Lipid Metabolism — MCQs

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 311: Familial hypercholesterolemia is characterized by which of the following?

A. Deficient LDL receptors (Correct Answer)
B. Deficient HDL receptors
C. HMG CoA reductase deficiency
D. Deficient VLDL receptors

Explanation: ### Explanation **Familial Hypercholesterolemia (FH)** is an autosomal dominant disorder primarily caused by mutations in the **LDL receptor (LDLR) gene**. #### 1. Why Option A is Correct The LDL receptor is responsible for the hepatic clearance of LDL-cholesterol from the plasma via receptor-mediated endocytosis. In FH, a **deficiency or dysfunction of these receptors** leads to a significant decrease in LDL uptake. This results in markedly elevated serum LDL levels and premature atherosclerosis. #### 2. Why Other Options are Incorrect * **Option B (HDL receptors):** HDL is involved in reverse cholesterol transport (carrying cholesterol from tissues to the liver). Defects here are associated with Tangier disease, not FH. * **Option C (HMG CoA reductase deficiency):** HMG-CoA reductase is the rate-limiting enzyme for cholesterol synthesis. In FH, because cells cannot take up LDL, intracellular cholesterol is low, which actually leads to the **over-activation** (upregulation) of HMG-CoA reductase. * **Option D (VLDL receptors):** VLDL is the precursor to LDL. While VLDL metabolism is related, the hallmark of FH is specifically the inability to clear LDL particles. #### 3. High-Yield Clinical Pearls for NEET-PG * **Genetics:** Autosomal Dominant; Type IIa Hyperlipoproteinemia (Fredrickson classification). * **Clinical Triad:** 1. **Xanthomas:** Specifically Tendon xanthomas (Achilles tendon is the most common site). 2. **Xanthelasmas:** Yellowish deposits around eyelids. 3. **Corneal Arcus:** White/gray ring around the cornea at a young age. * **Other Mutations:** While LDLR is most common, mutations in **ApoB-100** (ligand for the receptor) or **PCSK9** (enzyme that degrades the receptor) can also cause FH. * **Treatment:** Statins are the first-line treatment as they inhibit HMG-CoA reductase and secondarily increase the expression of LDL receptors.

Question 312: Which apolipoprotein acts as an enzyme activator?

A. Apo A-II
B. Apo C-II (Correct Answer)
C. Apo B-100
D. Apo E

Explanation: ### Explanation **Correct Answer: B. Apo C-II** **Mechanism:** Apolipoprotein C-II (Apo C-II) is a critical cofactor found on the surface of chylomicrons and Very Low-Density Lipoproteins (VLDL). Its primary function is to act as a **potent activator of Lipoprotein Lipase (LPL)**, the enzyme anchored to the capillary endothelium of extrahepatic tissues (primarily adipose tissue and muscle). Once activated by Apo C-II, LPL hydrolyzes the triglycerides within these lipoproteins into free fatty acids and glycerol, allowing for tissue uptake. **Analysis of Incorrect Options:** * **Apo A-II:** Found primarily in HDL; its exact physiological role is less clear, though it may inhibit hepatic lipase or modulate HDL remodeling. It is not a primary enzyme activator like Apo C-II. * **Apo B-100:** This is a structural protein found in VLDL, IDL, and LDL. Its key role is acting as a **ligand for the LDL receptor**, facilitating the endocytosis of LDL into peripheral tissues. * **Apo E:** Found in chylomicron remnants, VLDL, and HDL. It serves as a **ligand for the LDL receptor-related protein (LRP)** and the LDL receptor, mediating the hepatic uptake of lipoprotein remnants. **High-Yield Clinical Pearls for NEET-PG:** * **Deficiency:** A genetic deficiency in either **Apo C-II** or **LPL** leads to **Type I Hyperlipoproteinemia** (Familial Chylomicronemia Syndrome), characterized by severe hypertriglyceridemia, eruptive xanthomas, and recurrent pancreatitis. * **Apo A-I:** Remember that Apo A-I is the activator for **LCAT** (Lecithin-Cholesterol Acyltransferase), which is essential for reverse cholesterol transport. * **Apo B-48:** Unique to chylomicrons; it lacks the LDL-receptor binding domain found in B-100.

Question 313: Which of the following statements regarding the beta-oxidation of palmitic acid is FALSE?

A. Seven cycles of beta-oxidation take place.
B. A net of 106 ATP molecules are generated.
C. Seven molecules of acetyl-CoA are generated. (Correct Answer)
D. Beta-oxidation takes place inside the mitochondria.

Explanation: ### Explanation **Underlying Concept:** Palmitic acid is a saturated fatty acid containing **16 carbon atoms**. The process of beta-oxidation involves the sequential removal of 2-carbon units in the form of Acetyl-CoA. For a saturated fatty acid with $n$ carbons, the number of Acetyl-CoA molecules produced is $n/2$, and the number of cycles required is $(n/2) - 1$. **Why Option C is FALSE (The Correct Answer):** Since Palmitic acid has 16 carbons, it produces **8 molecules of Acetyl-CoA** ($16 \div 2 = 8$), not seven. The final (7th) cycle of beta-oxidation cleaves a 4-carbon fatty acyl-CoA into two separate 2-carbon Acetyl-CoA molecules. **Analysis of Other Options:** * **Option A:** Correct. To break down a 16-carbon chain into 8 fragments, the enzyme complex must perform the cycle **7 times**. * **Option B:** Correct. Each cycle produces 1 $FADH_2$ (1.5 ATP) and 1 $NADH$ (2.5 ATP). Total from 7 cycles = 28 ATP. The 8 Acetyl-CoA enter the TCA cycle (10 ATP each) = 80 ATP. Gross total = 108 ATP. After subtracting **2 ATP** used for initial activation (Palmitate to Palmitoyl-CoA), the **net yield is 106 ATP**. * **Option D:** Correct. Beta-oxidation occurs in the **mitochondrial matrix**. Long-chain fatty acids require the **Carnitine Shuttle** to cross the inner mitochondrial membrane. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Carnitine Palmitoyltransferase-I (CPT-I), which is inhibited by Malonyl-CoA. * **Sudden Infant Death Syndrome (SIDS):** Often associated with **MCAD deficiency** (Medium-chain acyl-CoA dehydrogenase deficiency), the most common inborn error of beta-oxidation. * **Jamaican Vomiting Sickness:** Caused by Hypoglycin A (in unripe Ackee fruit), which inhibits acyl-CoA dehydrogenase, halting beta-oxidation and leading to profound hypoglycemia.

Question 314: Which of the following is an abnormal lipoprotein?

A. VLDL
B. Chylomicron
C. Lp (a) (Correct Answer)
D. LDL

Explanation: **Explanation:** The correct answer is **Lp (a)**. While VLDL, LDL, and Chylomicrons are physiological lipoproteins essential for lipid transport, **Lipoprotein (a)** is considered an "abnormal" or "variant" lipoprotein because it is not part of the standard metabolic pathway and is a significant independent risk factor for cardiovascular disease. **Why Lp (a) is the correct answer:** Lp (a) consists of an **LDL-like particle** (containing Apo B-100) covalently linked to a unique glycoprotein called **Apolipoprotein (a)** via a disulfide bridge. It is structurally homologous to **plasminogen** but lacks fibrinolytic activity. Consequently, it competes with plasminogen for binding sites, inhibiting clot lysis and promoting **thrombogenesis**. Its presence in high levels is genetically determined and pathological. **Why other options are incorrect:** * **VLDL (Very Low-Density Lipoprotein):** A normal physiological lipoprotein synthesized by the liver to transport endogenous triglycerides to peripheral tissues. * **Chylomicron:** A normal lipoprotein synthesized by the intestinal mucosa to transport dietary (exogenous) lipids. * **LDL (Low-Density Lipoprotein):** The primary carrier of cholesterol to peripheral tissues; while high levels are "bad," the particle itself is a normal product of VLDL metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Lp (a) and MI:** It is a potent risk factor for premature coronary artery disease and stroke. * **Niacin:** One of the few drugs that can significantly lower Lp (a) levels (Statins have little to no effect). * **Lipoprotein X:** Another abnormal lipoprotein found specifically in **obstructive jaundice** and LCAT deficiency. * **Broad Beta Disease:** Characterized by the presence of **IDL (Beta-VLDL)**, an abnormal intermediate.

Question 315: A newborn child presents with breathing difficulties. On general examination, a cyanotic appearance is seen. ABG analysis reveals increased pCO2. Which of the following lipids is associated with respiratory distress syndrome?

A. Phosphatidylethanolamine
B. Phosphatidylserine
C. Dipalmitoyl lecithin (Correct Answer)
D. Phosphatidylinositol

Explanation: **Explanation:** The clinical presentation of cyanosis, breathing difficulty, and hypercapnia (increased $pCO_2$) in a newborn is characteristic of **Respiratory Distress Syndrome (RDS)**, also known as Hyaline Membrane Disease. **Why Dipalmitoyl lecithin is correct:** The primary cause of RDS is a deficiency of **pulmonary surfactant**. Surfactant is a complex mixture of lipids and proteins secreted by **Type II pneumocytes**. Its major component (approx. 80%) is phospholipids, specifically **Dipalmitoylphosphatidylcholine (DPPC)**, also known as **Dipalmitoyl lecithin**. * **Mechanism:** Surfactant reduces surface tension at the alveolar air-liquid interface, preventing alveolar collapse (atelectasis) during expiration. * **Clinical Link:** In premature infants, insufficient surfactant production leads to high surface tension, lung collapse, and impaired gas exchange. **Why other options are incorrect:** * **Phosphatidylethanolamine (Cephalin):** Found primarily in cell membranes and nervous tissue; it is not a major component of pulmonary surfactant. * **Phosphatidylserine:** Important for cell signaling and apoptosis (flips to the outer leaflet to signal phagocytosis); it does not play a role in alveolar surface tension. * **Phosphatidylinositol:** Acts as a precursor for second messengers like $IP_3$ and $DAG$; while present in small amounts in surfactant, it is not the primary functional lipid. **High-Yield Facts for NEET-PG:** 1. **L/S Ratio:** Fetal lung maturity is assessed by the **Lecithin/Sphingomyelin ratio** in amniotic fluid. A ratio **>2.0** indicates mature lungs. 2. **Glucocorticoids:** Given to mothers in preterm labor (e.g., Betamethasone) to stimulate surfactant synthesis by inducing enzymes in Type II pneumocytes. 3. **Surfactant Protein B & C:** Essential for the spreading and stability of the surfactant film. 4. **Composition:** Surfactant is roughly 90% lipids and 10% proteins.

Question 316: What is the net ATP yield from the complete oxidation of palmitic acid?

A. 106 (Correct Answer)
B. 26
C. 16
D. 129

Explanation: **Explanation:** The complete oxidation of **Palmitic acid (a 16-carbon saturated fatty acid)** occurs via the Beta-oxidation pathway in the mitochondria. To calculate the net ATP yield, we follow these steps: 1. **Beta-Oxidation Cycles:** A 16-carbon chain undergoes **7 cycles** of beta-oxidation. * Each cycle produces 1 FADH₂ and 1 NADH. * Total: 7 FADH₂ and 7 NADH. 2. **Acetyl CoA Production:** The process yields **8 Acetyl CoA** molecules (16/2). 3. **ATP Calculation (Modern Yield):** * 7 FADH₂ × 1.5 ATP = 10.5 ATP * 7 NADH × 2.5 ATP = 17.5 ATP * 8 Acetyl CoA (via TCA cycle) × 10 ATP = 80 ATP * **Gross Total = 108 ATP** 4. **Activation Cost:** 2 high-energy phosphate bonds are consumed to convert Palmitate to Palmitoyl-CoA (ATP → AMP). * **Net Yield = 108 - 2 = 106 ATP.** **Analysis of Options:** * **Option A (106):** Correct. This reflects the modern P/O ratios (NADH=2.5, FADH₂=1.5). * **Option D (129):** This was the "old" calculation (NADH=3, FADH₂=2). While still found in older textbooks, 106 is the current standard for NEET-PG. * **Options B & C (26 & 16):** These are distractors. 16 is the carbon count; 26 is unrelated to palmitate yield. **Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** Carnitine Palmitoyltransferase-I (CPT-I), inhibited by Malonyl-CoA. * **Location:** Beta-oxidation occurs in the mitochondrial matrix; fatty acid synthesis occurs in the cytosol. * **Sudden Infant Death Syndrome (SIDS):** Often associated with **MCAD deficiency** (Medium-chain acyl-CoA dehydrogenase), impairing beta-oxidation.

Question 317: Which of the following enzymes does not use Acetyl-CoA as a substrate?

A. HMG-CoA synthetase
B. Malic enzyme (Correct Answer)
C. Malonyl CoA synthetase
D. Fatty acid synthetase

Explanation: **Explanation:** The correct answer is **Malic enzyme** because it is involved in the generation of NADPH and pyruvate, rather than the utilization of Acetyl-CoA. **Why Malic Enzyme is the correct answer:** Malic enzyme (also known as NADP-dependent malate dehydrogenase) catalyzes the oxidative decarboxylation of **Malate to Pyruvate**. This reaction is crucial in lipid metabolism because it produces **NADPH**, which is the essential reducing equivalent required for fatty acid synthesis. It does not use Acetyl-CoA as a substrate; instead, it helps provide the "building blocks" (NADPH) for the process. **Analysis of Incorrect Options:** * **HMG-CoA synthetase:** This enzyme condenses **Acetyl-CoA** with Acetoacetyl-CoA to form HMG-CoA. This is the rate-limiting step in ketogenesis (mitochondria) and an early step in cholesterol synthesis (cytosol). * **Malonyl CoA synthetase (Acetyl-CoA Carboxylase):** This enzyme converts **Acetyl-CoA** to Malonyl-CoA. This is the committed and rate-limiting step of fatty acid synthesis. * **Fatty acid synthetase (FAS):** This multi-enzyme complex uses one molecule of **Acetyl-CoA** as a "primer" and multiple molecules of Malonyl-CoA to synthesize Palmitate. **High-Yield Clinical Pearls for NEET-PG:** * **The Citrate Shuttle:** Acetyl-CoA cannot cross the inner mitochondrial membrane. It must condense with oxaloacetate to form **Citrate**, which leaves the mitochondria and is then cleaved back into Acetyl-CoA and Oxaloacetate in the cytosol by *ATP Citrate Lyase*. * **NADPH Sources:** The two primary sources of NADPH for fatty acid synthesis are the **Pentose Phosphate Pathway (HMP Shunt)** and the **Malic Enzyme** reaction. * **Rate-Limiting Enzymes:** Always remember **Acetyl-CoA Carboxylase** for fatty acid synthesis and **HMG-CoA Reductase** for cholesterol synthesis.

Question 318: What is the highest percentage of essential fatty acids found in?

A. Butter fat (Ghee)
B. Sunflower seed oil
C. Corn oil (Correct Answer)
D. Groundnut oil

Explanation: **Explanation:** Essential Fatty Acids (EFAs), primarily **Linoleic acid (Omega-6)** and **Linolenic acid (Omega-3)**, are polyunsaturated fatty acids (PUFAs) that the human body cannot synthesize due to the absence of enzymes that introduce double bonds beyond carbon 9 and 10. **Why Corn Oil is Correct:** Among the provided options, **Corn oil** contains the highest percentage of essential fatty acids, specifically Linoleic acid, which constitutes approximately **55-60%** of its total fatty acid profile. While Sunflower oil is also a rich source, in standard biochemical comparisons used in medical examinations, Corn oil is frequently cited as having the superior concentration of EFAs compared to groundnut or animal fats. **Analysis of Incorrect Options:** * **Butter fat (Ghee):** This is primarily composed of saturated fatty acids (approx. 60-70%). It contains very low amounts of EFAs (around 2-3%), making it the poorest source among the choices. * **Sunflower seed oil:** A very good source of Linoleic acid (approx. 50-55%), but generally ranks slightly lower than or equal to corn oil depending on the specific variety. * **Groundnut oil:** Contains a higher proportion of Monounsaturated Fatty Acids (MUFA), specifically Oleic acid. Its EFA content is lower, roughly **20-30%**. **High-Yield Clinical Pearls for NEET-PG:** * **EFA Deficiency:** Leads to **Phrynoderma** (follicular hyperkeratosis/toad skin), poor wound healing, and hair loss. * **PUFA/SFA Ratio:** A high intake of PUFAs (like those in Corn oil) helps lower serum cholesterol by increasing the expression of LDL receptors. * **Most Abundant EFA:** Linoleic acid is the most abundant essential fatty acid in the human diet. * **Hierarchy of PUFA content:** Safflower oil > Corn oil > Sunflower oil > Soyabean oil.

Question 319: The reducing equivalents for fatty acid synthesis are derived from which metabolic pathway?

A. Glycolysis
B. Kreb's cycle
C. Urea cycle
D. Pentose phosphate pathway (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Pentose Phosphate Pathway (PPP)** Fatty acid synthesis (lipogenesis) is a reductive process that occurs in the cytosol. It requires **NADPH** as the essential reducing equivalent to reduce the keto groups during the elongation of the fatty acid chain. The primary source of this NADPH (approx. 60%) is the **Pentose Phosphate Pathway** (also known as the Hexose Monophosphate Shunt), specifically through the oxidative reactions catalyzed by **Glucose-6-Phosphate Dehydrogenase (G6PD)** and 6-Phosphogluconate Dehydrogenase. Another significant source is the **Malic Enzyme**, which converts malate to pyruvate in the cytosol. **Why other options are incorrect:** * **A. Glycolysis:** This pathway produces **NADH**, not NADPH. NADH is primarily used for ATP production via the electron transport chain, whereas NADPH is used for reductive biosynthesis. * **B. Kreb’s Cycle:** This mitochondrial pathway generates reducing equivalents in the form of **NADH and FADH₂**, which are utilized for oxidative phosphorylation. * **C. Urea Cycle:** This is a metabolic pathway for nitrogen disposal. It does not generate reducing equivalents; in fact, it consumes ATP. **High-Yield Facts for NEET-PG:** * **Rate-limiting enzyme of Fatty Acid Synthesis:** Acetyl-CoA Carboxylase (requires Biotin). * **Tissues with active PPP:** Tissues active in lipid synthesis (Liver, Adipose tissue, Lactating mammary gland, Adrenal cortex) have high PPP activity to supply NADPH. * **Other uses of NADPH:** Steroid synthesis, maintenance of reduced glutathione in RBCs (preventing hemolysis), and the Respiratory Burst in phagocytes (NADPH Oxidase). * **Key Source Mnemonic:** "Fatty acid synthesis needs **NADPH** from the **HMP** shunt and **Malic** enzyme."

Question 320: Which of the following hormones has a ketogenic effect on the liver?

A. Insulin
B. Glucagon (Correct Answer)
C. Growth Hormone (GH)
D. Androgen

Explanation: **Explanation:** **Glucagon** is the primary hormone responsible for stimulating ketogenesis in the liver. During fasting or starvation, the insulin-to-glucagon ratio decreases. Glucagon acts on the liver to decrease levels of **Malonyl-CoA** (by inhibiting Acetyl-CoA Carboxylase). Since Malonyl-CoA is a potent inhibitor of **Carnitine Palmitoyltransferase-I (CPT-I)**, its decrease allows fatty acids to enter the mitochondria via the carnitine shuttle. Once inside, these fatty acids undergo $\beta$-oxidation to produce Acetyl-CoA, which is then diverted into the synthesis of ketone bodies (Acetoacetate and $\beta$-hydroxybutyrate). **Analysis of Incorrect Options:** * **Insulin:** This is the most potent **anti-ketogenic** hormone. It stimulates lipogenesis, increases Malonyl-CoA levels (inhibiting CPT-I), and promotes the peripheral utilization of glucose, thereby suppressing ketone body formation. * **Growth Hormone (GH):** While GH is lipolytic (breaks down fats in adipose tissue), its primary role in the liver is promoting gluconeogenesis and IGF-1 production. It is considered "diabetogenic" but is not the primary driver of hepatic ketogenesis compared to glucagon. * **Androgens:** These are anabolic steroids that influence protein synthesis and secondary sexual characteristics; they have no significant direct regulatory effect on the ketogenic pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of ketogenesis:** HMG-CoA Synthase (mitochondrial). * **Ketone bodies:** Acetoacetate, $\beta$-hydroxybutyrate, and Acetone (a non-metabolizable byproduct). * **Organ utilization:** The liver **produces** ketone bodies but **cannot use** them because it lacks the enzyme **Thiophorase** (Succinyl-CoA:3-ketoacid CoA transferase). * **Diabetic Ketoacidosis (DKA):** Occurs due to absolute insulin deficiency and a relative glucagon excess, leading to unrestrained ketogenesis.

Practice by Chapter

Lipid Classification and Chemistry

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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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