Lipid Metabolism — MCQs

A 4-month-old child presented with extreme tiredness, irritable mood, poor appetite, and fasting hypoglycemia associated with vomiting and muscle weakness. Blood tests revealed elevated levels of free fatty acids but low levels of acylcarnitine. A muscle biopsy demonstrated significant fatty acid infiltration in the cytoplasm. What is the most likely molecular defect in this child?

Lipid Metabolism Indian Medical PG Practice Questions and MCQs

Question 361: A 10-year-old boy admitted for cleft lip surgery suddenly developed acute abdominal pain and was found to have xanthomas and milky plasma on investigation. Which lipoprotein is increased?

A. VLDL Remnant
B. Chylomicron (Correct Answer)
C. Triglycerides
D. Cholesterol

Explanation: **Explanation:** The clinical presentation of **milky plasma** (lipemic serum) and **acute abdominal pain** (suggestive of acute pancreatitis) in a young child is a classic hallmark of **Type I Hyperlipoproteinemia** (Familial Chylomicronemia Syndrome). 1. **Why Chylomicron is correct:** Chylomicrons are the largest, least dense lipoproteins, primarily composed of dietary triglycerides. When there is a deficiency in **Lipoprotein Lipase (LPL)** or its cofactor **Apo C-II**, chylomicrons cannot be cleared from the blood. This massive accumulation causes the plasma to appear "milky" or "creamy." The high levels of chylomicrons can lead to capillary plugging in the pancreas, causing acute pancreatitis (abdominal pain) and the formation of eruptive xanthomas. 2. **Why other options are incorrect:** * **VLDL Remnant:** These are increased in Type III Hyperlipoproteinemia (Dysbetalipoproteinemia). While they cause xanthomas (specifically palmar xanthomas), they do not typically cause the classic "milky plasma" seen in Type I. * **Triglycerides:** While triglycerides are indeed elevated, the question asks which **lipoprotein** is increased. Triglycerides are a lipid component, not a lipoprotein particle. * **Cholesterol:** Cholesterol is primarily carried by LDL. Elevated LDL (Type IIa) leads to xanthelasmas and tendon xanthomas but does not cause milky plasma or acute pancreatitis. **High-Yield Clinical Pearls for NEET-PG:** * **Refrigeration Test:** In Type I, a creamy layer forms on top of the plasma when left overnight, while the layer below remains clear. * **Defect:** Mutation in *LPL* gene (most common) or *APOC2* gene. * **Risk:** The primary life-threatening complication is **Acute Pancreatitis**, not atherosclerosis. * **Treatment:** Strict fat-restricted diet (medium-chain triglycerides are preferred as they bypass chylomicron formation).

Question 362: What is the normal level of serum cholesterol?

A. 100-140 mg/100 ml
B. 260-360 mg/100 ml
C. 150-250 mg/100 ml (Correct Answer)
D. 80-120 mg/ml

Explanation: **Explanation:** The normal range for total serum cholesterol in a healthy adult is typically cited as **150–250 mg/dL** (or mg/100 ml) in standard medical biochemistry textbooks (like Vasudevan or Satyanarayana), which are the primary references for NEET-PG. 1. **Why Option C is correct:** Cholesterol is an essential structural component of cell membranes and a precursor for steroid hormones, bile acids, and Vitamin D. While modern clinical guidelines (like NCEP-ATP III) suggest that "desirable" levels are below 200 mg/dL to reduce cardiovascular risk, the physiological "normal range" established in academic biochemistry remains 150–250 mg/dL. 2. **Why other options are incorrect:** * **Option A (100-140 mg/dl):** This range is too low (hypocholesterolemia), often seen in malabsorption, hyperthyroidism, or severe liver disease. * **Option B (260-360 mg/dl):** This indicates hypercholesterolemia, significantly increasing the risk of atherosclerosis and coronary artery disease. * **Option D (80-120 mg/ml):** This is physiologically impossible and likely a unit error (mg/ml instead of mg/dL). **High-Yield Clinical Pearls for NEET-PG:** * **Transport:** Cholesterol is transported in the blood primarily by **LDL** (Bad cholesterol) and **HDL** (Good cholesterol). * **Rate-limiting Enzyme:** **HMG-CoA Reductase** is the key enzyme in cholesterol synthesis, which is inhibited by **Statins**. * **Clinical Correlation:** Levels >250 mg/dL are associated with **Xanthomas** (lipid deposits in skin/tendons) and increased risk of Myocardial Infarction. * **Conversion:** To convert mg/dL to mmol/L, divide by 38.6.

Question 363: HDL3 is converted to HDL2 by which enzyme?

A. CETP
B. LCAT (Correct Answer)
C. Both
D. PLTP

Explanation: **Explanation:** The conversion of **HDL3 to HDL2** is a critical step in **Reverse Cholesterol Transport (RCT)**. **Why LCAT is the correct answer:** HDL3 is a small, dense, protein-rich particle. The enzyme **LCAT (Lecithin-Cholesterol Acyltransferase)**, which is activated by **Apo A-I**, catalyzes the transfer of a fatty acid from lecithin to free cholesterol on the surface of HDL3. This creates **cholesterol esters**, which are highly hydrophobic and move into the core of the particle. As the core expands with these esters, the HDL3 particle increases in size and decreases in density, transforming into the larger, spherical **HDL2**. **Analysis of Incorrect Options:** * **CETP (Cholesteryl Ester Transfer Protein):** This protein facilitates the exchange of cholesteryl esters from HDL2 to VLDL/LDL in exchange for triglycerides. This process actually helps convert HDL2 back into HDL3 (the reverse of the question). * **PLTP (Phospholipid Transfer Protein):** This protein transfers phospholipids from triglyceride-rich lipoproteins (like VLDL) to HDL, helping in HDL remodeling, but it is not the primary driver of the HDL3 to HDL2 maturation. **High-Yield Clinical Pearls for NEET-PG:** * **HDL2** is considered the most cardioprotective form of HDL. * **Hepatic Lipase (HL)** converts HDL2 back to HDL3 by hydrolyzing triglycerides and phospholipids. * **Fish-Eye Disease:** A partial LCAT deficiency where only alpha-LCAT is affected. * **Tangier Disease:** Caused by a mutation in the **ABCA1 transporter**, leading to a near-total absence of HDL.

Question 364: All of the following statements about LDL are true EXCEPT:

A. It delivers cholesterol to cells
B. It contains only one apolipoprotein
C. It is a marker for cardiovascular disease
D. It contains Apo-B48 (Correct Answer)

Explanation: **Explanation:** The correct answer is **D (It contains Apo-B48)** because this statement is factually incorrect. **Apolipoprotein B-100 (Apo-B100)** is the primary structural protein found in LDL, VLDL, and IDL. In contrast, **Apo-B48** is unique to **chylomicrons** and their remnants, synthesized exclusively in the intestinal mucosal cells. **Analysis of Options:** * **Option A (Delivers cholesterol to cells):** This is true. LDL is the primary carrier of cholesterol in the blood. It transports cholesterol from the liver to peripheral tissues via receptor-mediated endocytosis (LDL receptors). * **Option B (Contains only one apolipoprotein):** This is true. Unlike other lipoproteins that carry multiple types of apolipoproteins (like HDL), mature LDL particles contain **only one** molecule of **Apo-B100**. * **Option C (Marker for cardiovascular disease):** This is true. LDL is often termed "bad cholesterol." High levels lead to cholesterol deposition in arterial walls, leading to atherosclerosis and increased risk of myocardial infarction. **High-Yield NEET-PG Pearls:** * **Apo-B100 vs. Apo-B48:** Both are products of the same gene. Apo-B48 is formed via **RNA editing** (C to U conversion by cytidine deaminase), which creates a premature stop codon, resulting in a protein that is 48% the length of Apo-B100. * **LDL Formation:** LDL is not secreted directly; it is the "end product" of VLDL metabolism (VLDL → IDL → LDL). * **Friedewald Equation:** LDL Cholesterol = Total Cholesterol – [HDL + (Triglycerides/5)]. (Note: This is invalid if TG >400 mg/dL).

Question 365: In primary familial hypercholesterolemia, what is the defect?

A. LDL receptors (Correct Answer)
B. Apolipoprotein
C. Apolipoprotein B
D. VLDL

Explanation: **Explanation:** **Primary Familial Hypercholesterolemia (Type IIa Hyperlipoproteinemia)** is an autosomal dominant disorder characterized by a significant elevation in serum LDL cholesterol. **Why LDL Receptors are the correct answer:** The primary defect lies in the **LDL receptor (LDLR) gene**. These receptors are responsible for the hepatic uptake of LDL particles from the circulation via receptor-mediated endocytosis. A deficiency or dysfunction of these receptors leads to decreased clearance of LDL, resulting in markedly elevated plasma cholesterol levels and premature atherosclerosis. **Analysis of Incorrect Options:** * **Apolipoprotein E:** Defects in Apo E (specifically the E2 isoform) lead to **Type III Hyperlipoproteinemia** (Dysbetalipoproteinemia), characterized by the accumulation of chylomicron remnants and IDL. * **Apolipoprotein B:** While a mutation in Apo B-100 (the ligand for the LDL receptor) can cause "Familial Defective Apo B-100," it is a distinct clinical entity. In the classic "Primary Familial Hypercholesterolemia," the receptor itself is the primary site of defect. * **VLDL:** VLDL is a transport lipoprotein. Elevated VLDL is seen in Type IV Hypertriglyceridemia, not primarily in Type IIa. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Look for **Tendon Xanthomas** (especially the Achilles tendon), **Xanthelasma** (eyelids), and **Corneal Arcus** in a young patient with high cholesterol. * **Genetics:** Homozygous individuals present in childhood with myocardial infarction before age 20; Heterozygotes present in their 30s-40s. * **Classification:** It is classified as **Type IIa** under the Fredrickson classification. * **Treatment:** Statins are the first-line treatment as they upregulate the expression of remaining functional LDL receptors.

Question 366: Which of the following is a glycolipid?

A. Cerebroside (Correct Answer)
B. Plasmalogen
C. Sphingomyelin
D. Phosphatidylcholine

Explanation: **Explanation:** Lipids are classified based on their chemical composition. **Glycolipids** (also known as glycosphingolipids) are compound lipids that contain a carbohydrate (sugar) moiety and a sphingosine backbone, but lack a phosphate group. **1. Why Cerebroside is correct:** Cerebrosides are the simplest form of glycolipids. They consist of a **ceramide** (sphingosine + fatty acid) attached to a single sugar unit, usually **glucose** (glucocerebroside) or **galactose** (galactocerebroside). They are essential components of nerve cell membranes and the myelin sheath. **2. Why the other options are incorrect:** * **Plasmalogen:** This is a **phospholipid** (specifically an ether lipid) characterized by an ether bond at the C1 position of glycerol. It is found abundantly in cardiac tissue. * **Sphingomyelin:** While it contains a sphingosine backbone like glycolipids, it is classified as a **phospholipid** because it contains a phosphate group and a nitrogenous base (choline). It is the only sphingolipid that is also a phospholipid. * **Phosphatidylcholine (Lecithin):** This is the most abundant **phospholipid** in the cell membrane. It consists of glycerol, two fatty acids, a phosphate group, and choline. **High-Yield Clinical Pearls for NEET-PG:** * **Gaucher’s Disease:** Caused by a deficiency of *beta-glucosidase*, leading to the accumulation of glucocerebrosides. * **Krabbe’s Disease:** Caused by a deficiency of *beta-galactosidase*, leading to the accumulation of galactocerebrosides. * **Gangliosides:** Complex glycolipids containing sialic acid (**NANA**). Accumulation of GM2 ganglioside occurs in **Tay-Sachs disease**. * **Marker:** Sphingomyelin is a major component of the myelin sheath; the L/S ratio (Lecithin/Sphingomyelin) in amniotic fluid is a marker for fetal lung maturity.

Question 367: Which of the following vitamins provides the cofactor for reduction reactions in fatty acid synthesis?

A. Folate
B. Niacin (Correct Answer)
C. Riboflavin
D. Thiamin

Explanation: **Explanation:** The correct answer is **Niacin (Vitamin B3)**. Fatty acid synthesis (Lipogenesis) is a reductive process that occurs in the cytosol. The key reducing equivalent required for this process is **NADPH** (Nicotinamide Adenine Dinucleotide Phosphate). NADPH is derived from **Niacin**, which forms the nicotinamide ring of the molecule. During the elongation cycle of fatty acid synthesis, the enzyme *Ketoacyl reductase* and *Enoyl reductase* utilize NADPH to donate electrons, reducing the growing carbon chain. **Why other options are incorrect:** * **Folate (B9):** Primarily involved in one-carbon metabolism (e.g., DNA synthesis and amino acid metabolism), not fatty acid reduction. * **Riboflavin (B2):** Forms FAD and FMN. While these are redox cofactors, they are primarily involved in oxidative pathways like Beta-oxidation (degradation) of fatty acids and the TCA cycle. * **Thiamin (B1):** Acts as TPP (Thiamin Pyrophosphate), a cofactor for oxidative decarboxylation (e.g., Pyruvate Dehydrogenase) and the transketolase reaction in the HMP shunt. **High-Yield Clinical Pearls for NEET-PG:** * **Sources of NADPH:** The primary source for fatty acid synthesis is the **Pentose Phosphate Pathway (HMP Shunt)** via the enzyme G6PD. Another source is the **Malic Enzyme**, which converts malate to pyruvate. * **Location:** Fatty acid synthesis occurs in the **cytosol**, whereas beta-oxidation occurs in the **mitochondria**. * **Rate-limiting step:** The conversion of Acetyl-CoA to Malonyl-CoA by **Acetyl-CoA Carboxylase (ACC)**, which requires **Biotin (B7)**. * **Niacin Deficiency:** Leads to **Pellagra** (Dermatitis, Diarrhea, Dementia, Death).

Question 368: Fish eye disease is a genetic disorder characterized by which of the following?

A. Classic form of LCAT deficiency
B. Partial LCAT deficiency (Correct Answer)
C. Tangier disease
D. Familial hyperalphalipoproteinemia

Explanation: **Explanation:** **Fish Eye Disease (FED)** is a rare genetic disorder caused by a **partial deficiency of the enzyme Lecithin-Cholesterol Acyltransferase (LCAT)**. 1. **Why the correct answer is right:** LCAT is responsible for esterifying free cholesterol into cholesterol esters. In FED, there is a selective loss of **alpha-LCAT activity** (which acts on HDL), while **beta-LCAT activity** (acting on VLDL/LDL) is preserved. This leads to a significant reduction in HDL-cholesterol levels and the deposition of unesterified cholesterol in the corneal stroma, giving the eyes a characteristic "boiled fish" appearance. 2. **Why incorrect options are wrong:** * **Classic LCAT Deficiency:** This involves a **complete** (both alpha and beta) deficiency of the enzyme. Unlike FED, it presents with more severe systemic features, including hemolytic anemia and progressive renal failure, alongside corneal opacities. * **Tangier Disease:** This is caused by a mutation in the **ABCA1 transporter** gene. While it also features extremely low HDL, its hallmark clinical sign is **enlarged, orange-colored tonsils** and hepatosplenomegaly, not the specific corneal pattern of FED. * **Familial Hyperalphalipoproteinemia:** This is a condition characterized by **elevated** HDL levels (often due to CETP deficiency), which is cardioprotective, the opposite of the low-HDL state seen in FED. **High-Yield Clinical Pearls for NEET-PG:** * **LCAT Activator:** Apo A-I is the primary activator of LCAT. * **Key Difference:** FED = Partial deficiency (Corneal opacities only); Classic LCAT deficiency = Complete deficiency (Cornea + Anemia + Renal failure). * **HDL Metabolism:** LCAT is essential for the maturation of discoidal nascent HDL into spherical mature HDL (Reverse Cholesterol Transport).

Question 369: Increased formation of ketone bodies during fasting is a result of which of the following?

A. Increased oxidation of fatty acids as a source of fuel (Correct Answer)
B. Decreased formation of acetyl CoA in the liver
C. Decreased levels of glucagon
D. Increased glycogenesis in muscle

Explanation: ### Explanation **1. Why Option A is Correct:** During fasting, the insulin-to-glucagon ratio decreases, triggering **lipolysis** in adipose tissue. This releases free fatty acids (FFAs) into the bloodstream, which are taken up by the liver and undergo **$\beta$-oxidation**. This process generates a massive surplus of **Acetyl CoA**. In the fasting state, oxaloacetate (OAA) is diverted toward gluconeogenesis to maintain blood glucose. The resulting shortage of OAA prevents Acetyl CoA from entering the TCA cycle. Consequently, the excess Acetyl CoA is shunted into the **ketogenesis pathway** (HMG-CoA synthase pathway) to produce acetoacetate, $\beta$-hydroxybutyrate, and acetone. Thus, increased fatty acid oxidation is the primary driver of ketogenesis. **2. Why Other Options are Incorrect:** * **Option B:** Ketogenesis requires an *increase* in Acetyl CoA levels, not a decrease. * **Option C:** Fasting is characterized by *increased* glucagon levels. Glucagon stimulates hormone-sensitive lipase, which provides the fatty acid precursors for ketone bodies. * **Option D:** During fasting, the body undergoes *glycogenolysis* (breakdown) rather than glycogenesis (synthesis) to provide energy. **3. NEET-PG High-Yield Pearls:** * **Rate-limiting enzyme:** Mitochondrial **HMG-CoA Synthase** is the rate-limiting step in ketogenesis. (Note: Cytosolic HMG-CoA synthase is used for cholesterol synthesis). * **Site of Synthesis:** Ketone bodies are synthesized exclusively in the **liver mitochondria**, but the liver **cannot utilize** them because it lacks the enzyme **Thiophorase** (Succinyl-CoA:3-ketoacid CoA transferase). * **Ketone Body Ratio:** The ratio of $\beta$-hydroxybutyrate to acetoacetate depends on the NADH/NAD+ ratio in the mitochondria. * **Clinical Sign:** Acetone is a non-metabolizable side product excreted via the lungs, giving the characteristic "fruity odor" to the breath in diabetic ketoacidosis.

Question 370: A 4-month-old child presented with extreme tiredness, irritable mood, poor appetite, and fasting hypoglycemia associated with vomiting and muscle weakness. Blood tests revealed elevated levels of free fatty acids but low levels of acylcarnitine. A muscle biopsy demonstrated significant fatty acid infiltration in the cytoplasm. What is the most likely molecular defect in this child?

A. Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency
B. Carnitine transporter deficiency (Correct Answer)
C. Acetyl-CoA carboxylase deficiency
D. Carnitine palmitoyltransferase II (CPT II) deficiency

Explanation: ### Explanation **Correct Option: B. Carnitine transporter deficiency** The clinical presentation of **fasting hypoglycemia**, muscle weakness, and fatty infiltration of tissues points toward a defect in **fatty acid oxidation (β-oxidation)**. In **Carnitine Transporter Deficiency** (Primary Carnitine Deficiency), the plasma membrane transporter (OCTN2) responsible for moving carnitine into cells is defective. This leads to: 1. **Low serum acylcarnitine:** Since carnitine cannot enter cells or be reabsorbed by the kidneys, it is lost in the urine, leaving no substrate to form acylcarnitines. 2. **Elevated Free Fatty Acids (FFAs):** FFAs are released from adipose tissue during fasting but cannot enter the mitochondria for oxidation. 3. **Hypoketotic Hypoglycemia:** Without β-oxidation, there is no Acetyl-CoA to drive ketogenesis or provide energy for gluconeogenesis. 4. **Lipid Accumulation:** Unused FFAs are converted back to triglycerides and stored in the cytoplasm of muscles and the liver. --- ### Why the other options are incorrect: * **A. MCAD Deficiency:** This is the most common fatty acid oxidation disorder. However, it typically presents with **elevated** medium-chain acylcarnitines (C6-C10) in the blood, not low levels. * **C. Acetyl-CoA Carboxylase Deficiency:** This enzyme is involved in fatty acid **synthesis**, not oxidation. Deficiency would not cause fasting hypoglycemia or lipid accumulation in muscles. * **D. CPT II Deficiency:** While this also causes muscle weakness and hypoglycemia, it typically presents with **elevated** long-chain acylcarnitines because carnitine has already been attached to the fatty acid by CPT I. --- ### High-Yield NEET-PG Pearls: * **The Carnitine Shuttle:** Required for long-chain fatty acids (>12 carbons) to enter the mitochondria. * **Hallmark of β-oxidation defects:** Hypoketotic hypoglycemia (low glucose + low ketones). * **Primary vs. Secondary:** Primary deficiency has low carnitine/acylcarnitine; CPT-I deficiency has **high** free carnitine (as it can't be used). * **Treatment:** High carbohydrate diet and oral L-carnitine supplementation.

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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