Lipid Metabolism Practice Questions

Q: Acetyl CoA carboxylase is the rate-limiting enzyme of which of the following metabolic pathways?

Fatty acid synthesis. **Explanation:** **Acetyl CoA Carboxylase (ACC)** is the key regulatory and rate-limiting enzyme in **Fatty Acid Synthesis (Lipogenesis)**. It catalyzes the ATP-dependent carboxylation of Acetyl CoA to Malonyl CoA. This step is crucial because Malonyl CoA serves as the primary building block for the fatty acid chain and also acts as a potent inhibitor of Carnitine Palmitoyltransferase-I (CPT-I), thereby preventing the simultaneous breakdown of fats (Beta-oxidation). **Analysis of Incorrect Options:** * **A. Cholesterol synthesis:** The rate-limiting enzyme is **HMG-CoA Reductase**, which converts HMG-CoA to Mevalonate. * **C. Urea synthesis:** The rate-limiting enzyme is **Carbamoyl Phosphate Synthetase I (CPS-I)**, located in the mitochondria. * **D. Gluconeogenesis:** The primary rate-limiting enzyme is **Fructose-1,6-bisphosphatase**. Other key regulatory enzymes include Pyruvate carboxylase and PEP carboxykinase. **High-Yield Facts for NEET-PG:** * **Activators of ACC:** Citrate (allosteric feed-forward activator) and Insulin (promotes dephosphorylation/activation). * **Inhibitors of ACC:** Long-chain fatty acyl-CoA (Palmitoyl CoA - feedback inhibition) and Glucagon/Epinephrine (promote phosphorylation/inactivation via AMPK). * **Cofactor:** ACC requires **Biotin (Vitamin B7)** for its activity. * **Location:** Fatty acid synthesis occurs in the **Cytosol**, primarily in the liver, adipose tissue, and lactating mammary glands.

Q: All of the following about prostaglandins are TRUE, EXCEPT:

PGE1 and PGE2 increase in bronchial asthma. **Explanation:** **Why Option D is the Correct Answer (The "Except"):** In bronchial asthma, the primary lipid mediators involved are **Leukotrienes (LTC4, LTD4, and LTE4)**, which are potent bronchoconstrictors. Regarding prostaglandins, **PGE2** actually acts as a **bronchodilator** and has anti-inflammatory properties in the airway. While PGF2$\alpha$ and PGD2 are bronchoconstrictors, the statement that PGE1 and PGE2 increase to cause asthma is physiologically incorrect. In fact, aspirin-induced asthma occurs because the inhibition of the COX pathway shunts arachidonic acid toward the lipoxygenase (LOX) pathway, increasing leukotriene production. **Analysis of Other Options:** * **Option A:** Prostaglandins are eicosanoids (20-carbon compounds) derived primarily from **arachidonic acid** (an omega-6 fatty acid) via the Cyclooxygenase (COX) pathway. * **Option B:** They were first isolated from **human semen** by Ulf von Euler in the 1930s. He mistakenly believed they originated from the prostate gland (hence the name "prostaglandins"), though they are actually produced by the seminal vesicles. * **Option C:** Structurally, all prostaglandins are characterized by a **20-carbon carboxylic acid** chain containing a **5-carbon (cyclopentane) ring**. **NEET-PG High-Yield Pearls:** * **Rate-limiting step:** The release of arachidonic acid from membrane phospholipids by **Phospholipase A2** (inhibited by Corticosteroids). * **PGE2 Clinical Use:** Used for cervical ripening and induction of labor (Dinoprostone). * **PGI2 (Prostacyclin):** Produced by vascular endothelium; causes vasodilation and inhibits platelet aggregation. * **TXA2 (Thromboxane):** Produced by platelets; causes vasoconstriction and promotes platelet aggregation (antagonistic to PGI2).

Q: The main component of hyperlipidemia constituting a major risk factor for atherosclerosis is:

Low-density lipoprotein (LDL) cholesterol. **Explanation:** **1. Why LDL Cholesterol is the Correct Answer:** Low-density lipoprotein (LDL) is the primary carrier of cholesterol from the liver to peripheral tissues. It is considered the most significant risk factor for atherosclerosis because of its small size and high cholesterol content. LDL particles can easily penetrate the arterial endothelium, where they become oxidized. These **oxidized LDL particles** are engulfed by macrophages to form **foam cells**, which are the hallmark of early atherosclerotic plaques (fatty streaks). High levels of LDL are directly correlated with an increased risk of Coronary Artery Disease (CAD). **2. Why the Other Options are Incorrect:** * **HDL (Option A):** Known as "Good Cholesterol," HDL mediates **reverse cholesterol transport**, moving cholesterol from peripheral tissues back to the liver. High levels are cardioprotective, not a risk factor. * **IDL (Option B):** IDL is a transient intermediate formed during the conversion of VLDL to LDL. While it is pro-atherogenic, it is not the "main" component or the primary marker used for risk stratification in clinical practice. * **VLDL (Option C):** VLDL primarily transports endogenous triglycerides. While elevated VLDL (hypertriglyceridemia) is a risk factor, it is less directly linked to the initiation of the atherosclerotic plaque compared to LDL. **3. NEET-PG High-Yield Pearls:** * **Friedewald Formula:** LDL = Total Cholesterol – [HDL + (Triglycerides/5)]. (Note: This is invalid if TG >400 mg/dL). * **Apolipoproteins:** LDL contains **Apo B-100**, which acts as a ligand for the LDL receptor. * **Lp(a):** Lipoprotein (a) is an independent genetic risk factor for atherosclerosis; it is essentially an LDL particle with an added Apo(a) protein. * **Statins:** The first-line treatment for hyperlipidemia, they work by inhibiting HMG-CoA reductase, leading to an up-regulation of LDL receptors on hepatocytes.

Q: Which of the following is true about bile acids?

All of the above.. ### Explanation Bile acids are essential polar derivatives of cholesterol synthesized in the liver, playing a crucial role in lipid digestion and absorption. **1. Why Option D is correct:** All the provided statements are biochemically accurate: * **Derived from Cholesterol (Option B):** The synthesis of bile acids is the primary pathway for cholesterol excretion. Through a series of reactions, the hydrophobic cholesterol molecule is converted into amphipathic bile acids. * **Rate-Limiting Enzyme (Option A):** The conversion of cholesterol to **7$\alpha$-hydroxycholesterol** by the enzyme **7$\alpha$-hydroxylase** (a cytochrome P450 enzyme) is the committed and rate-limiting step. This enzyme is feedback-inhibited by bile acids and induced by cholesterol. * **Primary Bile Acids (Option C):** These are synthesized directly in the liver. The two main primary bile acids are **Cholic acid** and **Chenodeoxycholic acid**. **2. Understanding the Process:** * **Primary vs. Secondary:** Primary bile acids (Cholic, Chenodeoxycholic) are synthesized in the liver. Once they reach the intestine, bacterial action (dehydroxylation) converts them into **secondary bile acids**: Deoxycholic acid (from Cholic) and Lithocholic acid (from Chenodeoxycholic). * **Conjugation:** Before secretion, bile acids are conjugated with **Glycine or Taurine** to form bile salts, which increases their solubility at intestinal pH. **3. High-Yield Clinical Pearls for NEET-PG:** * **Enterohepatic Circulation:** Approximately 95% of bile salts are reabsorbed in the **terminal ileum** and returned to the liver. * **Bile Acid Sequestrants (Cholestyramine):** These drugs bind bile acids in the gut, preventing reabsorption. This forces the liver to use more cholesterol to synthesize new bile acids, thereby lowering LDL levels. * **Vitamin C Deficiency:** 7$\alpha$-hydroxylase requires Vitamin C; thus, scurvy can lead to cholesterol accumulation and gallstone formation.

Q: Which of the following is not considered a "bad cholesterol"?

HDL. ### Explanation **Correct Answer: A. HDL** **Why HDL is the "Good Cholesterol":** High-Density Lipoprotein (HDL) is termed "good cholesterol" because of its role in **Reverse Cholesterol Transport**. It picks up excess cholesterol from peripheral tissues and blood vessel walls (including atherosclerotic plaques) and transports it back to the liver for excretion in bile. High levels of HDL are cardio-protective as they reduce the risk of atherosclerosis and coronary artery disease. **Why the other options are "Bad Cholesterol":** These lipoproteins are considered "bad" because they transport lipids from the liver to the peripheral tissues, contributing to plaque formation: * **LDL (Low-Density Lipoprotein):** The primary carrier of cholesterol to tissues. It is the most potent pro-atherogenic lipoprotein; high levels lead to cholesterol deposition in arterial walls. * **VLDL (Very Low-Density Lipoprotein):** Secreted by the liver to transport endogenous triglycerides. It is a precursor to LDL. * **IDL (Intermediate-Density Lipoprotein):** Formed during the conversion of VLDL to LDL. Like LDL, it is enriched in cholesterol esters and contributes to atherosclerosis. **High-Yield NEET-PG Pearls:** * **Apo-A1:** The primary apoprotein associated with **HDL** (activates LCAT). * **Apo-B100:** The characteristic apoprotein for **VLDL, IDL, and LDL**. * **LCAT (Lecithin-Cholesterol Acyltransferase):** The enzyme responsible for esterifying cholesterol within HDL, converting it from discoid to spherical form. * **Friedewald Formula:** Used to calculate LDL: $LDL = Total\ Cholesterol – (HDL + TG/5)$. (Note: This is invalid if TG >400 mg/dL).

Q: Which enzyme defect causes Tay-Sachs disease?

Hexosaminidase A. ### Explanation **Correct Option: C. Hexosaminidase A** Tay-Sachs disease is a lysosomal storage disorder (Sphingolipidosis) caused by a deficiency of the enzyme **Hexosaminidase A**. This enzyme is responsible for the degradation of **GM2 gangliosides**. Its deficiency leads to the toxic accumulation of GM2 gangliosides in the neurons of the brain and spinal cord, resulting in progressive neurodegeneration. **Analysis of Incorrect Options:** * **A. Beta-glucosidase (Glucocerebrosidase):** Deficiency causes **Gaucher disease**, the most common lysosomal storage disorder, characterized by hepatosplenomegaly and "wrinkled paper" cytoplasm in macrophages. * **B. Alpha-galactosidase:** Deficiency causes **Fabry disease**, an X-linked disorder characterized by angiokeratomas, peripheral neuropathy, and renal failure. * **D. Beta-galactosidase:** Deficiency causes **GM1 gangliosidosis** or **Krabbé disease** (specifically Galactosylceramidase). **Clinical Pearls for NEET-PG:** * **Cherry-red spot on Macula:** A classic finding in Tay-Sachs. Note that it is also seen in Niemann-Pick disease; however, **hepatosplenomegaly is absent** in Tay-Sachs (present in Niemann-Pick). * **Onion-skin appearance:** Histology of lysosomes shows whorled membranes. * **Genetics:** Autosomal Recessive inheritance; common in Ashkenazi Jews. * **Clinical Presentation:** Startle response (hyperacusis), developmental regression, and seizures, usually appearing between 3–6 months of age. * **Mnemonic:** Tay-Sa**X** lacks He**X**osaminidase A.

Question 1

Acetyl CoA carboxylase is the rate-limiting enzyme of which of the following metabolic pathways?

Accepted Answer

Fatty acid synthesis

Answer

Cholesterol synthesis

Answer

Urea synthesis

Answer

Gluconeogenesis

Question 2

All of the following about prostaglandins are TRUE, EXCEPT:

Accepted Answer

PGE1 and PGE2 increase in bronchial asthma

Answer

Synthesized from arachidonic acid

Answer

First isolated from semen

Answer

Prostaglandins contain a cyclopentane ring

Question 3

The main component of hyperlipidemia constituting a major risk factor for atherosclerosis is:

Accepted Answer

Low-density lipoprotein (LDL) cholesterol

Answer

High-density lipoprotein (HDL) cholesterol

Answer

Intermediate-density lipoprotein (IDL) cholesterol

Answer

Very low-density lipoprotein (VLDL) cholesterol

Question 4

Which of the following is true about bile acids?

Accepted Answer

All of the above.

Answer

7 alpha-hydroxylase is the rate-limiting enzyme in their synthesis.

Answer

They are derived from cholesterol.

Answer

Cholic acid is a primary bile acid.

Question 5

Which of the following is not considered a "bad cholesterol"?

Accepted Answer

HDL

Answer

LDL

Answer

VLDL

Answer

IDL

Question 6

In diabetics, what factor limits the synthesis of triglycerides in adipose tissue?

Accepted Answer

Glycerol-3-P

Answer

NADPH

Answer

ATP

Answer

Acetyl CoA

Question 7

What is the most important source of reducing equivalents for fatty acid synthesis in the liver?

Accepted Answer

HMP pathway

Answer

Glycolysis

Answer

TCA cycle

Answer

Uronic acid pathway

Question 8

Which enzyme defect causes Tay-Sachs disease?

Accepted Answer

Hexosaminidase A

Answer

Beta-glucosidase

Answer

Alpha-galactosidase

Answer

Beta-galactosidase

Question 9

A child presented with altered sensorium and icterus three days after a viral illness for which he was treated with aspirin. What is the underlying biochemical defect?

Accepted Answer

Beta oxidation of fatty acids

Answer

Glucose-6-phosphatase deficiency

Answer

Pyruvate dehydrogenase deficiency

Answer

Urea cycle defect

Question 10

What is the total ATP generated by the complete oxidation of stearic acid?

Accepted Answer

148

Answer

114

Answer

131

Answer

None of the above

Lipid Metabolism — MCQs

Lipid Metabolism — MCQs

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