Lipid Metabolism Practice Questions

Q: The chyle from the intestine is rich with chylomicrons. Which of the following forms the protein core of chylomicrons?

Triglyceride + cholesterol + phospholipid. ### Explanation **Concept Overview:** Chylomicrons are the largest and least dense lipoproteins, synthesized in the intestinal mucosal cells to transport dietary (exogenous) lipids into the circulation. A lipoprotein particle is structured with a **hydrophobic core** and a **hydrophilic shell**. **Why Option C is Correct:** The "protein core" or the internal lipid cargo of a chylomicron consists primarily of **Triglycerides** (about 85-90%) and **Cholesteryl esters**. While phospholipids and free cholesterol are typically found on the surface membrane to provide solubility, the question refers to the overall lipid composition that constitutes the bulk of the particle. In the context of this question, the chylomicron is a complex assembly of **Triglycerides + Cholesterol + Phospholipids** (along with specific apolipoproteins like Apo B-48). **Analysis of Incorrect Options:** * **Options A, B, and D:** These are incomplete. Chylomicrons are never composed of a single lipid type. While triglycerides are the predominant component, they must be packaged with cholesterol and phospholipids to form a stable lipoprotein particle capable of entering the lymphatic system (chyle). **NEET-PG High-Yield Pearls:** * **Apolipoprotein Marker:** **Apo B-48** is the unique structural protein for chylomicrons (synthesized via RNA editing of the Apo B gene in the intestine). * **Route of Transport:** Chylomicrons are too large to enter capillaries directly; they enter **lacteals** (lymphatics), forming **chyle**, and reach the blood via the **thoracic duct**. * **Activation:** They receive **Apo C-II** and **Apo E** from HDL in the plasma. Apo C-II is essential for activating **Lipoprotein Lipase (LPL)** to release free fatty acids. * **Appearance:** Post-prandial plasma appears milky due to high chylomicron content (Chylomicronemia).

Q: An 8-month-old child presented with reduced appetite, abdominal distension and pain, and psychomotor retardation. The child was normal at birth, and both parents are normal. On examination: Hepatosplenomegaly, moderate lymphadenopathy, abnormal posturing of the limbs, trunk, and face, impaired voluntary rapid eye movements, cherry-red spot on fundus examination, and bony defects. Lymph node histopathology and electron microscopy findings were noted. Which of the following enzymes is most likely deficient in the above disease?

Sphingomyelinase. The clinical presentation points toward **Niemann-Pick Disease (NPD) Type A**, a lysosomal storage disorder characterized by the deficiency of the enzyme **Sphingomyelinase**. ### **Why Sphingomyelinase is Correct** The combination of **hepatosplenomegaly** and a **cherry-red spot** on the fundus is the classic "hallmark" for Niemann-Pick Type A. While Tay-Sachs also presents with a cherry-red spot, it lacks organomegaly. The "abnormal posturing" and "impaired rapid eye movements" (supranuclear gaze palsy) are characteristic neurological findings. The accumulation of sphingomyelin in the reticuloendothelial system leads to lymphadenopathy and bony defects (due to marrow expansion). Histopathology typically reveals "foam cells" (lipid-laden macrophages). ### **Why Other Options are Incorrect** * **Hexosaminidase A (Tay-Sachs Disease):** Presents with a cherry-red spot and psychomotor regression, but **never** features hepatosplenomegaly or bony defects. * **Alpha-galactosidase A (Fabry Disease):** Presents with angiokeratomas, peripheral neuropathy (burning pain), and renal failure. It does not typically cause a cherry-red spot or early infancy psychomotor retardation. * **Glucocerebrosidase (Gaucher Disease):** The most common lysosomal storage disorder. While it features massive hepatosplenomegaly and bony "Erlenmeyer flask" deformities, it **does not** present with a cherry-red spot. ### **NEET-PG High-Yield Pearls** * **Niemann-Pick Type A:** Deficiency of Sphingomyelinase; Cherry-red spot + Hepatosplenomegaly; Foam cells on biopsy. * **Tay-Sachs:** Deficiency of Hexosaminidase A; Cherry-red spot + **No** Hepatosplenomegaly; Onion-skin lysosomes on EM. * **Gaucher:** Deficiency of Glucocerebrosidase; Hepatosplenomegaly + Bony crises; **Crumpled tissue paper** appearance of macrophages. * **Cherry-red spot differential:** Niemann-Pick, Tay-Sachs, Sandhoff disease, Sialidosis, and Central Retinal Artery Occlusion (CRAO).

Q: Insulin inhibits ketogenesis by all except:

Increasing b-oxidation. **Explanation:** Ketogenesis occurs primarily in the liver during states of low insulin and high glucagon (starvation or uncontrolled diabetes). Insulin is a potent **anti-ketogenic** hormone. **Why Option D is the correct answer:** Insulin **inhibits** beta-oxidation; it does not increase it. Insulin stimulates the synthesis of **Malonyl-CoA** (via Acetyl-CoA Carboxylase). Malonyl-CoA inhibits **Carnitine Palmitoyltransferase-I (CPT-I)**, the rate-limiting enzyme that transports fatty acids into the mitochondria. By blocking this transport, insulin prevents beta-oxidation, thereby depriving the ketogenesis pathway of its substrate (Acetyl-CoA). **Analysis of Incorrect Options:** * **A. Inhibiting lipolysis:** Insulin inhibits Hormone-Sensitive Lipase (HSL) in adipose tissue. This reduces the release of free fatty acids (FFAs) into the blood, leaving the liver with no raw material for ketone bodies. * **B. Increased esterification:** Insulin promotes the conversion of FFAs into triglycerides (esterification) rather than allowing them to enter the ketogenic pathway. * **C. Directing acetyl-CoA to TCA cycle:** By promoting the fed state, insulin ensures that oxaloacetate is available to condense with Acetyl-CoA to enter the TCA cycle, rather than diverting Acetyl-CoA toward HMG-CoA synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of Ketogenesis:** HMG-CoA Synthase (Mitochondrial). * **Ketone bodies:** Acetoacetate, 3-hydroxybutyrate, and Acetone (non-metabolizable). * **Key Regulator:** Malonyl-CoA is the "gatekeeper," preventing the simultaneous occurrence of fatty acid synthesis and beta-oxidation (Reciprocal Regulation). * **Organ utilization:** The brain, heart, and skeletal muscle can use ketones, but the **liver cannot** (due to lack of Thiophorase/succinyl-CoA:3-ketoacid CoA-transferase).

Q: Which lipoprotein has the highest cholesterol content?

LDL (Low-density lipoprotein). **Explanation:** Lipoproteins are classified based on their density and relative composition of lipids (triglycerides, cholesterol, phospholipids) and proteins. **Why LDL is the correct answer:** Low-density lipoprotein (LDL) is the primary carrier of cholesterol in the blood. It contains the highest percentage of **cholesterol and cholesterol esters** (approximately 50% of its total weight). LDL is formed from the metabolism of VLDL and IDL; as triglycerides are removed by lipoprotein lipase, the particle becomes enriched with cholesterol. Its primary function is to transport cholesterol from the liver to peripheral tissues. **Why the other options are incorrect:** * **Chylomicrons:** These have the lowest density and the largest size. They are composed predominantly of **triglycerides (85-90%)** derived from dietary intake. * **VLDL:** These are synthesized in the liver and primarily transport **endogenous triglycerides (55-65%)**. * **IDL:** This is a transient intermediate formed during the conversion of VLDL to LDL. While it contains more cholesterol than VLDL, it still has a higher triglyceride-to-cholesterol ratio compared to LDL. **High-Yield NEET-PG Clinical Pearls:** * **"Bad Cholesterol":** LDL is termed "bad" because high levels are strongly associated with atherosclerosis and coronary artery disease. * **Apolipoprotein B-100:** This is the characteristic structural protein found in VLDL, IDL, and LDL. * **Friedewald Equation:** Used to calculate LDL cholesterol: $LDL = Total\ Cholesterol - HDL - (Triglycerides/5)$. (Note: This is invalid if TG >400 mg/dL). * **HDL:** Known as "Good Cholesterol," it has the highest **protein** content and is involved in reverse cholesterol transport.

Q: A patient presents with palmar xanthomas, indicating an increased risk of atherosclerosis and coronary artery disease. Lipid profile reveals elevated triacylglycerols and cholesterol, with increased levels of IDL and chylomicrons. What is the pathophysiology of this condition?

Apo E deficiency. ### Explanation The clinical presentation of **Palmar Xanthomas** (xanthoma striatum palmare) combined with elevated IDL and chylomicron remnants is pathognomonic for **Type III Hyperlipoproteinemia** (also known as Dysbetalipoproteinemia or Broad Beta Disease). **1. Why Apo E deficiency is correct:** Apolipoprotein E (Apo E) serves as the essential ligand for the hepatic uptake of **chylomicron remnants** and **IDL** (VLDL remnants) via the LDL-receptor-related protein (LRP) and the LDL receptor. In Apo E deficiency (specifically the E2/E2 homozygous isoform), these remnants cannot be cleared by the liver. They accumulate in the blood, leading to elevated cholesterol and triglycerides, and deposit in the palmar creases, causing characteristic xanthomas. **2. Why other options are incorrect:** * **LDL deficiency:** This would lead to Abetalipoproteinemia, characterized by malabsorption and low cholesterol, not xanthomas or elevated lipids. * **VLDL overproduction:** This is seen in Type IV Hypertriglyceridemia. While it increases triglycerides, it does not typically cause palmar xanthomas or significant IDL accumulation. * **Apo C-2 deficiency:** Apo C-2 is a cofactor for Lipoprotein Lipase (LPL). Deficiency leads to Type I Hyperlipoproteinemia (Chylomicronemia syndrome), characterized by eruptive xanthomas and pancreatitis, but not typically palmar xanthomas or premature atherosclerosis. **3. NEET-PG High-Yield Pearls:** * **Pathognomonic Sign:** Palmar xanthomas = Type III Hyperlipoproteinemia. * **Electrophoresis:** Shows a "Broad Beta Band" due to the overlap of IDL and VLDL. * **Genetics:** Most commonly associated with the **Apo E2 isoform** (which has low affinity for receptors), while Apo E4 is associated with Alzheimer’s disease. * **Risk:** High risk for both Coronary Artery Disease (CAD) and Peripheral Vascular Disease.

Q: A 6-month-old infant is brought to the emergency department with lethargy, vomiting, and poor feeding. The episode occurred after prolonged fasting. Laboratory results reveal hypoglycemia, low ketone levels (hypoketosis), and mild hepatomegaly. These findings suggest a defect in fat metabolism. Which of the following is the most likely underlying disorder?

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency. ***Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency*** - This disorder is the most common defect of **fatty acid oxidation** and is characterized by the inability to break down medium-chain fatty acids during fasting, leading to severe **hypoglycemia**. - The hallmark finding is **hypoketosis** (low ketones), as the inability to generate acetyl-CoA from fatty acid breakdown means the substrate required for **ketogenesis** is unavailable. *Hers disease* - This is **Glycogen Storage Disease type VI**, involving a deficiency in **liver glycogen phosphorylase**, leading to impaired glycogenolysis and fasting hypoglycemia. - However, GSD type VI does not impair **beta-oxidation**; hence, patients usually maintain the ability to produce adequate **ketone bodies** during periods of fasting. *Hereditary fructose intolerance* - This disorder is a defect of **aldolase B** and causes symptoms (vomiting, lethargy, hypoglycemia) only after the introduction of **dietary fructose** or sucrose. - The acute symptoms are due to **phosphate trapping** and subsequent inhibition of gluconeogenesis, which is dependent on dietary exposure rather than prolonged fasting alone. *Glycogen storage disease type I (Von Gierke disease)* - This condition, caused by a deficiency of **glucose-6-phosphatase**, leads to profound fasting hypoglycemia, significant hepatomegaly, and **lactic acidosis**. - Unlike MCAD deficiency, Von Gierke disease primarily causes defects in glucose release but generally maintains or even increases **ketone body production** (hyperketosis) because fat breakdown is often accelerated.

Q: A man presents with high VLDL and chylomicrons, with eruptive xanthomas. What is the most likely diagnosis?

Familial hypertriglyceridemia. ***Familial hypertriglyceridemia (Type IV or V hyperlipidemia)*** - This condition presents with **elevated VLDL and chylomicrons**, causing severe hypertriglyceridemia [1] - **Eruptive xanthomas** are pathognomonic for triglyceride levels >1000 mg/dL, appearing as small yellow papules on extensor surfaces and buttocks - Patients are at risk for **acute pancreatitis** due to extreme triglyceride elevation - Treatment includes **fibrates, omega-3 fatty acids, and strict dietary fat restriction** [2] *Familial hyperlipidemia* - This is a non-specific term that could refer to any inherited lipid disorder - Not a precise diagnostic entity used in clinical practice *Familial dysbetalipoproteinemia (Type III)* - Characterized by elevated **IDL (intermediate-density lipoproteins)**, not primarily VLDL + chylomicrons [1] - Presents with **palmar xanthomas (yellowish discoloration of palmar creases)** and tuberoeruptive xanthomas [1] - Different lipid pattern and xanthoma distribution than described *Familial hypercholesterolemia* - Characterized by elevated **LDL cholesterol**, not triglycerides [1] - Presents with **tendinous xanthomas, xanthelasma, and corneal arcus** [1] - Does not cause eruptive xanthomas or chylomicronemia

Q: Which source and cell produce testosterone?

Cholesterol and Leydig cells. ***Cholesterol and Leydig cells***- **Testosterone** is a steroid hormone, and like all steroid hormones (glucocorticoids, mineralocorticoids, estrogen), it is derived from the precursor molecule, **cholesterol**.- The primary source of testosterone production in the male testes is the **Leydig cells** (interstitial cells), stimulated by **Luteinizing Hormone (LH)**.*Cholesterol and Sertoli cells*- While **cholesterol** is the accurate precursor (source), **Sertoli cells** are mainly responsible for supporting **spermatogenesis** and producing **androgen-binding protein (ABP)** and **inhibin**.- Sertoli cells regulate the testicular microenvironment and are primarily stimulated by **FSH**, not for testosterone synthesis.*Dihydrotestosterone and Leydig cells*- **Dihydrotestosterone (DHT)** is a potent *metabolite* formed from testosterone via the enzyme **5-alpha reductase**, not the precursor for testosterone synthesis itself.- Although **Leydig cells** are the site of testosterone production, the initial source must be **cholesterol**, making DHT incorrect.*FSH and Leydig cells*- **FSH** (Follicle-Stimulating Hormone) primarily targets the **Sertoli cells** to promote sperm development and inhibin production.- The pituitary hormone that stimulates **Leydig cells** to synthesize testosterone from cholesterol is **Luteinizing Hormone (LH)**.

Q: A 90 kg obese man is taking a carbohydrate-rich diet. Which of the following enzymes/intermediates will be elevated in him? 1. Malonyl CoA 2. Acetyl CoA Carboxylase 3. PDH 4. Citrate Lyase

$1,2,3,4$. ***Correct Answer: 1,2,3,4 (All of them)*** In an obese person consuming a carbohydrate-rich diet, **high insulin levels** drive maximum flux through **lipogenesis (fatty acid synthesis)**. This results in elevation of ALL enzymes and intermediates in the pathway: - **PDH (Pyruvate Dehydrogenase)**: Activated by insulin to convert pyruvate → Acetyl CoA in mitochondria - **Citrate Lyase**: Elevated to cleave citrate → cytosolic Acetyl CoA (substrate for fatty acid synthesis) - **Acetyl CoA Carboxylase (ACC)**: The **rate-limiting enzyme** of fatty acid synthesis, activated by insulin (dephosphorylation) and citrate - **Malonyl CoA**: The **first committed intermediate** in fatty acid synthesis, product of ACC action on Acetyl CoA All four components work sequentially in the pathway from glucose → fatty acids, and all are upregulated in this metabolic state. *Incorrect Option: 1,2,3* This incorrectly excludes **Citrate Lyase**, which is essential for providing cytosolic Acetyl CoA from mitochondrial citrate. Without elevated Citrate Lyase activity, fatty acid synthesis cannot proceed efficiently despite high carbohydrate intake. *Incorrect Option: 2,3,4* This incorrectly excludes **Malonyl CoA**, the direct product of Acetyl CoA Carboxylase and the committed intermediate for fatty acid synthesis. When ACC is highly active (as it would be with high insulin), Malonyl CoA concentration must be elevated. *Incorrect Option: 1,3,4* This incorrectly excludes **Acetyl CoA Carboxylase (ACC)**, the **rate-limiting enzyme** of the entire fatty acid synthesis pathway. In a high carbohydrate/high insulin state, ACC is maximally activated by dephosphorylation and allosteric activation by citrate, making this a critical error.

Question 1

The chyle from the intestine is rich with chylomicrons. Which of the following forms the protein core of chylomicrons?

Accepted Answer

Triglyceride + cholesterol + phospholipid

Answer

Triglyceride only

Answer

Triglyceride + cholesterol

Answer

Only cholesterol

Question 2

Which of the following regulates lipolysis in adipocytes?

Accepted Answer

Activation of triglyceride lipase as a result of hormone-stimulated increases in cyclic AMP levels

Answer

Activation of fatty acid synthesis mediated by cyclic AMP

Answer

Glycerol phosphorylation to prevent futile esterification of fatty acids

Answer

Activation of cyclic AMP production by insulin

Question 3

An 8-month-old child presented with reduced appetite, abdominal distension and pain, and psychomotor retardation. The child was normal at birth, and both parents are normal. On examination: Hepatosplenomegaly, moderate lymphadenopathy, abnormal posturing of the limbs, trunk, and face, impaired voluntary rapid eye movements, cherry-red spot on fundus examination, and bony defects. Lymph node histopathology and electron microscopy findings were noted. Which of the following enzymes is most likely deficient in the above disease?

Accepted Answer

Sphingomyelinase

Answer

Hexosaminidase A

Answer

Alpha-galactosidase A

Answer

Glucocerebrosidase

Question 4

Insulin inhibits ketogenesis by all except:

Accepted Answer

Increasing b-oxidation

Answer

Inhibiting lipolysis

Answer

Increased esterification of fatty acids

Answer

Directing acetyl-CoA to the TCA cycle

Question 5

Which lipoprotein has the highest cholesterol content?

Accepted Answer

LDL (Low-density lipoprotein)

Answer

VLDL (Very low-density lipoprotein)

Answer

Chylomicrons

Answer

IDL (Intermediate-density lipoprotein)

Question 6

A patient presents with palmar xanthomas, indicating an increased risk of atherosclerosis and coronary artery disease. Lipid profile reveals elevated triacylglycerols and cholesterol, with increased levels of IDL and chylomicrons. What is the pathophysiology of this condition?

Accepted Answer

Apo E deficiency

Answer

LDL deficiency

Answer

VLDL overproduction

Answer

Apo C-2 deficiency

Question 7

A 6-month-old infant is brought to the emergency department with lethargy, vomiting, and poor feeding. The episode occurred after prolonged fasting. Laboratory results reveal hypoglycemia, low ketone levels (hypoketosis), and mild hepatomegaly. These findings suggest a defect in fat metabolism. Which of the following is the most likely underlying disorder?

Accepted Answer

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency

Answer

Glycogen storage disease type I (Von Gierke disease)

Answer

Hers disease

Answer

Hereditary fructose intolerance

Question 8

A man presents with high VLDL and chylomicrons, with eruptive xanthomas. What is the most likely diagnosis?

Accepted Answer

Familial hypertriglyceridemia

Answer

Familial hyperlipidemia

Answer

Familial dysbetalipoproteinemia

Answer

Familial hypercholesterolemia

Question 9

Which source and cell produce testosterone?

Accepted Answer

Cholesterol and Leydig cells

Answer

Dihydrotestosterone and Leydig cells

Answer

FSH and Leydig cells

Answer

Cholesterol and Sertoli cells

Question 10

A 90 kg obese man is taking a carbohydrate-rich diet. Which of the following enzymes/intermediates will be elevated in him? 1. Malonyl CoA 2. Acetyl CoA Carboxylase 3. PDH 4. Citrate Lyase

Accepted Answer

$1,2,3,4$

Answer

$1,2,3$

Answer

$1,3,4$

Answer

$2,3,4$

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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