Lipid Metabolism — MCQs

A 2-year-old girl presented with recurrent abdominal pain. On investigation, her blood was noted to be milky white. Yellowish-white papules were seen on the dorsum of her hands. Fundoscopic examination revealed opalescent retinal vessels. Her fasting triglycerides were >1000 mg/dL, while her cholesterol level was normal. What is the diagnosis?

Q147Easy

Besides uncontrolled diabetes, ketone bodies are seen in which of the following conditions?

Q148Easy

What is the first intermediate product of the lipoxygenase (LOX) pathway?

Q149Easy

What is the chemical formula C17H31COOH?

Q150Easy

Which of the following statements is true about secondary bile acids?

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 141: Which of the following enzymes does not participate in fatty acid synthesis?

A. Hydratase
B. Reductase
C. Transacylase
D. Dehydrogenase (Correct Answer)

Explanation: ### Explanation **Fatty Acid Synthesis (Lipogenesis)** is a reductive process that occurs in the cytoplasm, primarily catalyzed by the multi-enzyme complex **Fatty Acid Synthase (FAS)**. The synthesis follows a repetitive four-step cycle: Condensation, Reduction, Dehydration, and Reduction. **Why Dehydrogenase is the Correct Answer:** Dehydrogenases are enzymes that catalyze the removal of hydrogen atoms, typically transferring them to electron acceptors like $NAD^+$ or $FAD$. This process is characteristic of **$\beta$-oxidation (Fatty Acid Breakdown)**, which occurs in the mitochondria. In contrast, fatty acid synthesis requires the *addition* of electrons, utilizing **Reductases** and **NADPH** as the reducing power. Therefore, Dehydrogenase does not participate in the synthetic pathway. **Analysis of Incorrect Options:** * **Transacylase:** These enzymes (Acetyl CoA-ACP transacylase and Malonyl CoA-ACP transacylase) are essential for "priming" the reaction by transferring acetyl and malonyl groups to the Fatty Acid Synthase complex. * **Reductase:** Synthesis involves two reduction steps catalyzed by **$\beta$-ketoacyl reductase** and **Enoyl reductase**. These steps utilize NADPH to saturate the carbon chain. * **Hydratase:** While the synthesis step is technically a **Dehydratase** (removing water to form a double bond), the term "Hydratase" refers to the manipulation of water across double bonds. In the context of the FAS complex, the 3-hydroxyacyl dehydratase is a core component. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** Synthesis occurs in the **Cytosol**, while $\beta$-oxidation occurs in the **Mitochondria**. * **Reducing Agent:** **NADPH** is the essential electron donor for synthesis (primarily sourced from the HMP Shunt). * **Rate-Limiting Enzyme:** **Acetyl-CoA Carboxylase (ACC)**, which requires **Biotin** as a cofactor. * **End Product:** The primary end product of the FAS complex is **Palmitate** (a 16-carbon saturated fatty acid).

Question 142: Cholesterol is not a precursor for the synthesis of which of the following?

A. Vitamin D
B. Progesterone
C. Bile acids
D. Lipocortin (Correct Answer)

Explanation: **Explanation:** The correct answer is **Lipocortin**. Cholesterol is a 27-carbon steroid molecule that serves as the essential structural precursor for all steroid hormones, bile acids, and Vitamin D. **Lipocortin** (also known as Annexin A1), however, is a **protein**. It is synthesized in response to glucocorticoids and acts by inhibiting Phospholipase A2, thereby reducing the release of arachidonic acid and exerting anti-inflammatory effects. Since it is a protein, it is synthesized via translation of mRNA, not from a cholesterol backbone. **Why other options are incorrect:** * **Vitamin D:** Cholecalciferol is synthesized from **7-dehydrocholesterol** (an intermediate in cholesterol synthesis) in the skin via UV light exposure. * **Progesterone:** This is a steroid hormone. All steroid hormones (progestagens, glucocorticoids, mineralocorticoids, androgens, and estrogens) are derived from cholesterol via the rate-limiting step of converting cholesterol to **pregnenolone**. * **Bile acids:** Cholic acid and chenodeoxycholic acid are synthesized directly from cholesterol in the liver. The rate-limiting enzyme is **7-alpha-hydroxylase**. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of Cholesterol synthesis:** HMG-CoA Reductase (inhibited by Statins). * **Rate-limiting enzyme of Bile Acid synthesis:** 7-alpha-hydroxylase. * **Steroidogenesis:** The conversion of cholesterol to pregnenolone occurs in the mitochondria by the enzyme **Desmolase** (CYP11A1). * **Lipocortin Mechanism:** It is the primary mediator of the anti-inflammatory action of steroids (Glucocorticoids → ↑ Lipocortin → ↓ Phospholipase A2 → ↓ Prostaglandins/Leukotrienes).

Question 143: All of the following statements about apoproteins are true except?

A. Apoprotein A-I activates LCAT
B. Apoprotein C-I activates lipoprotein lipase
C. Apoprotein C-II inhibits lipoprotein lipase (Correct Answer)
D. Apoprotein C-II activates lipoprotein lipase

Explanation: ### Explanation This question tests your knowledge of the functional roles of apolipoproteins in lipid metabolism, a high-yield area for NEET-PG. **1. Why Option C is the Correct Answer (The "Except" Statement):** Apoprotein **C-II** is a mandatory cofactor that **activates** Lipoprotein Lipase (LPL). LPL is the enzyme responsible for hydrolyzing triglycerides in chylomicrons and VLDL into free fatty acids and glycerol. Therefore, the statement that Apo C-II *inhibits* LPL is physiologically incorrect, making it the right choice for this "except" question. **2. Analysis of Other Options:** * **Option A (True):** **Apo A-I** is the major protein component of HDL. It acts as a potent activator of **LCAT** (Lecithin-Cholesterol Acyltransferase), which esterifies cholesterol, allowing HDL to sequester cholesterol from peripheral tissues (Reverse Cholesterol Transport). * **Option B (True):** While Apo C-II is the primary activator, **Apo C-I** is also known to activate LPL, though its role is less dominant in clinical discussions compared to C-II. * **Option D (True):** As established, Apo C-II is the essential activator of LPL. A deficiency in either LPL or Apo C-II leads to **Type I Hyperlipoproteinemia** (Familial Chylomicronemia Syndrome). **3. High-Yield Clinical Pearls for NEET-PG:** * **Apo B-48:** Found in Chylomicrons (derived from the intestine; lacks the LDL receptor-binding domain). * **Apo B-100:** Found in VLDL, IDL, and LDL (ligand for the LDL receptor). * **Apo E:** Essential for the hepatic uptake of chylomicron remnants and IDL via the LDL receptor-related protein (LRP). * **Apo (a):** A specific protein linked to Apo B-100 by a disulfide bond to form **Lipoprotein(a)**; high levels are a significant risk factor for atherosclerosis.

Question 144: Raised serum level of lipoprotein-(a) is a predictor of?

A. Cirrhosis of liver
B. Rheumatic arthritis
C. Atherosclerosis (Correct Answer)
D. Cervical cancer

Explanation: **Explanation:** **Lipoprotein(a) [Lp(a)]** is a specialized lipoprotein consisting of a Low-Density Lipoprotein (LDL) particle covalently linked to a unique glycoprotein called **Apolipoprotein(a)** via a disulfide bridge. **Why Atherosclerosis is the Correct Answer:** Lp(a) is a potent independent risk factor for **Atherosclerosis** and coronary artery disease (CAD) due to two primary mechanisms: 1. **Pro-atherogenic:** Like LDL, it deposits cholesterol into the arterial walls. 2. **Pro-thrombotic:** Apo(a) shares significant structural homology with **plasminogen**. It competitively inhibits plasminogen activation, thereby inhibiting fibrinolysis (clot breakdown) and promoting thrombosis at the site of atherosclerotic plaques. **Why Other Options are Incorrect:** * **Cirrhosis of liver:** Lp(a) is synthesized in the liver. In advanced cirrhosis, the synthetic function of the liver declines, typically leading to *decreased* levels of lipoproteins, not raised levels. * **Rheumatic arthritis:** While chronic inflammation can alter lipid profiles, Lp(a) is not a specific diagnostic or predictive marker for RA. * **Cervical cancer:** There is no established clinical correlation between serum Lp(a) levels and the pathogenesis or prediction of cervical cancer. **High-Yield Clinical Pearls for NEET-PG:** * **Structural Analogy:** Apo(a) is a "kringle-containing" protein similar to plasminogen. * **Genetic Determination:** Lp(a) levels are largely genetically determined and are not significantly affected by diet or most statins. * **Treatment:** Niacin (Vitamin B3) and newer PCSK9 inhibitors are known to lower Lp(a) levels. * **Normal Range:** Levels <30 mg/dL are considered normal; levels >50 mg/dL significantly increase cardiovascular risk.

Question 145: All of the following statements about estrogen are TRUE EXCEPT:

A. Decreases HDL (Correct Answer)
B. Increases triglycerides
C. Increases turnover of LDL receptors
D. Increases apolipoprotein A

Explanation: **Explanation:** The correct answer is **A (Decreases HDL)**. Estrogen is fundamentally **cardioprotective** because it improves the lipid profile; therefore, it **increases HDL** (the "good" cholesterol) rather than decreasing it. **1. Why Option A is the Correct (False) Statement:** Estrogen increases the expression of **Apolipoprotein A-I**, which is the primary structural protein of HDL. It also inhibits hepatic lipase activity, the enzyme responsible for HDL degradation. By increasing production and decreasing clearance, estrogen significantly raises plasma HDL levels. **2. Analysis of Other Options:** * **Option B (Increases triglycerides):** Estrogen stimulates the hepatic synthesis of VLDL (Very Low-Density Lipoprotein), which is rich in triglycerides. This is why oral contraceptive pills or hormone replacement therapy can sometimes lead to mild hypertriglyceridemia. * **Option C (Increases turnover of LDL receptors):** Estrogen upregulates the expression of LDL receptors in the liver. This increases the "turnover" (clearance) of LDL from the blood, leading to a decrease in plasma LDL levels. * **Option D (Increases apolipoprotein A):** As mentioned, estrogen directly promotes the synthesis of Apo A-I and Apo A-II, which are the building blocks of HDL particles. **High-Yield Clinical Pearls for NEET-PG:** * **Post-menopausal shift:** After menopause, the loss of estrogen leads to an increase in LDL and a decrease in HDL, contributing to the increased risk of coronary artery disease in older women. * **Lipoprotein (a):** Estrogen is known to **decrease** levels of Lp(a), an independent risk factor for atherosclerosis. * **Bile Composition:** Estrogen increases the saturation of cholesterol in bile (by increasing cholesterol secretion), which explains why females and those on OCPs have a higher incidence of **gallstones**.

Question 146: A 2-year-old girl presented with recurrent abdominal pain. On investigation, her blood was noted to be milky white. Yellowish-white papules were seen on the dorsum of her hands. Fundoscopic examination revealed opalescent retinal vessels. Her fasting triglycerides were >1000 mg/dL, while her cholesterol level was normal. What is the diagnosis?

A. Familial chylomicronemia syndrome (Correct Answer)
B. Familial defective Apo B
C. Sitosterolemia
D. Familial dysbetalipoproteinemia

Explanation: **Explanation:** The clinical presentation described is a classic case of **Familial Chylomicronemia Syndrome (FCS)**, also known as **Type I Hyperlipoproteinemia**. **1. Why the Correct Answer is Right:** FCS is caused by a deficiency in **Lipoprotein Lipase (LPL)** or its cofactor, **Apo C-II**. This leads to an inability to clear chylomicrons from the blood. * **Milky white blood (Lactescent serum):** Due to massive accumulation of chylomicrons (which are triglyceride-rich). * **Hypertriglyceridemia (>1000 mg/dL):** Chylomicrons primarily carry exogenous triglycerides. * **Eruptive Xanthomas:** The "yellowish-white papules" on the dorsum of hands/buttocks. * **Lipemia Retinalis:** The "opalescent retinal vessels" seen on fundoscopy. * **Abdominal Pain:** Recurrent episodes are often due to **acute pancreatitis**, a major complication of severe hypertriglyceridemia. **2. Why Incorrect Options are Wrong:** * **Familial Defective Apo B-100 (Type IIa):** Characterized by high **LDL cholesterol**, not triglycerides. It presents with xanthelasmas and tendon xanthomas, not milky serum. * **Sitosterolemia:** A rare plant sterol storage disease. It presents with tendon xanthomas and premature atherosclerosis, but triglyceride levels are typically normal. * **Familial Dysbetalipoproteinemia (Type III):** Caused by **Apo E deficiency**, leading to high IDL and Chylomicron remnants. It presents with **palmar xanthomas** and elevated levels of *both* cholesterol and triglycerides. **3. High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Test:** The "Refrigeration Test" shows a **creamy layer on top** with clear infranatant. * **Genetics:** Autosomal Recessive. * **Management:** Strict fat-restricted diet (Chylomicrons are formed from dietary fat). * **Key Enzyme:** LPL is anchored to capillary endothelium by heparan sulfate; its activity increases after an injection of heparin.

Question 147: Besides uncontrolled diabetes, ketone bodies are seen in which of the following conditions?

A. Starvation (Correct Answer)
B. Dehydration
C. Hyperglycemia
D. Infections

Explanation: **Explanation:** Ketone bodies (Acetoacetate, 3-hydroxybutyrate, and Acetone) are produced by the liver through **ketogenesis** when the body shifts from carbohydrate metabolism to fat oxidation for energy. **1. Why Starvation is Correct:** In both uncontrolled diabetes and starvation, there is a **low Insulin-to-Glucagon ratio**. In starvation, the lack of dietary glucose leads to glycogen depletion. To maintain energy, the body triggers lipolysis in adipose tissue, releasing free fatty acids (FFAs). These FFAs undergo β-oxidation in the liver to produce **Acetyl-CoA**. Because oxaloacetate is diverted toward gluconeogenesis to maintain blood glucose, Acetyl-CoA cannot enter the TCA cycle and is instead diverted to form ketone bodies. **2. Why Incorrect Options are Wrong:** * **Dehydration:** While dehydration can coexist with diabetic ketoacidosis (DKA), it is a fluid-volume status issue and does not biochemically trigger ketogenesis. * **Hyperglycemia:** High blood glucose alone does not cause ketosis. In Type 2 Diabetes, patients are often hyperglycemic but have enough insulin to suppress ketogenesis. Ketosis only occurs when there is a functional "intracellular starvation." * **Infections:** While infections can *precipitate* DKA in a diabetic patient by increasing stress hormones (cortisol/epinephrine), infection itself is not a primary metabolic cause of ketone body production. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Ketogenesis:** Liver (Mitochondria). * **Site of Ketolysis:** Extrahepatic tissues (Brain, Heart, Muscle). **The liver cannot use ketone bodies** because it lacks the enzyme **Thiophorase** (Succinyl-CoA:3-ketoacid CoA transferase). * **Rate-limiting enzyme:** HMG-CoA Synthase. * **Rothera’s Test:** Detects Acetoacetate and Acetone (not 3-hydroxybutyrate).

Question 148: What is the first intermediate product of the lipoxygenase (LOX) pathway?

A. Leukotrienes (LTs)
B. 12-Hydroperoxyeicosatetraenoic acid (HPETE) (Correct Answer)
C. Hydroxyeicosatetraenoic acid (HETE)
D. Lipoxins

Explanation: **Explanation:** The **Lipoxygenase (LOX) pathway** is a major metabolic route for arachidonic acid, primarily occurring in leukocytes, platelets, and macrophages. **Why Option B is correct:** The initial step in the LOX pathway involves the incorporation of oxygen into arachidonic acid by the enzyme lipoxygenase. This reaction produces **Hydroperoxyeicosatetraenoic acids (HPETEs)** as the **first intermediate products**. Depending on the specific enzyme (5-LOX, 12-LOX, or 15-LOX), different HPETEs are formed. For example, 12-LOX (found in platelets) produces 12-HPETE. These HPETEs are unstable and are rapidly converted into more stable compounds. **Why other options are incorrect:** * **Option A (Leukotrienes):** These are secondary products derived from 5-HPETE. For instance, 5-HPETE is converted to Leukotriene A4 (LTA4), which then forms other LTs (LTB4, LTC4, etc.). * **Option C (HETE):** HPETEs are chemically reduced by peroxidase enzymes to form the more stable **Hydroxyeicosatetraenoic acids (HETEs)**. Thus, HETEs are subsequent products, not the first intermediates. * **Option D (Lipoxins):** These are anti-inflammatory mediators formed via the action of 15-LOX followed by 5-LOX. They are end-products of the pathway, not initial intermediates. **High-Yield Clinical Pearls for NEET-PG:** * **5-LOX Inhibitor:** Zileuton (used in chronic asthma management). * **LT Receptor Antagonists:** Montelukast and Zafirlukast (block CysLT1 receptors). * **Slow-Reacting Substance of Anaphylaxis (SRS-A):** A mixture of LTC4, LTD4, and LTE4, responsible for prolonged bronchoconstriction. * **Chemotaxis:** LTB4 is a potent chemoattractant for neutrophils (Mnemonic: **B**4 = **B**rings neutrophils).

Question 149: What is the chemical formula C17H31COOH?

A. Linoleic acid (Correct Answer)
B. Linolenic acid
C. Arachidonic acid
D. Timnodonic acid

Explanation: ### Explanation **1. Why Linoleic Acid is Correct:** The chemical formula **C₁₇H₃₁COOH** represents a fatty acid with a total of **18 carbons** (17 in the chain + 1 in the carboxyl group). To determine the degree of saturation, we compare it to a saturated fatty acid of the same length (Stearic acid, C₁₇H₃₅COOH). * Linoleic acid is an **18:2 (Δ9, 12)** fatty acid. * Each double bond removes two hydrogen atoms. Since it has two double bonds, it has four fewer hydrogens than stearic acid (35 - 4 = 31). * Therefore, C₁₇H₃₁COOH is the formula for Linoleic acid, an essential **Omega-6 (ω-6)** fatty acid. **2. Why the Other Options are Incorrect:** * **Linolenic acid (B):** This is an **18:3** fatty acid (Omega-3). With three double bonds, it would have six fewer hydrogens than stearic acid, resulting in **C₁₇H₂₉COOH**. * **Arachidonic acid (C):** This is a **20:4** fatty acid (Omega-6). It contains 20 carbons, so its formula is **C₁₉H₃₁COOH**. * **Timnodonic acid (D):** Also known as **EPA (Eicosapentaenoic acid)**, it is a **20:5** fatty acid (Omega-3). Its formula is **C₁₉H₂₉COOH**. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Essential Fatty Acids (EFA):** Humans lack the enzymes (**Δ12 and Δ15 desaturases**) to introduce double bonds beyond carbon 9; thus, Linoleic and Linolenic acids must be obtained from the diet. * **Arachidonic Acid:** It is "semi-essential" because it can be synthesized from Linoleic acid. It is the precursor for **prostaglandins, thromboxanes, and leukotrienes**. * **Deficiency:** EFA deficiency leads to **Phrynoderma** (follicular hyperkeratosis or "toad skin"). * **Mnemonic:** **L**inoleic (2 double bonds), **L**inolenic (3 double bonds) — "Alphabetical order: 2 comes before 3."

Question 150: Which of the following statements is true about secondary bile acids?

A. Primary bile acids conjugated with taurine and glycine form secondary bile acids.
B. Deconjugated by bacteria in the liver.
C. Reabsorbed by enterohepatic circulation. (Correct Answer)
D. They are formed from bilirubin.

Explanation: **Explanation:** Bile acid metabolism is a high-yield topic for NEET-PG. To understand the correct answer, one must distinguish between primary and secondary bile acids. **1. Why Option C is Correct:** Secondary bile acids (Deoxycholic acid and Lithocholic acid) are formed in the **colon** by the action of intestinal bacteria on primary bile acids. Like primary bile acids, they are largely **reabsorbed** (primarily in the ileum) and transported back to the liver via the portal blood. This recycling process is known as **enterohepatic circulation**, which ensures that the body maintains a sufficient bile acid pool without constant de novo synthesis. **2. Why the Other Options are Incorrect:** * **Option A:** Conjugation with taurine or glycine occurs in the **liver** to form bile salts (e.g., Glycocholic acid). This process makes them more water-soluble but does not turn them into "secondary" bile acids. * **Option B:** Deconjugation and dehydroxylation are performed by **intestinal bacteria in the gut**, not in the liver. The liver's role is synthesis and conjugation. * **Option D:** Bile acids are synthesized from **cholesterol**, not bilirubin. Bilirubin is a breakdown product of heme and is a bile pigment, not a bile acid. **Clinical Pearls for NEET-PG:** * **Primary Bile Acids:** Cholic acid and Chenodeoxycholic acid (Mnemonic: **C**ome **C**lean). * **Secondary Bile Acids:** Deoxycholic acid and Lithocholic acid (Mnemonic: **D**irty **L**ittle secrets—formed by bacteria). * **Rate-limiting enzyme:** Cholesterol 7-alpha-hydroxylase (inhibited by bile acids via feedback). * **Cholestyramine:** A bile acid sequestrant that interrupts enterohepatic circulation to lower LDL cholesterol.

Practice by Chapter

Lipid Classification and Chemistry

Practice Questions

Fatty Acid Oxidation

Practice Questions

Ketone Body Metabolism

Practice Questions

Fatty Acid Synthesis

Practice Questions

Metabolism of Triacylglycerols

Practice Questions

Phospholipid Metabolism

Practice Questions

Cholesterol Metabolism and Biosynthesis

Practice Questions

Bile Acids and Bile Salts

Practice Questions

Lipoprotein Metabolism and Transport

Practice Questions

Dyslipidemias and Atherosclerosis

Practice Questions

Prostaglandins and Eicosanoids

Practice Questions

Fatty Liver and Lipotropic Factors

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free