUE, EXCEPT:

Accepted Answer

PGE1 and PGE2 increase in bronchial asthma

Answer

Synthesized from arachidonic acid

Answer

First isolated from semen

Answer

Prostaglandins contain a cyclopentane ring

Question 9

The main component of hyperlipidemia constituting a major risk factor for atherosclerosis is:

Accepted Answer

Low-density lipoprotein (LDL) cholesterol

Answer

High-density lipoprotein (HDL) cholesterol

Answer

Intermediate-density lipoprotein (IDL) cholesterol

Answer

Very low-density lipoprotein (VLDL) cholesterol

Question 10

Which reaction requires HMG–COA reductase activity?

Accepted Answer

β-hydroxy–β-methyl glutaryl COA → Mevalonic acid

Answer

Acetyl COA + CO2 + ATP → Malonyl COA + ADP + Pi

Answer

L-methylmalonyl-CoA → Succinyl CoA

Answer

Lactose + H2O → Glucose + Galactose

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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