Lipid Metabolism — MCQs
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What is the precursor of all steroid hormones?
Stored triacylglycerol and cholesterol are released by which enzyme?
A 38-year-old man presents with a history of an infection. Ocular examination reveals small opaque rings on the lower edge of the iris in the anterior chamber. Nodular lesions are found on his Achilles tendon. Successful therapy should be aimed at increasing which of the following gene products in hepatocyte cell membranes?
Deficiency in alpha-oxidation of fatty acids leads to which of the following conditions?
A 58-year-old man had a myocardial infarction 1 year ago. He now wants to prevent another acute coronary event and is advised to begin a program of exercise and to change his diet. A reduction in the level of which of the following serum laboratory findings 1 year later would best indicate the success of his diet and exercise regimen?
Which of the following enzymes is involved in the synthesis of both cholesterol and ketone bodies?
Deficiency of beta-glucosidase causes which of the following conditions?
Which of the following fatty acids is a precursor of arachidonic acid?
Enzyme replacement therapy is most effective for which of the following inherited metabolic disorders?
Lipoprotein X is elevated in which of the following conditions?
Lipid Metabolism Indian Medical PG Practice Questions and MCQs
Question 321: What is the precursor of all steroid hormones?
- A. Pregnenolone (Correct Answer)
- B. Deoxycorticosterone
- C. Androstenedione
- D. Dehydroepiandrosterone
Explanation: **Explanation:** The synthesis of all steroid hormones (mineralocorticoids, glucocorticoids, and sex steroids) begins with **Cholesterol**. However, the first committed steroid intermediate in this biosynthetic pathway is **Pregnenolone**. **Why Pregnenolone is correct:** Cholesterol (a 27-carbon molecule) is transported into the mitochondria by the **StAR protein**. There, the enzyme **Cholesterol side-chain cleavage enzyme (P450scc / Desmolase)** converts cholesterol into Pregnenolone (a 21-carbon molecule). Because all subsequent steroid pathways—whether in the adrenal cortex or gonads—diverge from this single molecule, Pregnenolone is recognized as the universal precursor or "grandparent" of all steroid hormones. **Analysis of Incorrect Options:** * **B. Deoxycorticosterone:** This is an intermediate specifically in the mineralocorticoid pathway (leading to Aldosterone). It is formed downstream from pregnenolone. * **C. Androstenedione:** This is an intermediate in the androgen pathway, synthesized from either DHEA or progesterone. It serves as a precursor to testosterone and estrone, but not to corticosteroids. * **D. Dehydroepiandrosterone (DHEA):** This is a weak androgen produced from 17-hydroxypregnenolone. While it is a precursor for sex steroids, it is not a precursor for mineralocorticoids or glucocorticoids. **High-Yield NEET-PG Pearls:** * **Rate-limiting step:** The conversion of cholesterol to pregnenolone by **Desmolase** is the rate-limiting step in steroidogenesis. * **ACTH Action:** ACTH stimulates steroid synthesis primarily by increasing the activity of Desmolase and the StAR protein. * **Location:** This initial step occurs in the **mitochondria**, while subsequent steps occur in the Smooth Endoplasmic Reticulum (SER).
Question 322: Stored triacylglycerol and cholesterol are released by which enzyme?
- A. Lysosomal lipase
- B. Lipoprotein lipase
- C. LCAT
- D. Hormone sensitive lipase (Correct Answer)
Explanation: **Explanation:** **1. Why Hormone-Sensitive Lipase (HSL) is correct:** Hormone-Sensitive Lipase is the rate-limiting enzyme for **lipolysis** in adipose tissue. It hydrolyzes stored triacylglycerols (TAGs) into free fatty acids and glycerol. Crucially, HSL also possesses **cholesteryl esterase** activity, allowing it to release free cholesterol from stored cholesteryl esters. It is activated by glucagon and epinephrine (via cAMP/Protein Kinase A phosphorylation) during fasting or stress, and inhibited by insulin. **2. Why the other options are incorrect:** * **Lysosomal Lipase:** This enzyme (acid lipase) degrades lipids that enter the cell via endocytosis (e.g., LDL particles). While it breaks down TAGs and cholesterol esters, it is not the primary regulator for mobilizing systemic energy stores from adipocytes. * **Lipoprotein Lipase (LPL):** Found on the endothelial surface of capillaries, LPL acts on **circulating** lipids (Chylomicrons and VLDL) to provide fatty acids to peripheral tissues. It does not act on stored intracellular lipids. * **LCAT (Lecithin-Cholesterol Acyltransferase):** This enzyme is involved in cholesterol **esterification** within HDL particles in the plasma (converting free cholesterol to cholesterol esters). It does not release stored lipids. **Clinical Pearls for NEET-PG:** * **Perilipin:** In resting adipocytes, lipids are protected by the protein perilipin. Phosphorylation of perilipin by PKA allows HSL to access the lipid droplet. * **Insulin’s Role:** Insulin is the most potent inhibitor of HSL; this is why diabetic ketoacidosis (DKA) occurs in insulin deficiency—uninhibited HSL leads to massive fatty acid release and subsequent ketone body formation. * **Product of Lipolysis:** Glycerol released by HSL cannot be reused by adipocytes (due to lack of **glycerol kinase**) and must go to the liver for gluconeogenesis.
Question 323: A 38-year-old man presents with a history of an infection. Ocular examination reveals small opaque rings on the lower edge of the iris in the anterior chamber. Nodular lesions are found on his Achilles tendon. Successful therapy should be aimed at increasing which of the following gene products in hepatocyte cell membranes?
- A. Apolipoprotein B-100
- B. Apolipoprotein B-100 receptor (Correct Answer)
- C. Apolipoprotein E
- D. Apolipoprotein E receptor
Explanation: ### Explanation **Clinical Diagnosis: Familial Hypercholesterolemia (Type IIa Hyperlipoproteinemia)** The patient presents with classic signs of severe hypercholesterolemia: **Arcus senilis** (opaque rings in the iris) and **Tendon xanthomas** (nodular lesions on the Achilles tendon). These findings indicate a defect in the clearance of LDL-cholesterol from the plasma. #### 1. Why the Correct Answer is Right The primary defect in Familial Hypercholesterolemia is a deficiency or dysfunction of the **LDL receptor** (also known as the **Apolipoprotein B-100/E receptor**). * LDL particles contain **Apo B-100**, which acts as the ligand for the LDL receptor on hepatocytes. * Increasing the expression of these receptors on hepatocyte membranes enhances the uptake of LDL from the circulation, thereby lowering plasma cholesterol levels. * Statins, the mainstay of therapy, work by inhibiting HMG-CoA reductase, which triggers a compensatory **upregulation of LDL receptors** (Apo B-100 receptors). #### 2. Why the Incorrect Options are Wrong * **Apolipoprotein B-100:** This is the structural protein of VLDL and LDL. Increasing it would likely increase the production of atherogenic particles rather than clearing them. * **Apolipoprotein E:** While Apo E is a ligand for several receptors (including the LDL receptor), the specific pathology of tendon xanthomas is linked to the LDL receptor-Apo B-100 pathway. * **Apolipoprotein E receptor (LRP):** This receptor primarily clears chylomicron remnants and IDL. While it recognizes Apo E, it does not significantly clear LDL (which lacks Apo E). #### 3. NEET-PG High-Yield Pearls * **Tendon Xanthoma:** Pathognomonic for Familial Hypercholesterolemia (Type IIa). * **LDL Receptor Ligands:** It recognizes both **Apo B-100** (on LDL) and **Apo E** (on VLDL/IDL). * **PCSK9 Inhibitors:** A modern class of drugs that increase the number of LDL receptors by preventing their degradation, used in resistant cases of this condition. * **Xanthelasma:** Yellowish deposits around the eyelids, also seen in this condition.
Question 324: Deficiency in alpha-oxidation of fatty acids leads to which of the following conditions?
- A. Dicarboxylic aciduria
- B. Zellweger syndrome
- C. Jamaican vomiting sickness
- D. Refsum disease (Correct Answer)
Explanation: **Explanation:** **Correct Answer: D. Refsum disease** Alpha-oxidation is a specialized pathway occurring in peroxisomes, primarily required for the breakdown of **Phytanic acid** (a branched-chain fatty acid found in dairy and chlorophyll). Unlike most fatty acids, phytanic acid has a methyl group at the beta-carbon, which blocks beta-oxidation. Alpha-oxidation removes one carbon atom from the carboxyl end to bypass this block. Refsum disease is caused by a deficiency in the enzyme **Phytanoyl-CoA hydroxylase**. This leads to the toxic accumulation of phytanic acid in tissues, manifesting clinically as retinitis pigmentosa, peripheral neuropathy, cerebellar ataxia, and ichthyosis. **Analysis of Incorrect Options:** * **A. Dicarboxylic aciduria:** This occurs when **beta-oxidation** is impaired (e.g., MCAD deficiency). The body resorts to **omega-oxidation** in the endoplasmic reticulum, producing dicarboxylic acids that are excreted in the urine. * **B. Zellweger syndrome:** This is a **peroxisomal biogenesis disorder** where peroxisomes are absent or non-functional. While it affects alpha and beta oxidation, it is a generalized defect rather than a specific deficiency of the alpha-oxidation pathway itself. * **C. Jamaican vomiting sickness:** Caused by **Hypoglycin A** (from unripe ackee fruit), which inhibits **Acyl-CoA dehydrogenase**, thereby blocking beta-oxidation and leading to profound hypoglycemia. **High-Yield Clinical Pearls for NEET-PG:** * **Dietary Management:** The primary treatment for Refsum disease is the strict avoidance of green leafy vegetables (chlorophyll) and ruminant fats. * **Location:** Alpha-oxidation occurs exclusively in **peroxisomes**. * **Key Enzyme:** Phytanoyl-CoA hydroxylase (Deficient in Classic Refsum). * **Refsum vs. Zellweger:** Refsum is a single enzyme defect; Zellweger is a total organelle (peroxisome) failure.
Question 325: A 58-year-old man had a myocardial infarction 1 year ago. He now wants to prevent another acute coronary event and is advised to begin a program of exercise and to change his diet. A reduction in the level of which of the following serum laboratory findings 1 year later would best indicate the success of his diet and exercise regimen?
- A. Calcium
- B. Cholesterol (Correct Answer)
- C. Glucose
- D. Potassium
Explanation: **Explanation:** The primary goal of secondary prevention after a myocardial infarction (MI) is to reduce the progression of atherosclerosis. **Cholesterol**, specifically Low-Density Lipoprotein (LDL), is the most significant modifiable risk factor for coronary artery disease (CAD). 1. **Why Cholesterol is Correct:** Atherosclerosis is driven by the accumulation of cholesterol in the arterial walls, leading to plaque formation. Diet (reducing saturated fats) and aerobic exercise effectively lower serum LDL and total cholesterol while increasing HDL ("good" cholesterol). A reduction in serum cholesterol directly correlates with a decreased risk of recurrent coronary events and improved cardiovascular outcomes. 2. **Why Other Options are Incorrect:** * **Calcium:** While coronary artery calcification is a marker of atherosclerosis, serum calcium levels are tightly regulated by the parathyroid hormone and do not reflect the success of a cardiovascular diet or exercise regimen. * **Glucose:** While managing blood sugar is vital for diabetic patients, a reduction in glucose is not the *best* specific indicator for preventing a second MI in a general post-MI patient compared to lipid profile improvement. * **Potassium:** Serum potassium levels are critical for cardiac rhythm but are influenced by renal function and medications (like ACE inhibitors or diuretics), not by long-term lifestyle changes aimed at atherosclerosis. **High-Yield Clinical Pearls for NEET-PG:** * **Target LDL:** For post-MI patients (Very High Risk), the current goal is often LDL <55 mg/dL. * **Exercise Effect:** Exercise primarily increases **HDL** and lowers **Triglycerides** by increasing the activity of **Lipoprotein Lipase (LPL)**. * **Dietary Impact:** Saturated fats downregulate LDL receptors; reducing them increases receptor expression, lowering serum LDL.
Question 326: Which of the following enzymes is involved in the synthesis of both cholesterol and ketone bodies?
- A. HMG CoA reductase
- B. HMG CoA synthase (Correct Answer)
- C. HMG CoA lyase
- D. Thiolase
Explanation: **Explanation:** The enzyme **HMG-CoA synthase** is the correct answer because it catalyzes the condensation of Acetoacetyl-CoA and Acetyl-CoA to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). This step is a common intermediate in two distinct metabolic pathways: 1. **Ketogenesis:** Occurs in the **mitochondria** of liver cells. 2. **Cholesterol Synthesis:** Occurs in the **cytosol** (and endoplasmic reticulum) of various tissues. **Analysis of Options:** * **HMG-CoA Reductase (Option A):** This is the **rate-limiting enzyme** for cholesterol synthesis only. It converts HMG-CoA to mevalonate. It is not involved in ketogenesis. * **HMG-CoA Lyase (Option C):** This enzyme is specific to **ketogenesis**. It cleaves HMG-CoA into Acetoacetate and Acetyl-CoA in the mitochondria. It plays no role in cholesterol synthesis. * **Thiolase (Option D):** While Thiolase is involved in the initial step of both pathways (combining two Acetyl-CoA molecules to form Acetoacetyl-CoA), HMG-CoA synthase is the specific enzyme that defines the "HMG" pathway shared by both. **High-Yield Clinical Pearls for NEET-PG:** * **Compartmentalization:** Remember that the liver is the only organ that can produce ketone bodies, but it **cannot utilize them** because it lacks the enzyme *Thiophorase* (Succinyl-CoA:3-ketoacid CoA transferase). * **Pharmacology Link:** HMG-CoA Reductase is the target of **Statins**, which are used to treat hypercholesterolemia. * **Rate-limiting steps:** * Ketogenesis: HMG-CoA Synthase (Mitochondrial). * Cholesterol Synthesis: HMG-CoA Reductase (Cytosolic).
Question 327: Deficiency of beta-glucosidase causes which of the following conditions?
- A. Gaucher's disease (Correct Answer)
- B. Fabry's disease
- C. Krabbe's disease
- D. GM1 gangliosidosis
Explanation: **Explanation:** The question pertains to **Sphingolipidoses**, a subgroup of Lysosomal Storage Disorders characterized by the deficiency of specific lysosomal enzymes required for the degradation of sphingolipids. **1. Why Gaucher’s Disease is Correct:** Gaucher’s disease is caused by a deficiency of the enzyme **$\beta$-glucosidase** (also known as **glucocerebrosidase**). This deficiency leads to the accumulation of **glucocerebroside** (glucosylceramide) in the reticuloendothelial system. It is the most common lysosomal storage disorder. **2. Analysis of Incorrect Options:** * **Fabry’s Disease:** Caused by a deficiency of **$\alpha$-galactosidase A**, leading to the accumulation of ceramide trihexoside. It is unique as it is **X-linked recessive**, while most others are autosomal recessive. * **Krabbe’s Disease:** Caused by a deficiency of **$\beta$-galactosidase** (galactocerebrosidase), leading to the accumulation of galactocerebroside and psychosine, which is toxic to myelin-producing cells. * **GM1 Gangliosidosis:** Caused by a deficiency of **$\beta$-galactosidase**, leading to the accumulation of GM1 gangliosides and keratan sulfate. **3. NEET-PG High-Yield Clinical Pearls:** * **Gaucher Cells:** Pathognomonic "wrinkled tissue paper" or "crumpled silk" appearance of the macrophage cytoplasm. * **Clinical Triad (Gaucher):** Hepatosplenomegaly, bone involvement (Erlenmeyer flask deformity of the femur, bone crises), and pancytopenia. * **Enzyme Replacement Therapy (ERT):** Recombinant glucocerebrosidase (Imiglucerase) is the standard treatment for Type 1 Gaucher’s disease. * **Cherry Red Spot:** Seen in Tay-Sachs and Niemann-Pick, but **absent** in Gaucher’s disease.
Question 328: Which of the following fatty acids is a precursor of arachidonic acid?
- A. Linoleic acid (Correct Answer)
- B. Linolenic acid
- C. Oleic acid
- D. Palmitic acid
Explanation: **Explanation:** **1. Why Linoleic Acid is correct:** Arachidonic acid (C20:4, ω-6) is a semi-essential fatty acid. It can be synthesized in the human body only if its precursor, **Linoleic acid (C18:2, ω-6)**, is available. The conversion occurs in the endoplasmic reticulum through a series of elongation and desaturation reactions: *Linoleic acid → γ-Linolenic acid → Dihomo-γ-linolenic acid → Arachidonic acid.* Since humans cannot introduce double bonds beyond carbon 9, Linoleic acid (an essential fatty acid) must be obtained from the diet to produce arachidonic acid. **2. Why the other options are incorrect:** * **Linolenic acid (α-Linolenic acid):** This is an **ω-3** fatty acid. It serves as the precursor for Eicosapentaenoic acid (EPA) and Docosahexaenoic acid (DHA), not arachidonic acid. * **Oleic acid:** This is a monounsaturated **ω-9** fatty acid (C18:1). While it can be synthesized endogenously, it cannot be converted into ω-6 fatty acids like arachidonic acid. * **Palmitic acid:** This is a 16-carbon **saturated** fatty acid. It is the first fatty acid produced by the Fatty Acid Synthase (FAS) complex and serves as a precursor for longer-chain saturated and monounsaturated fats, but not polyunsaturated fatty acids (PUFAs) like arachidonic acid. **High-Yield Clinical Pearls for NEET-PG:** * **Essential Fatty Acids (EFA):** Only Linoleic and α-Linolenic acid are truly essential. Arachidonic acid becomes essential only if Linoleic acid is deficient in the diet. * **Prostaglandin Synthesis:** Arachidonic acid is the primary substrate for the **Cyclooxygenase (COX)** and **Lipoxygenase (LOX)** pathways, leading to the production of Prostaglandins, Thromboxanes, and Leukotrienes (Group 2 eicosanoids). * **Vanishing EFA:** Deficiency of EFAs leads to **Phrynoderma** (follicular hyperkeratosis) and poor wound healing.
Question 329: Enzyme replacement therapy is most effective for which of the following inherited metabolic disorders?
- A. Niemann-Pick disease
- B. Gangliosidosis
- C. Gaucher's disease (Correct Answer)
- D. Phenylketonuria
Explanation: **Explanation:** **Gaucher’s disease** is the correct answer because it was the first lysosomal storage disorder (LSD) for which **Enzyme Replacement Therapy (ERT)** was successfully developed and remains the gold standard of treatment. The disease is caused by a deficiency of **β-glucocerebrosidase**, leading to the accumulation of glucosylceramide in macrophages (Gaucher cells). ERT (using recombinant enzymes like Imiglucerase) is highly effective in reversing systemic manifestations such as hepatosplenomegaly and hematological abnormalities (anemia, thrombocytopenia). **Analysis of Incorrect Options:** * **Niemann-Pick Disease:** While ERT (Olipudase alfa) has recently been approved for Type B (non-neuropathic), it is not effective for Type A due to the enzyme's inability to cross the blood-brain barrier. Historically and clinically, Gaucher’s remains the classic example of ERT success. * **Gangliosidosis (e.g., Tay-Sachs):** These primarily affect the Central Nervous System (CNS). Current ERT cannot cross the blood-brain barrier, making it ineffective for the neurodegenerative components of these diseases. * **Phenylketonuria (PKU):** This is an amino acid metabolism disorder, not a lysosomal storage disease. It is primarily managed through **dietary restriction** (low phenylalanine diet) rather than ERT. **High-Yield Clinical Pearls for NEET-PG:** * **Gaucher’s Hallmark:** "Crinkled paper" appearance of macrophage cytoplasm. * **Most Common LSD:** Gaucher’s disease is the most common lysosomal storage disorder. * **Bone Involvement:** Look for "Erlenmeyer flask deformity" of the distal femur and avascular necrosis in Gaucher’s cases. * **ERT Limitation:** The major hurdle for ERT in LSDs is the **Blood-Brain Barrier**; hence, ERT is most effective for non-neuropathic (Type 1) Gaucher’s.
Question 330: Lipoprotein X is elevated in which of the following conditions?
- A. Primary biliary cirrhosis (Correct Answer)
- B. Indian childhood cirrhosis
- C. Hypercholesterolemia
- D. Alcoholic cirrhosis
Explanation: **Explanation:** **Lipoprotein X (LpX)** is an abnormal, pathological low-density lipoprotein (LDL) that appears in the plasma of patients with **cholestasis**. Unlike normal lipoproteins, LpX lacks Apolipoprotein B-100 and is primarily composed of unesterified cholesterol and phospholipids (lecithin). 1. **Why Primary Biliary Cirrhosis (PBC) is correct:** PBC is a chronic cholestatic liver disease characterized by the destruction of intrahepatic bile ducts. In cholestasis, there is a **regurgitation of biliary lipids** (specifically lecithin and cholesterol) into the bloodstream. These lipids aggregate with albumin and Apolipoprotein C to form LpX. Therefore, LpX is a highly specific marker for obstructive jaundice and PBC. 2. **Why the other options are incorrect:** * **Indian Childhood Cirrhosis (ICC):** This is primarily a copper-overload liver disease. While it leads to cirrhosis, it is not classically defined by the specific lipoprotein abnormalities seen in obstructive cholestasis. * **Hypercholesterolemia:** This refers to elevated levels of normal LDL or VLDL. LpX is an *abnormal* lipoprotein and is not a feature of familial or dietary hypercholesterolemia. * **Alcoholic Cirrhosis:** While it involves liver damage, it typically presents with elevated VLDL or HDL abnormalities rather than the specific formation of LpX, which requires significant biliary obstruction. **High-Yield Pearls for NEET-PG:** * **LCAT Deficiency:** Besides cholestasis, LpX is also found in patients with **Lecithin-Cholesterol Acyltransferase (LCAT) deficiency**. * **Diagnostic Significance:** LpX is the most specific biochemical marker for differentiating obstructive (post-hepatic) jaundice from parenchymal liver disease. * **Composition:** It is unique because it contains **Apo C and Albumin**, but lacks Apo B. * **Pseudohyponatremia:** High levels of LpX can cause a "falsely low" sodium reading on certain lab tests.
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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