Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 461: Acetyl CoA carboxylase is the rate-limiting enzyme of which of the following metabolic pathways?

A. Cholesterol synthesis
B. Fatty acid synthesis (Correct Answer)
C. Urea synthesis
D. Gluconeogenesis

Explanation: **Explanation:** **Acetyl CoA Carboxylase (ACC)** is the key regulatory and rate-limiting enzyme in **Fatty Acid Synthesis (Lipogenesis)**. It catalyzes the ATP-dependent carboxylation of Acetyl CoA to Malonyl CoA. This step is crucial because Malonyl CoA serves as the primary building block for the fatty acid chain and also acts as a potent inhibitor of Carnitine Palmitoyltransferase-I (CPT-I), thereby preventing the simultaneous breakdown of fats (Beta-oxidation). **Analysis of Incorrect Options:** * **A. Cholesterol synthesis:** The rate-limiting enzyme is **HMG-CoA Reductase**, which converts HMG-CoA to Mevalonate. * **C. Urea synthesis:** The rate-limiting enzyme is **Carbamoyl Phosphate Synthetase I (CPS-I)**, located in the mitochondria. * **D. Gluconeogenesis:** The primary rate-limiting enzyme is **Fructose-1,6-bisphosphatase**. Other key regulatory enzymes include Pyruvate carboxylase and PEP carboxykinase. **High-Yield Facts for NEET-PG:** * **Activators of ACC:** Citrate (allosteric feed-forward activator) and Insulin (promotes dephosphorylation/activation). * **Inhibitors of ACC:** Long-chain fatty acyl-CoA (Palmitoyl CoA - feedback inhibition) and Glucagon/Epinephrine (promote phosphorylation/inactivation via AMPK). * **Cofactor:** ACC requires **Biotin (Vitamin B7)** for its activity. * **Location:** Fatty acid synthesis occurs in the **Cytosol**, primarily in the liver, adipose tissue, and lactating mammary glands.

Question 462: All of the following about prostaglandins are TRUE, EXCEPT:

A. Synthesized from arachidonic acid
B. First isolated from semen
C. Prostaglandins contain a cyclopentane ring
D. PGE1 and PGE2 increase in bronchial asthma (Correct Answer)

Explanation: **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).

Question 463: The main component of hyperlipidemia constituting a major risk factor for atherosclerosis is:

A. High-density lipoprotein (HDL) cholesterol
B. Intermediate-density lipoprotein (IDL) cholesterol
C. Low-density lipoprotein (LDL) cholesterol (Correct Answer)
D. Very low-density lipoprotein (VLDL) cholesterol

Explanation: **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.

Question 464: Which of the following is true about bile acids?

A. 7 alpha-hydroxylase is the rate-limiting enzyme in their synthesis.
B. They are derived from cholesterol.
C. Cholic acid is a primary bile acid.
D. All of the above. (Correct Answer)

Explanation: ### Explanation Bile acids are essential polar derivatives of cholesterol synthesized in the liver, playing a crucial role in lipid digestion and absorption. **1. Why Option D is correct:** All the provided statements are biochemically accurate: * **Derived from Cholesterol (Option B):** The synthesis of bile acids is the primary pathway for cholesterol excretion. Through a series of reactions, the hydrophobic cholesterol molecule is converted into amphipathic bile acids. * **Rate-Limiting Enzyme (Option A):** The conversion of cholesterol to **7$\alpha$-hydroxycholesterol** by the enzyme **7$\alpha$-hydroxylase** (a cytochrome P450 enzyme) is the committed and rate-limiting step. This enzyme is feedback-inhibited by bile acids and induced by cholesterol. * **Primary Bile Acids (Option C):** These are synthesized directly in the liver. The two main primary bile acids are **Cholic acid** and **Chenodeoxycholic acid**. **2. Understanding the Process:** * **Primary vs. Secondary:** Primary bile acids (Cholic, Chenodeoxycholic) are synthesized in the liver. Once they reach the intestine, bacterial action (dehydroxylation) converts them into **secondary bile acids**: Deoxycholic acid (from Cholic) and Lithocholic acid (from Chenodeoxycholic). * **Conjugation:** Before secretion, bile acids are conjugated with **Glycine or Taurine** to form bile salts, which increases their solubility at intestinal pH. **3. High-Yield Clinical Pearls for NEET-PG:** * **Enterohepatic Circulation:** Approximately 95% of bile salts are reabsorbed in the **terminal ileum** and returned to the liver. * **Bile Acid Sequestrants (Cholestyramine):** These drugs bind bile acids in the gut, preventing reabsorption. This forces the liver to use more cholesterol to synthesize new bile acids, thereby lowering LDL levels. * **Vitamin C Deficiency:** 7$\alpha$-hydroxylase requires Vitamin C; thus, scurvy can lead to cholesterol accumulation and gallstone formation.

Question 465: Which of the following is not considered a "bad cholesterol"?

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

Explanation: ### Explanation **Correct Answer: A. HDL** **Why HDL is the "Good Cholesterol":** High-Density Lipoprotein (HDL) is termed "good cholesterol" because of its role in **Reverse Cholesterol Transport**. It picks up excess cholesterol from peripheral tissues and blood vessel walls (including atherosclerotic plaques) and transports it back to the liver for excretion in bile. High levels of HDL are cardio-protective as they reduce the risk of atherosclerosis and coronary artery disease. **Why the other options are "Bad Cholesterol":** These lipoproteins are considered "bad" because they transport lipids from the liver to the peripheral tissues, contributing to plaque formation: * **LDL (Low-Density Lipoprotein):** The primary carrier of cholesterol to tissues. It is the most potent pro-atherogenic lipoprotein; high levels lead to cholesterol deposition in arterial walls. * **VLDL (Very Low-Density Lipoprotein):** Secreted by the liver to transport endogenous triglycerides. It is a precursor to LDL. * **IDL (Intermediate-Density Lipoprotein):** Formed during the conversion of VLDL to LDL. Like LDL, it is enriched in cholesterol esters and contributes to atherosclerosis. **High-Yield NEET-PG Pearls:** * **Apo-A1:** The primary apoprotein associated with **HDL** (activates LCAT). * **Apo-B100:** The characteristic apoprotein for **VLDL, IDL, and LDL**. * **LCAT (Lecithin-Cholesterol Acyltransferase):** The enzyme responsible for esterifying cholesterol within HDL, converting it from discoid to spherical form. * **Friedewald Formula:** Used to calculate LDL: $LDL = Total\ Cholesterol – (HDL + TG/5)$. (Note: This is invalid if TG >400 mg/dL).

Question 466: In diabetics, what factor limits the synthesis of triglycerides in adipose tissue?

A. NADPH
B. ATP
C. Acetyl CoA
D. Glycerol-3-P (Correct Answer)

Explanation: **Explanation:** In adipose tissue, the synthesis of triglycerides (triacylglycerols) requires two main building blocks: **Fatty Acyl-CoA** and **Glycerol-3-Phosphate (G3P)**. **Why Glycerol-3-P is the limiting factor:** Unlike the liver, adipose tissue lacks the enzyme **Glycerol Kinase**. Therefore, it cannot phosphorylate free glycerol to produce G3P. Instead, it must derive G3P from the glycolytic intermediate, Dihydroxyacetone phosphate (DHAP), via the enzyme *Glycerol-3-phosphate dehydrogenase*. This process is strictly dependent on **glucose uptake** into the adipocyte. In diabetes mellitus, insulin deficiency or resistance leads to decreased recruitment of **GLUT-4** transporters. This results in reduced glucose entry, decreased glycolysis, and a subsequent deficiency of Glycerol-3-P, thereby limiting triglyceride synthesis and storage. **Analysis of Incorrect Options:** * **NADPH:** While required for *de novo* fatty acid synthesis, its deficiency is not the primary limiting factor for esterification in diabetics; fatty acids are often abundantly available from the bloodstream (via VLDL or chylomicrons). * **ATP:** Although required for the activation of fatty acids to Acyl-CoA, cellular ATP levels are generally maintained sufficiently and do not act as the specific metabolic bottleneck in this context. * **Acetyl CoA:** This is a precursor for fatty acid synthesis. In diabetics, Acetyl CoA levels may actually be elevated due to increased beta-oxidation, but without G3P, it cannot be stored as fat. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Deficiency:** Adipose tissue lacks **Glycerol Kinase** (High-yield fact). * **Insulin Effect:** Insulin promotes TG synthesis by increasing GLUT-4 mediated glucose uptake and activating **Lipoprotein Lipase (LPL)**. * **Metabolic Consequence:** In diabetes, the inability to store TGs in adipose tissue, combined with increased lipolysis (due to lack of insulin's inhibitory effect on Hormone Sensitive Lipase), leads to high levels of circulating Free Fatty Acids (FFAs).

Question 467: What is the most important source of reducing equivalents for fatty acid synthesis in the liver?

A. Glycolysis
B. TCA cycle
C. Uronic acid pathway
D. HMP pathway (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. HMP pathway** **Why it is correct:** Fatty acid synthesis (Lipogenesis) is a reductive process that occurs in the cytosol. It requires a significant amount of **NADPH** as a reducing equivalent to convert keto groups to methylene groups. The **Hexose Monophosphate (HMP) Shunt** (also known as the Pentose Phosphate Pathway) is the primary source of this NADPH. Specifically, the oxidative phase of the HMP shunt, catalyzed by **Glucose-6-Phosphate Dehydrogenase (G6PD)**, generates the bulk of NADPH required for biosynthetic pathways in the liver, lactating mammary glands, and adipose tissue. **Why the other options are incorrect:** * **A. Glycolysis:** While glycolysis provides the substrate (Acetyl-CoA via pyruvate) for fatty acid synthesis, it produces **NADH**, not NADPH. NADH is primarily used for ATP production in the electron transport chain. * **B. TCA cycle:** The TCA cycle occurs in the mitochondria and primarily generates **NADH and FADH2** for oxidative phosphorylation. It does not directly provide reducing equivalents for cytosolic fatty acid synthesis. * **C. Uronic acid pathway:** This pathway is involved in the synthesis of glucuronic acid (for detoxification) and pentoses; it does not contribute significantly to the NADPH pool required for lipogenesis. **High-Yield Clinical Pearls for NEET-PG:** * **Alternative Source:** The second most important source of NADPH for fatty acid synthesis is the **Malic Enzyme** (which converts Malate to Pyruvate). * **Rate-Limiting Step:** The rate-limiting enzyme for fatty acid synthesis is **Acetyl-CoA Carboxylase (ACC)**, which requires Biotin. * **Location:** Lipogenesis occurs in the **cytosol**, but the Acetyl-CoA is transported out of the mitochondria in the form of **Citrate** (the "Citrate Shuttle"). * **Key Tissues:** HMP shunt activity is highest in tissues active in lipid/steroid synthesis (Liver, Adrenal cortex, Testes, Ovaries, and Mammary glands).

Question 468: Which enzyme defect causes Tay-Sachs disease?

A. Beta-glucosidase
B. Alpha-galactosidase
C. Hexosaminidase A (Correct Answer)
D. Beta-galactosidase

Explanation: ### Explanation **Correct Option: C. Hexosaminidase A** Tay-Sachs disease is a lysosomal storage disorder (Sphingolipidosis) caused by a deficiency of the enzyme **Hexosaminidase A**. This enzyme is responsible for the degradation of **GM2 gangliosides**. Its deficiency leads to the toxic accumulation of GM2 gangliosides in the neurons of the brain and spinal cord, resulting in progressive neurodegeneration. **Analysis of Incorrect Options:** * **A. Beta-glucosidase (Glucocerebrosidase):** Deficiency causes **Gaucher disease**, the most common lysosomal storage disorder, characterized by hepatosplenomegaly and "wrinkled paper" cytoplasm in macrophages. * **B. Alpha-galactosidase:** Deficiency causes **Fabry disease**, an X-linked disorder characterized by angiokeratomas, peripheral neuropathy, and renal failure. * **D. Beta-galactosidase:** Deficiency causes **GM1 gangliosidosis** or **Krabbé disease** (specifically Galactosylceramidase). **Clinical Pearls for NEET-PG:** * **Cherry-red spot on Macula:** A classic finding in Tay-Sachs. Note that it is also seen in Niemann-Pick disease; however, **hepatosplenomegaly is absent** in Tay-Sachs (present in Niemann-Pick). * **Onion-skin appearance:** Histology of lysosomes shows whorled membranes. * **Genetics:** Autosomal Recessive inheritance; common in Ashkenazi Jews. * **Clinical Presentation:** Startle response (hyperacusis), developmental regression, and seizures, usually appearing between 3–6 months of age. * **Mnemonic:** Tay-Sa**X** lacks He**X**osaminidase A.

Question 469: A child presented with altered sensorium and icterus three days after a viral illness for which he was treated with aspirin. What is the underlying biochemical defect?

A. Beta oxidation of fatty acids (Correct Answer)
B. Glucose-6-phosphatase deficiency
C. Pyruvate dehydrogenase deficiency
D. Urea cycle defect

Explanation: ### Explanation **Correct Option: A. Beta oxidation of fatty acids** The clinical presentation describes **Reye’s Syndrome**, a rare but severe condition typically occurring in children treated with **aspirin** during a viral prodrome (like Influenza or Varicella). The underlying biochemical mechanism involves **mitochondrial dysfunction**. Aspirin (salicylates) and its metabolites act as mitochondrial toxins that inhibit the enzymes involved in the **$\beta$-oxidation of fatty acids**. This leads to: 1. **Microvesicular steatosis:** Failure to oxidize fatty acids causes them to accumulate as small droplets in hepatocytes. 2. **Hypoglycemia:** Impaired $\beta$-oxidation reduces the production of Acetyl-CoA, which is essential for activating pyruvate carboxylase (the first step of gluconeogenesis). 3. **Hyperammonemia:** Mitochondrial damage disrupts the urea cycle, leading to cerebral edema and altered sensorium (encephalopathy). --- ### Why other options are incorrect: * **B. Glucose-6-phosphatase deficiency (Von Gierke Disease):** While it causes severe hypoglycemia and hepatomegaly, it is a genetic glycogen storage disease, not typically triggered by aspirin or viral illness. It presents with lactic acidosis and hyperuricemia. * **C. Pyruvate dehydrogenase deficiency:** This leads to chronic lactic acidosis and neurological impairment from birth, rather than acute hepatic failure following aspirin intake. * **D. Urea cycle defect:** Although hyperammonemia occurs in Reye’s syndrome, the primary insult is mitochondrial damage affecting multiple pathways (including $\beta$-oxidation), not a primary genetic deficiency of a urea cycle enzyme. --- ### High-Yield Clinical Pearls for NEET-PG: * **Pathognomonic finding:** Liver biopsy shows **microvesicular steatosis** (no inflammation). * **Biochemical markers:** Elevated ALT/AST, prolonged PT/INR, hyperammonemia, and non-ketotic hypoglycemia. * **Contraindication:** Aspirin is strictly avoided in children with viral fever; **Acetaminophen** is the preferred alternative. * **Exception:** Aspirin is still used in children for **Kawasaki disease**.

Question 470: What is the total ATP generated by the complete oxidation of stearic acid?

A. 114
B. 131
C. 148 (Correct Answer)
D. None of the above

Explanation: ### Explanation The complete oxidation of a fatty acid occurs via **$\beta$-oxidation** in the mitochondria. To calculate the ATP yield for **Stearic Acid (C18:0)**, we follow these steps: **1. Activation Step:** Stearic acid is converted to Stearyl-CoA. This process consumes the equivalent of **2 ATP** (ATP → AMP + PPi). **2. $\beta$-oxidation Cycles:** A fatty acid with $n$ carbon atoms undergoes $(n/2 - 1)$ cycles. * For C18, there are **8 cycles**. * Each cycle produces 1 FADH₂ (1.5 ATP) and 1 NADH (2.5 ATP) = **4 ATP per cycle**. * Total from cycles: $8 \times 4 = \mathbf{32\ ATP}$. **3. Acetyl-CoA Production:** A fatty acid with $n$ carbon atoms produces $n/2$ Acetyl-CoA molecules. * For C18, **9 Acetyl-CoA** are produced. * Each Acetyl-CoA enters the TCA cycle to yield 10 ATP (3 NADH, 1 FADH₂, 1 GTP). * Total from Acetyl-CoA: $9 \times 10 = \mathbf{90\ ATP}$. **4. Total Calculation:** * Gross Yield: $32 + 90 = 122$ ATP (using modern P:O ratios). * **Note on NEET-PG Scoring:** Many standard textbooks (like Harper’s) still use the **traditional P:O ratios** (NADH = 3, FADH₂ = 2). * Cycles: $8 \times 5 = 40$ ATP * Acetyl-CoA: $9 \times 12 = 108$ ATP * Gross: $148$ ATP. Net: $148 - 2 = 146$ ATP. * However, the question asks for **total generated** (Gross), making **148** the correct choice. --- ### Analysis of Incorrect Options * **Option A (114):** This is the net yield for Palmitic acid (C16) using modern ratios. * **Option B (131):** This is the net yield for Palmitic acid (C16) using traditional ratios ($129 + 2$ for activation). * **Option D:** Incorrect as 148 represents the gross ATP generated before subtracting the activation cost. --- ### High-Yield Clinical Pearls * **Palmitic Acid (C16):** The most common fatty acid; yields **129 Net ATP** (Traditional) or **106 Net ATP** (Modern). * **Odd-chain Fatty Acids:** These are the only lipids that are **glucogenic** because their final product is Propionyl-CoA, which enters the TCA cycle as Succinyl-CoA. * **Rate-limiting enzyme:** Carnitine Palmitoyltransferase-I (CPT-I), inhibited by Malonyl-CoA.

Practice by Chapter

Lipid Classification and Chemistry

Practice Questions

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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