Lipid Metabolism Practice Questions

Q: Which of the following is the receptor for LDL?

B100. **Explanation:** The correct answer is **A. B100**. Low-Density Lipoprotein (LDL) is the primary carrier of cholesterol in the blood. To enter cells, LDL must bind to specific **LDL receptors (LDLR)** located on the surface of hepatocytes and peripheral tissues [1]. The ligand on the LDL particle that facilitates this binding is **Apolipoprotein B-100** [1]. Once B100 binds to the receptor, the entire complex is internalized via clathrin-mediated endocytosis [1]. **Analysis of Incorrect Options:** * **B. B48:** This apoprotein is unique to **chylomicrons**. It is a truncated version of B100 produced in the intestine. It lacks the LDL receptor-binding domain, which is why chylomicrons are not cleared by the LDL receptor. * **C. ApoA1:** This is the major structural protein of **HDL** (High-Density Lipoprotein). It acts as an activator of the enzyme LCAT (Lecithin-cholesterol acyltransferase), essential for reverse cholesterol transport. * **D. ApoAII:** Also found primarily in **HDL**, its exact physiological role is less defined but it is involved in triglyceride metabolism and HDL stability. **High-Yield Clinical Pearls for NEET-PG:** * **Familial Hypercholesterolemia (Type IIa):** Caused by mutations in the **LDL receptor** or the **Apo B-100** ligand, leading to severely elevated LDL levels and premature atherosclerosis [1]. * **Apo E:** Known as the "Remnant Receptor" ligand; it is required for the uptake of Chylomicron remnants and VLDL remnants (IDL) by the liver. * **Apo C-II:** Acts as a cofactor for **Lipoprotein Lipase (LPL)**; deficiency leads to Type I Hyperlipoproteinemia (Hyperchylomicronemia). * **Friedewald Formula:** LDL = Total Cholesterol – HDL – (Triglycerides/5). (Note: Not valid if TG >400 mg/dL). **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 156-159.

Q: Fatty liver caused due to excess alcohol consumption is primarily due to an altered ratio of which of the following?

NADH/NAD. **Explanation:** The primary metabolic consequence of ethanol metabolism is an **increased NADH/NAD+ ratio** in the liver. Alcohol is metabolized in the cytosol by *Alcohol Dehydrogenase* and in the mitochondria by *Acetaldehyde Dehydrogenase*. Both reactions reduce NAD+ to NADH, leading to an overabundance of NADH. **Why NADH/NAD+ is the correct answer:** The high NADH/NAD+ ratio signals a "pseudo-fed" state, shifting the liver's metabolic equilibrium: 1. **Inhibition of Fatty Acid Oxidation:** High NADH inhibits $\beta$-oxidation (which requires NAD+), leading to the accumulation of fatty acids. 2. **Increased Lipogenesis:** Excess NADH promotes the conversion of Dihydroxyacetone phosphate (DHAP) to Glycerol-3-phosphate, providing the backbone for Triglyceride synthesis. 3. **Shift in Redox State:** Pyruvate is diverted to Lactate to regenerate NAD+, leading to lactic acidosis and inhibiting gluconeogenesis. **Why other options are incorrect:** * **NAD/NADH (Option A):** This is the inverse. In chronic alcoholism, NAD+ is depleted, not increased. * **NADP/NADPH & NADPH/NADP (Options B & D):** While NADPH is involved in fatty acid synthesis, the acute metabolic derangement caused by alcohol is driven by the NAD-dependent dehydrogenase enzymes, not the pentose phosphate pathway or NADPH-dependent reactions. **High-Yield Clinical Pearls for NEET-PG:** * **Key Enzyme:** Alcohol Dehydrogenase is the rate-limiting step (follows zero-order kinetics). * **Histology:** Alcohol-induced fatty liver (Steatosis) typically shows **macrovesicular steatosis**. * **Associated Findings:** The high NADH/NAD+ ratio also causes **hyperuricemia** (due to lactate competing with urate for excretion) and **fasting hypoglycemia**. * **Biochemical Marker:** Serum **Gamma-Glutamyl Transferase (GGT)** is a sensitive marker for chronic alcohol ingestion.

Q: Platelet-activating factor is biochemically a?

Glycerol ether phospholipid. ### Explanation **Platelet-activating factor (PAF)** is a potent lipid mediator involved in inflammation, platelet aggregation, and anaphylaxis. **Why Option C is correct:** Biochemically, PAF is a **glycerol ether phospholipid** (specifically, 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine). Its structure is unique due to two key features: 1. **Ether Linkage:** It contains an alkyl group attached to the carbon-1 (C1) of glycerol via an **ether bond**, rather than the typical ester bond found in most phospholipids. 2. **Acetyl Group:** It has an **acetyl residue** at the C2 position instead of a long-chain fatty acid. This makes it more water-soluble than other membrane lipids, allowing it to act as a signaling molecule. **Why other options are incorrect:** * **Options A & D (Prostaglandins & Leukotrienes):** These are **Eicosanoids**, derived from 20-carbon polyunsaturated fatty acids (like arachidonic acid). While they are also inflammatory mediators, they do not contain a glycerol backbone or phosphate group. * **Option B (Glycolipid):** Glycolipids (like cerebrosides or gangliosides) contain a carbohydrate component and a sphingosine or glycerol base, but they lack the phosphate group and the specific ether-linked alkyl chain characteristic of PAF. **High-Yield Clinical Pearls for NEET-PG:** * **Source:** PAF is synthesized by platelets, neutrophils, monocytes, and endothelial cells. * **Potency:** It is one of the most potent known biological molecules, active at concentrations as low as $10^{-11}$ mol/L. * **Functions:** It triggers platelet aggregation, induces systemic vasodilation (hypotension), and causes bronchoconstriction. * **Enzyme:** It is inactivated by the enzyme **PAF acetylhydrolase**, which removes the acetyl group at the C2 position.

Q: After a meal rich in dietary lipids, lipids are absorbed by the small intestine and transported in the lymph primarily as which of the following?

Chylomicrons. **Explanation:** **1. Why Chylomicrons are the Correct Answer:** Dietary lipids (exogenous lipids), primarily triacylglycerols, are emulsified by bile salts and digested by pancreatic lipase in the small intestine. Once absorbed by the enterocytes, they are re-esterified and packaged into **Chylomicrons**. These are the largest and least dense lipoproteins, characterized by the presence of **Apolipoprotein B-48**. Because they are too large to enter the blood capillaries directly, they are secreted into the **lacteals (lymphatic vessels)** of the intestinal villi and enter the systemic circulation via the thoracic duct. **2. Why the Other Options are Incorrect:** * **VLDLs (Option A):** These transport **endogenous** lipids (synthesized in the liver), not dietary lipids. They contain Apo B-100. * **Free Fatty Acids (Option B):** While FFAs are transported bound to albumin, this occurs primarily during **fasting** (lipolysis from adipose tissue), not during the post-prandial absorption phase. * **LDLs (Option D):** Known as "bad cholesterol," LDLs are metabolic products of VLDLs (via IDL) and function to transport cholesterol to peripheral tissues. **3. High-Yield NEET-PG Clinical Pearls:** * **Apo B-48 vs. Apo B-100:** Both are products of the same gene. Apo B-48 is produced in the intestine via **RNA editing** (C to U conversion creating a premature stop codon). * **Milky Plasma:** After a fatty meal, plasma appears milky (lipemia) due to the high concentration of chylomicrons. * **LPL Activation:** Chylomicrons acquire **Apo C-II** and **Apo E** from HDL in the blood. Apo C-II is essential for activating **Lipoprotein Lipase (LPL)** to release fatty acids to tissues. * **Abetalipoproteinemia:** A deficiency in Microsomal Triglyceride Transfer Protein (MTP) leads to an inability to form Chylomicrons and VLDLs, resulting in fat malabsorption and steatorrhea.

Q: Which of the following is not a step of beta-oxidation?

NADP dependent oxidation. **Explanation:** Beta-oxidation is the primary catabolic pathway for fatty acids, occurring within the mitochondrial matrix. It involves a repeating sequence of four reactions that shorten the fatty acid chain by two carbons (as Acetyl-CoA) in each cycle. **Why Option A is correct:** The oxidation steps in beta-oxidation utilize **NAD⁺** and **FAD** as electron acceptors, not NADP⁺. * **NAD⁺** is used in the third step (3-hydroxyacyl-CoA dehydrogenase). * **NADPH** is typically utilized in **reductive biosynthesis** (e.g., Fatty Acid Synthesis in the cytoplasm), not in catabolic oxidation. Therefore, NADP-dependent oxidation is not a part of beta-oxidation. **Analysis of Incorrect Options:** * **B. FAD dependent oxidation:** This is the **first step** of the cycle, catalyzed by *Acyl-CoA dehydrogenase*, which creates a double bond between C2 and C3 (trans-Δ²-enoyl-CoA). * **D. Hydration:** This is the **second step**, where *Enoyl-CoA hydratase* adds water across the double bond to form 3-L-hydroxyacyl-CoA. * **C. Thiolysis:** This is the **fourth and final step**, catalyzed by *Thiolase* (β-ketothiolase). It uses a molecule of CoA-SH to cleave the bond, releasing Acetyl-CoA and a fatty acyl-CoA shortened by two carbons. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** Remember the sequence as **OHOT** (Oxidation by FAD → Hydration → Oxidation by NAD → Thiolysis). * **Location:** Beta-oxidation occurs in the **Mitochondria**, whereas Fatty Acid Synthesis occurs in the **Cytosol**. * **Rate-limiting step:** The transport of fatty acids into the mitochondria via the **Carnitine Shuttle** (inhibited by Malonyl-CoA). * **Energy Yield:** Oxidation of one Palmitate (16C) molecule yields a net of **106 ATP**.

Q: Which of the following types of hypertriglyceridemia is associated with an increase in chylomicron and VLDL remnants?

Type III. **Explanation:** **Type III Hyperlipoproteinemia** (also known as **Dysbetalipoproteinemia** or Broad Beta Disease) is characterized by a deficiency in **Apolipoprotein E (Apo E)**. Apo E is the essential ligand required for the liver to recognize and clear **chylomicron remnants** and **VLDL remnants (IDL)** via the LDL-receptor-related protein (LRP). When Apo E is defective (specifically the E2/E2 phenotype), these remnants accumulate in the plasma, leading to elevated cholesterol and triglycerides. **Analysis of Incorrect Options:** * **Type I (Familial Chylomicronemia):** Caused by a deficiency in Lipoprotein Lipase (LPL) or Apo C-II. It results in a massive increase in **chylomicrons** only, not remnants. * **Type IIa (Familial Hypercholesterolemia):** Caused by a defect in LDL receptors. It leads to isolated elevation of **LDL** (cholesterol), with normal triglyceride levels. * **Type IV (Familial Hypertriglyceridemia):** Characterized by the overproduction or decreased clearance of **VLDL**. Remnant levels are typically not the primary finding. **High-Yield Clinical Pearls for NEET-PG:** * **Pathognomonic Sign:** **Palmar xanthomas** (xanthoma striatum palmare) are highly specific for Type III. * **Electrophoresis:** Shows a characteristic **"Broad Beta Band"** due to the presence of IDL. * **Genetics:** Associated with the **Apo E2 homozygote** genotype. * **Risk:** Significant increase in the risk of premature coronary artery disease and peripheral vascular disease.

Q: Phospholipase A2 acts on which of the following?

Phosphatidyl-inositol. **Explanation:** **Phospholipase A2 (PLA2)** is a specific esterase enzyme that hydrolyzes the ester bond at the **second carbon (C2)** of a phospholipid, releasing a free fatty acid (usually arachidonic acid) and a lysophospholipid. 1. **Why Phosphatidyl-inositol is correct:** Phosphatidyl-inositol is a classic **glycerophospholipid** found in cell membranes. PLA2 acts specifically on the glycerol backbone of such phospholipids. When PLA2 acts on phosphatidyl-inositol, it releases **Arachidonic acid**, which serves as the precursor for the synthesis of eicosanoids (prostaglandins, leukotrienes, and thromboxanes). This is a critical step in the inflammatory cascade. 2. **Why other options are incorrect:** * **Phosphoglyceric acid:** This is an intermediate in glycolysis (e.g., 3-phosphoglycerate), not a phospholipid substrate for phospholipases. * **Phosphate:** This is an inorganic ion or a functional group, not a complex lipid molecule. * **Calcium ion:** While PLA2 is a **calcium-dependent enzyme** (it requires $Ca^{2+}$ for its catalytic activity), calcium is a cofactor, not the substrate itself. **Clinical Pearls for NEET-PG:** * **Snake Venom:** The venom of cobras and vipers contains high concentrations of PLA2, which causes hemolysis by producing lysolecithin (a potent detergent that dissolves RBC membranes). * **Steroid Mechanism:** Glucocorticoids induce the synthesis of **Lipocortin (Annexin A1)**, which inhibits PLA2. This is the primary mechanism by which steroids exert their anti-inflammatory effect (by preventing the release of arachidonic acid). * **Pancreatitis:** Serum PLA2 levels are often elevated in acute pancreatitis and are associated with the development of pulmonary complications (ARDS).

Q: What are the products of complete hydrolysis of cardiolipin?

3 glycerol, 4 fatty acids, 2 phosphates. **Explanation:** **Cardiolipin** (Diphosphatidylglycerol) is a unique phospholipid found exclusively in the inner mitochondrial membrane. To understand its hydrolysis products, one must look at its chemical structure: it consists of **two molecules of phosphatidic acid** joined together by a **central glycerol bridge**. 1. **Why Option A is correct:** * **Glycerol:** Each phosphatidic acid unit contains one glycerol backbone (2 total), plus the central linking glycerol. Total = **3 Glycerols**. * **Fatty Acids:** Each phosphatidic acid unit carries two fatty acid chains. Total = **4 Fatty Acids**. * **Phosphate:** Each phosphatidic acid unit contains one phosphate group. Total = **2 Phosphates**. Therefore, complete hydrolysis yields 3 glycerol, 4 fatty acids, and 2 phosphates. 2. **Why other options are incorrect:** * **Option B:** Incorrect because it suggests only 1 phosphate; cardiolipin is a *di*phosphatidyl compound. * **Option C:** Incorrect because it suggests 3 fatty acids; a "double" phospholipid structure must have 4 acyl chains. * **Option D:** Incorrect because 5 glycerols would imply a much larger, non-existent polymer. **High-Yield Clinical Pearls for NEET-PG:** * **Mitochondrial Marker:** Cardiolipin is essential for the optimal function of enzymes in the **Electron Transport Chain** (especially Complex IV). * **Barth Syndrome:** An X-linked genetic disorder caused by a defect in cardiolipin metabolism (tafazzin mutation), leading to cardiomyopathy and muscle weakness. * **Syphilis Testing:** Cardiolipin is the antigen used in the **VDRL/RPR tests** to detect non-specific antibodies (reagin) in syphilis. * **Antiphospholipid Antibody Syndrome (APS):** Anti-cardiolipin antibodies are a hallmark finding, associated with arterial/venous thrombosis and recurrent miscarriages.

Q: Which of the following variants of congenital adrenal hyperplasia (CAH) presents with mineralocorticoid deficiency?

3-beta-hydroxysteroid dehydrogenase deficiency. ### Explanation **1. Why Option D is Correct:** In **3-beta-hydroxysteroid dehydrogenase (3β-HSD) deficiency**, there is a block in the conversion of Pregnenolone to Progesterone and 17-OH Pregnenolone to 17-OH Progesterone. This enzyme is essential for the synthesis of **all three classes** of adrenal steroids: mineralocorticoids, glucocorticoids, and sex steroids. Because the block occurs very early in the steroidogenesis pathway, the production of Aldosterone is severely impaired, leading to **mineralocorticoid deficiency** (salt-wasting, hyponatremia, and hyperkalemia). **2. Why the Other Options are Incorrect:** * **A. 11-beta-hydroxylase deficiency:** This block leads to an accumulation of **11-deoxycorticosterone (DOC)**. DOC is a potent mineralocorticoid; its excess causes hypertension and hypokalemia, rather than deficiency. * **B. 17-alpha-hydroxylase deficiency:** This variant results in a shunting of precursors toward the mineralocorticoid pathway. There is an overproduction of **Corticosterone and DOC**, leading to hypertension and hypokalemia. * **C. P450 oxidoreductase deficiency:** This is a complex "mixed" deficiency. While it can affect various enzymes, it typically presents with apparent mineralocorticoid excess or normal levels, rather than a primary salt-wasting mineralocorticoid deficiency. **3. Clinical Pearls for NEET-PG:** * **Salt-Wasting CAH:** Only 21-hydroxylase (75% of cases) and 3β-HSD deficiency typically present with salt-wasting (mineralocorticoid deficiency). * **Hypertension in CAH:** Think of the "1"s. If the enzyme starts with **1** (**1**1-β or **1**7-α), the patient will have hypertension due to mineralocorticoid excess. * **Virilization:** 3β-HSD deficiency is unique because it causes **ambiguous genitalia in both sexes** (mild virilization in females due to DHEA; undervirilization in males due to lack of potent testosterone).

Question 1

In starvation, ketosis occurs due to which of the following processes?

Accepted Answer

Increased beta-oxidation

Answer

Decreased acetyl CoA

Answer

Decreased lipolysis

Answer

Decreased fatty acid synthesis

Question 2

Which of the following is the receptor for LDL?

Accepted Answer

B100

Answer

B48

Answer

ApoA1

Answer

ApoAII

Question 3

Fatty liver caused due to excess alcohol consumption is primarily due to an altered ratio of which of the following?

Accepted Answer

NADH/NAD

Answer

NAD/NADH

Answer

NADP/NADPH

Answer

NADPH/NADP

Question 4

Platelet-activating factor is biochemically a?

Accepted Answer

Glycerol ether phospholipid

Answer

Prostaglandin

Answer

Glycolipid

Answer

Leukotrienes

Question 5

After a meal rich in dietary lipids, lipids are absorbed by the small intestine and transported in the lymph primarily as which of the following?

Accepted Answer

Chylomicrons

Answer

Very-low-density lipoproteins (VLDLs)

Answer

Free fatty acids bound to albumin

Answer

Low-density lipoproteins (LDLs)

Question 6

Which of the following is not a step of beta-oxidation?

Accepted Answer

NADP dependent oxidation

Answer

FAD dependent oxidation

Answer

Thiolysis

Answer

Hydration

Question 7

Which of the following types of hypertriglyceridemia is associated with an increase in chylomicron and VLDL remnants?

Accepted Answer

Type III

Answer

Type I

Answer

Type IIa

Answer

Type IV

Question 8

Phospholipase A2 acts on which of the following?

Accepted Answer

Phosphatidyl-inositol

Answer

Phosphoglyceric acid

Answer

Phosphate

Answer

Calcium ion

Question 9

What are the products of complete hydrolysis of cardiolipin?

Accepted Answer

3 glycerol, 4 fatty acids, 2 phosphates

Answer

3 glycerol, 4 fatty acids, 1 phosphate

Answer

3 glycerol, 3 fatty acids, 2 phosphates

Answer

5 glycerol, 4 fatty acids, 2 phosphates

Question 10

Which of the following variants of congenital adrenal hyperplasia (CAH) presents with mineralocorticoid deficiency?

Accepted Answer

3-beta-hydroxysteroid dehydrogenase deficiency

Answer

11-beta-hydroxylase deficiency

Answer

17-alpha-hydroxylase deficiency

Answer

P450 oxidoreductase deficiency

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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