Lipid metabolism — MCQs

Q: Colipase is an enzyme found in secretions from which of the following? (Recent NEET Pattern 2018)

Pancreas. ***Pancreas*** - The pancreas is the primary source of **colipase** secretion in the digestive system. - Colipase is an essential co-factor for **pancreatic lipase**, helping it to anchor to the surface of **lipid droplets** and digest triglycerides in the presence of bile salts. - Colipase is secreted by pancreatic acinar cells along with other pancreatic enzymes into the duodenum. *Salivary glands* - The salivary glands (parotid, submandibular, sublingual) produce saliva containing enzymes like **amylase** and **lingual lipase**. - These glands do not secrete colipase, which is involved in fat digestion in the small intestine, not in the mouth. *Gallbladder* - The gallbladder stores and releases **bile**, which aids in fat emulsification. - Bile does not contain colipase; colipase is produced by the pancreas and works synergistically with bile salts during fat digestion. *Small intestine* - The small intestine is where most nutrient absorption occurs and produces some enzymes (e.g., brush border enzymes like maltase, lactase). - The small intestine does not synthesize colipase; it receives colipase from pancreatic secretions via the pancreatic duct. *Liver* - The liver produces bile, which is stored in the gallbladder and released into the duodenum to emulsify fats. - The liver does not produce colipase; this enzyme is specifically secreted by pancreatic acinar cells.

Q: Name the mechanism shown as $X$ below.

Nonenzymatic decarboxylation. ***Nonenzymatic decarboxylation*** - The image shows **acetoacetate converting into acetone**, with the release of **carbon dioxide (CO2)**. This process is a spontaneous, **nonenzymatic decarboxylation** that occurs in conditions like diabetic ketoacidosis. - The absence of an enzyme indicated at "X" further supports that this is a **nonenzymatic reaction**. *Thiolase* - **Thiolase** is an enzyme involved in the synthesis of acetoacetyl CoA, and also in the breakdown of fatty acids, but not in the conversion of acetoacetate to acetone. - This enzyme catalyzes the **condensation of two acetyl-CoA** molecules to form acetoacetyl-CoA or the cleavage of acetoacetyl-CoA. *HMG CoA Lyase* - **HMG CoA lyase** is an enzyme that catalyzes the cleavage of HMG-CoA to **acetoacetate and acetyl-CoA** during ketogenesis. - It works upstream in the ketone body synthesis pathway, not directly converting acetoacetate to acetone. *HMG CoA synthase* - **HMG CoA synthase** is an enzyme that catalyzes the condensation of **acetoacetyl-CoA and acetyl-CoA** to form HMG-CoA, the first committed step of ketogenesis. - It works upstream in the pathway, not in the conversion of acetoacetate to acetone. *L-beta hydroxyl butyrate dehydrogenase* - The image already labels **beta-hydroxybutyrate dehydrogenase**, which interconverts **acetoacetate and beta-hydroxybutyrate** using NADH and NAD+. - This enzyme is responsible for the reversible reduction of acetoacetate to beta-hydroxybutyrate, and not for the formation of acetone.

Q: Which is the location of HMG CoA synthase in cholesterol metabolism?

2. ***2*** - HMG CoA synthase is a key enzyme in the cholesterol synthesis pathway, specifically converting acetoacetyl-CoA to **hydroxymethylglutaryl-CoA (HMG-CoA)**. - This step occurs in the **cytosol**, which is indicated by '2' in the diagram. - The cytosolic form of HMG-CoA synthase is specifically involved in cholesterol biosynthesis. *1* - '1' points to the **nucleus**, which is primarily involved in DNA replication and transcription, not cholesterol synthesis. - While gene expression control for cholesterol synthesis occurs here (e.g., SREBP regulation), the enzymatic reaction itself does not take place in the nucleus. *3* - '3' indicates the **smooth endoplasmic reticulum (SER)**, which is involved in later steps of cholesterol synthesis, such as the conversion of squalene to cholesterol. - However, the initial steps catalyzed by HMG CoA synthase occur in the cytosol, not directly on the SER. *4* - '4' points to a **mitochondrion**, which is involved in energy production through the citric acid cycle and oxidative phosphorylation. - Note: There is a mitochondrial HMG-CoA synthase, but it is involved in **ketone body synthesis**, not cholesterol metabolism. *5* - '5' represents another cellular structure not involved in the HMG-CoA synthase step of cholesterol synthesis. - The cholesterol synthesis pathway begins in the cytosol with the formation of HMG-CoA before proceeding to other compartments.

Q: A patient has multiple tendon xanthomas. Serum cholesterol ( $398 \mathrm{mg} / \mathrm{dL}$ ) and LDL ( 220 $\mathrm{mg} / \mathrm{dL}$ ) were found to be elevated. What is the most likely defect?

LDL receptor defect. ***LDL receptor defect*** - **Tendon xanthomas** are a classic sign of **familial hypercholesterolemia**, which is most commonly caused by a genetic defect in the **LDL receptor**. - **Elevated LDL cholesterol** levels are a hallmark of this condition, as dysfunctional LDL receptors lead to impaired clearance of LDL particles from the blood. *Lipoprotein lipase deficiency* - This condition primarily causes severe **hypertriglyceridemia** and can lead to **eruptive xanthomas**, but not typically tendon xanthomas. - While cholesterol levels might be elevated, the defining feature would be very high triglyceride levels, often exceeding 1000 mg/dL. *Apo E defect* - A defect in **ApoE** (specifically the **ApoE2/E2 genotype**) is associated with **familial dysbetalipoproteinemia** (Type III hyperlipoproteinemia). - This condition causes elevated remnants of chylomicrons and VLDL, leading to **palmar xanthomas**, but less commonly tendon xanthomas, and often presents with high triglyceride levels in addition to cholesterol. *Apo B-100 defect* - **Familial defective apoB-100** can present similarly to familial hypercholesterolemia with elevated LDL cholesterol. - However, this is much **rarer** than LDL receptor defects (affecting ~1:700 vs 1:250-500 for LDL receptor mutations). - The clinical presentation and lipid profile overlap significantly, but LDL receptor defects remain the most common cause of this clinical picture. *LCAT deficiency* - **Lecithin-cholesterol acyltransferase (LCAT)** deficiency leads to an accumulation of **unesterified cholesterol** in plasma and tissues. - This typically presents with **corneal opacities**, **hemolytic anemia**, and proteinuria, rather than predominantly tendon xanthomas and isolated severe LDL elevation.

Q: A patient presents with xanthomas on the Achilles tendon. What is the most likely diagnosis?

Familial hypercholesterolemia. ***Familial hypercholesterolemia*** - **Xanthomas** on the **Achilles tendon** are a classic clinical sign of familial hypercholesterolemia, along with significantly elevated **LDL-C levels**. - This condition is an **autosomal dominant** genetic disorder characterized by defects in the **LDL receptor** pathway, leading to impaired clearance of LDL from the blood. - **Tendon xanthomas** (especially Achilles and extensor tendons) are pathognomonic for this condition. *Tangier's disease* - Characterized by very low or absent **HDL-C (high-density lipoprotein cholesterol)** levels, leading to **cholesterol ester accumulation** in various tissues. - While it can cause lipid deposition, its hallmark is **enlarged, orange tonsils** and peripheral neuropathy, not typically Achilles tendon xanthomas. *Familial hyperchylomicronemia* - This disorder primarily involves elevated **chylomicrons** and **triglycerides**, presenting with **eruptive xanthomas** (small, red-yellow papules) but not typically tendon xanthomas. - It is often associated with **pancreatitis** and **lipemia retinalis**. *Familial dysbetalipoproteinemia* - Characterized by elevated levels of **cholesterol** and **triglycerides** due to accumulation of remnant lipoproteins (IDL). - While it can cause **xanthomas**, these are typically **palmar xanthomas** (xanthoma striata palmaris) and **tuberoeruptive xanthomas**, less commonly Achilles tendon xanthomas. *Familial combined hyperlipidemia* - Most common familial lipid disorder, characterized by elevated **LDL-C** and/or **triglycerides** with variable phenotype. - While it causes premature coronary artery disease, it typically does **not** cause tendon xanthomas, which distinguishes it from familial hypercholesterolemia. - Xanthomas, if present, are usually **xanthelasma** (around eyelids) rather than tendon xanthomas.

Q: A patient with high triglycerides (TG) esterified with long-chain fatty acids (LCFA) presents with fatigue, and a biopsy of the muscle shows fat vacuoles. What is the most likely diagnosis?

Carnitine deficiency. ***Carnitine deficiency*** - **Carnitine** is essential for transporting **long-chain fatty acids (LCFAs)** into the mitochondria for beta-oxidation. - A deficiency leads to the accumulation of **LCFAs** as **triglycerides** in the cytoplasm, resulting in **fat vacuoles** in muscle and systemic fatigue due to impaired energy production. *Fatty acid synthase defect* - **Fatty acid synthase** is involved in the *de novo* synthesis of fatty acids, not their catabolism or transport. - A defect would impair fatty acid production, not lead to the accumulation of **triglycerides** from exogenous sources. *Lipoprotein lipase (LPL) defect* - **LPL** is crucial for cleaving **triglycerides** in circulating chylomicrons and VLDL, allowing fatty acids to be taken up by tissues. - A defect causes severe hypertriglyceridemia, but the primary issue in the muscle with fat vacuoles points towards a problem with intracellular fatty acid utilization rather than plasma triglyceride clearance. *LDL defect* - **LDL** is primarily responsible for transporting cholesterol to peripheral tissues. - Defects in **LDL** metabolism typically lead to hypercholesterolemia, not the accumulation of **triglycerides** or muscle fat vacuoles as described. *Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency* - **MCAD** deficiency is a fatty acid oxidation disorder affecting **medium-chain fatty acids** (C6-C12), not the **long-chain fatty acids** specifically mentioned in the stem. - It typically presents with hypoketotic hypoglycemia during fasting, often in infancy or childhood, rather than the characteristic muscle fat vacuole accumulation pattern seen with **carnitine deficiency**.

Q: A patient with tendon xanthomas, Increased LDL and cholesterol. What is the most probable diagnosis?

Type II Hyperlipoproteinemia. ***Type II Hyperlipoproteinemia*** - This type is characterized by significantly **elevated LDL and total cholesterol** due to a defect in LDL receptor function or APOB-100. - **Tendon xanthomas** are a classic physical finding in Type II hyperlipoproteinemia, specifically in familial hypercholesterolemia. *Type III Hyperlipoproteinemia* - This condition involves increased levels of **chylomicron remnants** and **VLDL remnants (IDL)**, leading to elevated cholesterol and triglycerides. - While xanthomas can occur (e.g., **palmar xanthomas**), tendon xanthomas are less typical, and the primary lipid abnormality isn't isolated LDL elevation. *Abetalipoproteinemia* - This is a rare autosomal recessive disorder resulting in the **absence of LDL, VLDL, and chylomicrons** in the blood. - Patients present with **fat malabsorption**, neurologic symptoms, and generally have very low or undetectable cholesterol and triglyceride levels, which is contrary to the clinical presentation. *Type I Hyperlipoproteinemia* - This disorder is characterized by a deficiency of **lipoprotein lipase (LPL)** or its cofactor, APO C-II, leading to extremely high levels of **chylomicrons** and **triglycerides**. - While eruptive xanthomas can be seen, **tendon xanthomas** are not a feature, and the primary abnormality is hypertriglyceridemia, not elevated LDL. *Type IV Hyperlipoproteinemia* - This condition is characterized by **elevated VLDL** and **triglycerides** with normal or slightly elevated LDL. - Xanthomas are generally not a feature, and the primary abnormality is hypertriglyceridemia rather than hypercholesterolemia with elevated LDL.

Lipid metabolism US Medical PG Practice Questions and MCQs

Question 1: Colipase is an enzyme found in secretions from which of the following? (Recent NEET Pattern 2018)

A. Salivary glands
B. Gallbladder
C. Pancreas (Correct Answer)
D. Small intestine
E. Liver

Explanation: ***Pancreas*** - The pancreas is the primary source of **colipase** secretion in the digestive system. - Colipase is an essential co-factor for **pancreatic lipase**, helping it to anchor to the surface of **lipid droplets** and digest triglycerides in the presence of bile salts. - Colipase is secreted by pancreatic acinar cells along with other pancreatic enzymes into the duodenum. *Salivary glands* - The salivary glands (parotid, submandibular, sublingual) produce saliva containing enzymes like **amylase** and **lingual lipase**. - These glands do not secrete colipase, which is involved in fat digestion in the small intestine, not in the mouth. *Gallbladder* - The gallbladder stores and releases **bile**, which aids in fat emulsification. - Bile does not contain colipase; colipase is produced by the pancreas and works synergistically with bile salts during fat digestion. *Small intestine* - The small intestine is where most nutrient absorption occurs and produces some enzymes (e.g., brush border enzymes like maltase, lactase). - The small intestine does not synthesize colipase; it receives colipase from pancreatic secretions via the pancreatic duct. *Liver* - The liver produces bile, which is stored in the gallbladder and released into the duodenum to emulsify fats. - The liver does not produce colipase; this enzyme is specifically secreted by pancreatic acinar cells.

Question 2: Name the mechanism shown as $X$ below.

A. Thiolase
B. Nonenzymatic decarboxylation (Correct Answer)
C. HMG CoA Lyase
D. L-beta hydroxyl butyrate dehydrogenase
E. HMG CoA synthase

Explanation: ***Nonenzymatic decarboxylation*** - The image shows **acetoacetate converting into acetone**, with the release of **carbon dioxide (CO2)**. This process is a spontaneous, **nonenzymatic decarboxylation** that occurs in conditions like diabetic ketoacidosis. - The absence of an enzyme indicated at "X" further supports that this is a **nonenzymatic reaction**. *Thiolase* - **Thiolase** is an enzyme involved in the synthesis of acetoacetyl CoA, and also in the breakdown of fatty acids, but not in the conversion of acetoacetate to acetone. - This enzyme catalyzes the **condensation of two acetyl-CoA** molecules to form acetoacetyl-CoA or the cleavage of acetoacetyl-CoA. *HMG CoA Lyase* - **HMG CoA lyase** is an enzyme that catalyzes the cleavage of HMG-CoA to **acetoacetate and acetyl-CoA** during ketogenesis. - It works upstream in the ketone body synthesis pathway, not directly converting acetoacetate to acetone. *HMG CoA synthase* - **HMG CoA synthase** is an enzyme that catalyzes the condensation of **acetoacetyl-CoA and acetyl-CoA** to form HMG-CoA, the first committed step of ketogenesis. - It works upstream in the pathway, not in the conversion of acetoacetate to acetone. *L-beta hydroxyl butyrate dehydrogenase* - The image already labels **beta-hydroxybutyrate dehydrogenase**, which interconverts **acetoacetate and beta-hydroxybutyrate** using NADH and NAD+. - This enzyme is responsible for the reversible reduction of acetoacetate to beta-hydroxybutyrate, and not for the formation of acetone.

Question 3: Which is the location of HMG CoA synthase in cholesterol metabolism?

A. 1
B. 2 (Correct Answer)
C. 3
D. 4
E. 5

Explanation: ***2*** - HMG CoA synthase is a key enzyme in the cholesterol synthesis pathway, specifically converting acetoacetyl-CoA to **hydroxymethylglutaryl-CoA (HMG-CoA)**. - This step occurs in the **cytosol**, which is indicated by '2' in the diagram. - The cytosolic form of HMG-CoA synthase is specifically involved in cholesterol biosynthesis. *1* - '1' points to the **nucleus**, which is primarily involved in DNA replication and transcription, not cholesterol synthesis. - While gene expression control for cholesterol synthesis occurs here (e.g., SREBP regulation), the enzymatic reaction itself does not take place in the nucleus. *3* - '3' indicates the **smooth endoplasmic reticulum (SER)**, which is involved in later steps of cholesterol synthesis, such as the conversion of squalene to cholesterol. - However, the initial steps catalyzed by HMG CoA synthase occur in the cytosol, not directly on the SER. *4* - '4' points to a **mitochondrion**, which is involved in energy production through the citric acid cycle and oxidative phosphorylation. - Note: There is a mitochondrial HMG-CoA synthase, but it is involved in **ketone body synthesis**, not cholesterol metabolism. *5* - '5' represents another cellular structure not involved in the HMG-CoA synthase step of cholesterol synthesis. - The cholesterol synthesis pathway begins in the cytosol with the formation of HMG-CoA before proceeding to other compartments.

Question 4: An adult male presented with a protruding abdomen, diarrhea, visual symptoms, and neurological manifestations. His LDL is low. Based on the peripheral smear finding shown in the image, what is the likely diagnosis?

A. Abetalipoproteinemia (Correct Answer)
B. EDTA changes
C. Uremia
D. Burns
E. Liver disease

Explanation: ***Abetalipoproteinemia*** - The image shows **acanthocytes (spur cells)**, characterized by irregularly spaced, blunt projections, which are a hallmark of **abetalipoproteinemia** due to abnormal lipid metabolism and membrane defects. - The clinical presentation of a **protruding abdomen (steatorrhea/malabsorption)**, **diarrhea**, **visual symptoms (retinopathy)**, **neurological manifestations (ataxia, peripheral neuropathy)**, and **low LDL** all strongly point to abetalipoproteinemia, a disorder affecting the synthesis of B-apolipoprotein and chylomicrons. *EDTA changes* - **EDTA changes** typically manifest as **rouleaux formation**, platelet satellite formation, or cell shrinkage, with red blood cell morphology generally remaining normal in terms of spur cell formation. - These changes are **artifactual** and are not associated with the patient's systemic symptoms like malabsorption, neurological issues, or specific lipid profile findings. *Uremia* - While **uremia** can cause various red blood cell abnormalities, including **burr cells (echinocytes)** with regularly spaced, pointed projections, it generally does not cause the irregularly shaped **acanthocytes** seen in the image. - The systemic symptoms of uremia would primarily involve **renal dysfunction (e.g., elevated BUN, creatinine)**, which are not mentioned, and not specifically the **visual or malabsorption symptoms** seen here. *Burns* - Severe **burns** can lead to red blood cell fragmentation, causing **schistocytes** or **microspherocytes** due to heat-induced damage. - Burns are not typically associated with the formation of **acanthocytes** or the constellation of symptoms (malabsorption, neurological, visual) and lipid profile (low LDL) described in this patient. *Liver disease* - Advanced **liver disease (cirrhosis)** can cause **spur cell anemia** with acanthocytes due to altered cholesterol-to-phospholipid ratio in RBC membranes. - However, the key distinguishing feature is the **low LDL** in this patient, which is characteristic of abetalipoproteinemia, whereas liver disease typically does not present with specifically **low LDL** as a prominent feature. - Additionally, the constellation of **visual symptoms (retinopathy)** and **neurological manifestations** with malabsorption are more consistent with the fat-soluble vitamin deficiency (A, E, K) seen in abetalipoproteinemia rather than isolated liver pathology.

Question 5: A patient came to the hospital with severe abdominal pain, and lipase levels were elevated. On imaging, a stone is found in the common bile duct (CBD). Which enzyme is most likely elevated in this condition?

A. ALT
B. GGT
C. LDH
D. AST
E. ALP (Correct Answer)

Explanation: ***ALP (Alkaline Phosphatase)*** - **ALP** is the **most characteristic enzyme elevation** in **biliary obstruction** from a CBD stone. - ALP is found in high concentrations in the **bile duct epithelium** and hepatocytes adjacent to bile ducts, and rises dramatically with **cholestasis** and **obstructive jaundice**. - In CBD stone obstruction, ALP typically rises **3-10 times normal**, making it the hallmark biochemical marker of this condition. - While lipase is elevated due to associated pancreatitis, **ALP elevation specifically indicates the biliary obstruction**. *GGT (Gamma-Glutamyl Transferase)* - **GGT** is also elevated in **cholestasis** and **bile duct obstruction**. - GGT often rises in parallel with ALP and helps confirm the hepatobiliary origin of ALP elevation (vs. bone source). - However, **ALP is more specific** and typically shows greater magnitude of elevation in acute CBD obstruction, making it the **most likely** elevated enzyme in this clinical context. *ALT (Alanine Aminotransferase)* - **ALT** may be **mildly to moderately elevated** if there is secondary hepatocellular injury from biliary obstruction. - However, ALT primarily indicates **hepatocyte damage** rather than cholestasis, and its elevation is typically **less pronounced** than ALP in obstructive biliary disease. - The pattern in CBD obstruction is **cholestatic** (high ALP) rather than **hepatocellular** (high ALT). *AST (Aspartate Aminotransferase)* - **AST** can be elevated in various conditions including liver, heart, and muscle damage. - Like ALT, it may show mild elevation in biliary obstruction but is **not the primary marker**. - AST is less specific than ALP for diagnosing CBD stone obstruction. *LDH (Lactate Dehydrogenase)* - **LDH** is a **non-specific marker** of tissue damage found in multiple organs. - While it may be elevated, it provides little diagnostic value when specific markers like **ALP and lipase** are available. - LDH does not help differentiate biliary obstruction from other causes of abdominal pain.

Question 6: A patient came to the emergency room with severe abdominal pain. The serum triglyceride level was $1500 \mathrm{mg} / \mathrm{dL}$. What is the most likely defect?

A. Apo B-48
B. Apo B-100
C. Apo C-II (Correct Answer)
D. LDL receptor
E. Lipoprotein lipase

Explanation: ***Apo C-II*** - **Apo C-II** is an essential cofactor for **lipoprotein lipase (LPL)**, which is responsible for hydrolyzing triglycerides from chylomicrons and VLDL. - A defect in Apo C-II leads to severely impaired triglyceride clearance, resulting in **chylomicronemia** and extremely high serum triglyceride levels (e.g., 1500 mg/dL), which can cause acute pancreatitis. - Both Apo C-II deficiency and LPL deficiency present similarly, but Apo C-II deficiency is the more specific answer when considering the **"defect"** terminology, as it represents the regulatory cofactor rather than the enzyme itself. *Apo B-48* - **Apo B-48** is a structural protein uniquely found on **chylomicrons**, synthesized in the intestine, and is essential for their formation and secretion. - A defect in Apo B-48 (e.g., in abetalipoproteinemia) would lead to the **absence of chylomicrons**, resulting in very low or undetectable triglyceride levels after a fat-containing meal, not high levels. *Apo B-100* - **Apo B-100** is the primary apolipoprotein of **VLDL, IDL, and LDL**, and it is crucial for VLDL assembly in the liver and for LDL receptor binding. - Defects in Apo B-100 leading to hyperlipidemia typically cause elevated LDL cholesterol (e.g., familial defective Apo B-100), rather than severe hypertriglyceridemia associated with chylomicronemia. *LDL receptor* - The **LDL receptor** is responsible for the uptake of **LDL particles** from the bloodstream, primarily in the liver. - A defect in the LDL receptor (e.g., in familial hypercholesterolemia) primarily causes **elevated LDL cholesterol** levels, but typically does not lead to the extreme hypertriglyceridemia seen in this patient. *Lipoprotein lipase* - **Lipoprotein lipase (LPL)** is the enzyme that hydrolyzes triglycerides in chylomicrons and VLDL particles. - A primary deficiency of LPL itself (Type I familial chylomicronemia) would also cause severe hypertriglyceridemia similar to Apo C-II deficiency. - However, Apo C-II deficiency is the more specific answer as it represents the **cofactor defect** that impairs LPL function, while direct LPL deficiency is a separate genetic entity.

Question 7: A patient has multiple tendon xanthomas. Serum cholesterol ( $398 \mathrm{mg} / \mathrm{dL}$ ) and LDL ( 220 $\mathrm{mg} / \mathrm{dL}$ ) were found to be elevated. What is the most likely defect?

A. Lipoprotein lipase deficiency
B. LDL receptor defect (Correct Answer)
C. Apo E defect
D. LCAT deficiency
E. Apo B-100 defect

Explanation: ***LDL receptor defect*** - **Tendon xanthomas** are a classic sign of **familial hypercholesterolemia**, which is most commonly caused by a genetic defect in the **LDL receptor**. - **Elevated LDL cholesterol** levels are a hallmark of this condition, as dysfunctional LDL receptors lead to impaired clearance of LDL particles from the blood. *Lipoprotein lipase deficiency* - This condition primarily causes severe **hypertriglyceridemia** and can lead to **eruptive xanthomas**, but not typically tendon xanthomas. - While cholesterol levels might be elevated, the defining feature would be very high triglyceride levels, often exceeding 1000 mg/dL. *Apo E defect* - A defect in **ApoE** (specifically the **ApoE2/E2 genotype**) is associated with **familial dysbetalipoproteinemia** (Type III hyperlipoproteinemia). - This condition causes elevated remnants of chylomicrons and VLDL, leading to **palmar xanthomas**, but less commonly tendon xanthomas, and often presents with high triglyceride levels in addition to cholesterol. *Apo B-100 defect* - **Familial defective apoB-100** can present similarly to familial hypercholesterolemia with elevated LDL cholesterol. - However, this is much **rarer** than LDL receptor defects (affecting ~1:700 vs 1:250-500 for LDL receptor mutations). - The clinical presentation and lipid profile overlap significantly, but LDL receptor defects remain the most common cause of this clinical picture. *LCAT deficiency* - **Lecithin-cholesterol acyltransferase (LCAT)** deficiency leads to an accumulation of **unesterified cholesterol** in plasma and tissues. - This typically presents with **corneal opacities**, **hemolytic anemia**, and proteinuria, rather than predominantly tendon xanthomas and isolated severe LDL elevation.

Question 8: A patient presents with xanthomas on the Achilles tendon. What is the most likely diagnosis?

A. Familial hypercholesterolemia (Correct Answer)
B. Tangier's disease
C. Familial hyperchylomicronemia
D. Familial dysbetalipoproteinemia
E. Familial combined hyperlipidemia

Explanation: ***Familial hypercholesterolemia*** - **Xanthomas** on the **Achilles tendon** are a classic clinical sign of familial hypercholesterolemia, along with significantly elevated **LDL-C levels**. - This condition is an **autosomal dominant** genetic disorder characterized by defects in the **LDL receptor** pathway, leading to impaired clearance of LDL from the blood. - **Tendon xanthomas** (especially Achilles and extensor tendons) are pathognomonic for this condition. *Tangier's disease* - Characterized by very low or absent **HDL-C (high-density lipoprotein cholesterol)** levels, leading to **cholesterol ester accumulation** in various tissues. - While it can cause lipid deposition, its hallmark is **enlarged, orange tonsils** and peripheral neuropathy, not typically Achilles tendon xanthomas. *Familial hyperchylomicronemia* - This disorder primarily involves elevated **chylomicrons** and **triglycerides**, presenting with **eruptive xanthomas** (small, red-yellow papules) but not typically tendon xanthomas. - It is often associated with **pancreatitis** and **lipemia retinalis**. *Familial dysbetalipoproteinemia* - Characterized by elevated levels of **cholesterol** and **triglycerides** due to accumulation of remnant lipoproteins (IDL). - While it can cause **xanthomas**, these are typically **palmar xanthomas** (xanthoma striata palmaris) and **tuberoeruptive xanthomas**, less commonly Achilles tendon xanthomas. *Familial combined hyperlipidemia* - Most common familial lipid disorder, characterized by elevated **LDL-C** and/or **triglycerides** with variable phenotype. - While it causes premature coronary artery disease, it typically does **not** cause tendon xanthomas, which distinguishes it from familial hypercholesterolemia. - Xanthomas, if present, are usually **xanthelasma** (around eyelids) rather than tendon xanthomas.

Question 9: A patient with high triglycerides (TG) esterified with long-chain fatty acids (LCFA) presents with fatigue, and a biopsy of the muscle shows fat vacuoles. What is the most likely diagnosis?

A. Carnitine deficiency (Correct Answer)
B. Fatty acid synthase defect
C. Lipoprotein lipase (LPL) defect
D. LDL defect
E. Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency

Explanation: ***Carnitine deficiency*** - **Carnitine** is essential for transporting **long-chain fatty acids (LCFAs)** into the mitochondria for beta-oxidation. - A deficiency leads to the accumulation of **LCFAs** as **triglycerides** in the cytoplasm, resulting in **fat vacuoles** in muscle and systemic fatigue due to impaired energy production. *Fatty acid synthase defect* - **Fatty acid synthase** is involved in the *de novo* synthesis of fatty acids, not their catabolism or transport. - A defect would impair fatty acid production, not lead to the accumulation of **triglycerides** from exogenous sources. *Lipoprotein lipase (LPL) defect* - **LPL** is crucial for cleaving **triglycerides** in circulating chylomicrons and VLDL, allowing fatty acids to be taken up by tissues. - A defect causes severe hypertriglyceridemia, but the primary issue in the muscle with fat vacuoles points towards a problem with intracellular fatty acid utilization rather than plasma triglyceride clearance. *LDL defect* - **LDL** is primarily responsible for transporting cholesterol to peripheral tissues. - Defects in **LDL** metabolism typically lead to hypercholesterolemia, not the accumulation of **triglycerides** or muscle fat vacuoles as described. *Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency* - **MCAD** deficiency is a fatty acid oxidation disorder affecting **medium-chain fatty acids** (C6-C12), not the **long-chain fatty acids** specifically mentioned in the stem. - It typically presents with hypoketotic hypoglycemia during fasting, often in infancy or childhood, rather than the characteristic muscle fat vacuole accumulation pattern seen with **carnitine deficiency**.

Question 10: A patient with tendon xanthomas, Increased LDL and cholesterol. What is the most probable diagnosis?

A. Type II Hyperlipoproteinemia (Correct Answer)
B. Type III Hyperlipoproteinemia
C. Abetalipoproteinemia
D. Type I Hyperlipoproteinemia
E. Type IV Hyperlipoproteinemia

Explanation: ***Type II Hyperlipoproteinemia*** - This type is characterized by significantly **elevated LDL and total cholesterol** due to a defect in LDL receptor function or APOB-100. - **Tendon xanthomas** are a classic physical finding in Type II hyperlipoproteinemia, specifically in familial hypercholesterolemia. *Type III Hyperlipoproteinemia* - This condition involves increased levels of **chylomicron remnants** and **VLDL remnants (IDL)**, leading to elevated cholesterol and triglycerides. - While xanthomas can occur (e.g., **palmar xanthomas**), tendon xanthomas are less typical, and the primary lipid abnormality isn't isolated LDL elevation. *Abetalipoproteinemia* - This is a rare autosomal recessive disorder resulting in the **absence of LDL, VLDL, and chylomicrons** in the blood. - Patients present with **fat malabsorption**, neurologic symptoms, and generally have very low or undetectable cholesterol and triglyceride levels, which is contrary to the clinical presentation. *Type I Hyperlipoproteinemia* - This disorder is characterized by a deficiency of **lipoprotein lipase (LPL)** or its cofactor, APO C-II, leading to extremely high levels of **chylomicrons** and **triglycerides**. - While eruptive xanthomas can be seen, **tendon xanthomas** are not a feature, and the primary abnormality is hypertriglyceridemia, not elevated LDL. *Type IV Hyperlipoproteinemia* - This condition is characterized by **elevated VLDL** and **triglycerides** with normal or slightly elevated LDL. - Xanthomas are generally not a feature, and the primary abnormality is hypertriglyceridemia rather than hypercholesterolemia with elevated LDL.

Practice by Chapter

Fatty acid oxidation (beta-oxidation)

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Fatty acid synthesis

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Ketone body metabolism

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Cholesterol synthesis and regulation

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Lipoprotein metabolism

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Phospholipid metabolism

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Eicosanoid synthesis and function

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Steroid hormone synthesis

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Adipose tissue metabolism

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Brown vs. white adipose tissue

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Disorders of lipid metabolism

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Integration with carbohydrate metabolism

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