Lipid Metabolism Practice Questions

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

Lipocortin. **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).

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

Apoprotein C-II inhibits lipoprotein lipase. ### 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.

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

Atherosclerosis. **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 50 mg/dL significantly increase cardiovascular risk.

Q: A 8-month-old infant is brought to the hospital presenting with loss of previously acquired motor skills. The infant also has hepatosplenomegaly. A liver biopsy reveals leukocytes with intracellular sphingomyelin accumulations. What is the possible diagnosis?

Niemann-Pick disease. ### Explanation **Correct Answer: C. Niemann-Pick disease** **1. Why it is correct:** Niemann-Pick disease (specifically Types A and B) is caused by a deficiency of the enzyme **Acid Sphingomyelinase**. This leads to the accumulation of **sphingomyelin** within the lysosomes of macrophages (foam cells) in various tissues. The clinical presentation of an infant with neurodegeneration (loss of motor skills) and **hepatosplenomegaly** is classic for Type A. The biopsy finding of intracellular sphingomyelin accumulation is the definitive biochemical marker for this condition. **2. Why other options are incorrect:** * **Gaucher disease:** While it presents with hepatosplenomegaly and bone involvement, the accumulated substance is **glucocerebroside** (due to glucocerebrosidase deficiency), and biopsy would show "crinkled paper" Gaucher cells. * **Tay-Sachs disease:** This involves **GM2 ganglioside** accumulation. While it causes neurodegeneration and a cherry-red spot, it is distinguished by the **absence of hepatosplenomegaly**. * **Pompe disease:** This is a Glycogen Storage Disease (Type II) caused by acid maltase deficiency. It presents with severe **cardiomegaly** and muscle weakness, not sphingomyelin accumulation. **3. NEET-PG High-Yield Pearls:** * **Cherry-red spot:** Seen in both Niemann-Pick and Tay-Sachs. Use the presence of **hepatosplenomegaly** to differentiate (Present in Niemann-Pick; Absent in Tay-Sachs). * **Histology:** Look for **"Foam cells"** (lipid-laden macrophages) in Niemann-Pick. * **Inheritance:** Most sphingolipidoses are Autosomal Recessive (except Fabry disease, which is X-linked Recessive). * **Niemann-Pick Type C:** Caused by a defect in cholesterol transport (NPC1/NPC2 genes), not primarily sphingomyelinase.

Q: All of the following statements about estrogen are TRUE EXCEPT:

Decreases HDL. **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**.

Q: 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?

Familial chylomicronemia syndrome. **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.

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

Starvation. **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).

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

12-Hydroperoxyeicosatetraenoic acid (HPETE). **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 1

An 8-year-old child is referred with a history of mononeuritis multiplex and hepatosplenomegaly. Examination of the oral cavity and pharynx revealed enlarged orange-colored tonsils. Which of the following statements regarding this condition is false?

Accepted Answer

It is an autosomal recessive condition.

Answer

It is a case of Tangier disease.

Answer

The plasma LDL-c levels will be low.

Answer

It is caused by ABCA1 deficiency.

Question 2

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

Accepted Answer

Lipocortin

Answer

Vitamin D

Answer

Progesterone

Answer

Bile acids

Question 3

All of the following statements about apoproteins are true except?

Accepted Answer

Apoprotein C-II inhibits lipoprotein lipase

Answer

Apoprotein A-I activates LCAT

Answer

Apoprotein C-I activates lipoprotein lipase

Answer

Apoprotein C-II activates lipoprotein lipase

Question 4

In starvation, there is ketosis due to which of the following?

Accepted Answer

Increased beta-oxidation

Answer

Decreased acetyl CoA

Answer

Decreased lipolysis

Answer

Decreased fatty acid synthesis

Question 5

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

Accepted Answer

Atherosclerosis

Answer

Cirrhosis of liver

Answer

Rheumatic arthritis

Answer

Cervical cancer

Question 6

A 8-month-old infant is brought to the hospital presenting with loss of previously acquired motor skills. The infant also has hepatosplenomegaly. A liver biopsy reveals leukocytes with intracellular sphingomyelin accumulations. What is the possible diagnosis?

Accepted Answer

Niemann-Pick disease

Answer

Gaucher disease

Answer

Tay-Sachs disease

Answer

Pompe disease

Question 7

All of the following statements about estrogen are TRUE EXCEPT:

Accepted Answer

Decreases HDL

Answer

Increases triglycerides

Answer

Increases turnover of LDL receptors

Answer

Increases apolipoprotein A

Question 8

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?

Accepted Answer

Familial chylomicronemia syndrome

Answer

Familial defective Apo B

Answer

Sitosterolemia

Answer

Familial dysbetalipoproteinemia

Question 9

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

Accepted Answer

Starvation

Answer

Dehydration

Answer

Hyperglycemia

Answer

Infections

Question 10

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

Accepted Answer

12-Hydroperoxyeicosatetraenoic acid (HPETE)

Answer

Leukotrienes (LTs)

Answer

Hydroxyeicosatetraenoic acid (HETE)

Answer

Lipoxins

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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