Biochemistry
10 questionsWhich of the following tests is most commonly used to detect glucose in urine?
Fructose intolerance is due to deficiency of which enzyme?
Which transporter is responsible for the transport of glucose in the pancreas?
Inhibition of glycolysis by increased supply of O2 is called ?
Which of the following is not a phospholipid ?
Ketone bodies are not used by?
Ketone body formation without glycosuria is seen in ?
What is essential for the transfer of fatty acid across the mitochondrial membrane?
Krabbe's disease is due to deficiency of ?
Which of the following lipoproteins is most strongly associated with an increased risk of cardiovascular diseases and is commonly referred to as "bad cholesterol"?
NEET-PG 2013 - Biochemistry NEET-PG Practice Questions and MCQs
Question 321: Which of the following tests is most commonly used to detect glucose in urine?
- A. a) Benedicts test
- B. c) Glucose-oxidase test (Correct Answer)
- C. b) Fehling solution
- D. d) None of the above
Explanation: ***Glucose-oxidase test*** - The **glucose-oxidase test** is a highly specific and sensitive enzymatic test used to detect **glucose** in urine. - It uses the enzyme glucose oxidase which specifically catalyzes the oxidation of glucose to gluconic acid and hydrogen peroxide, which then produces a color change. - This is the **most commonly used method** in modern clinical practice for detecting glucosuria due to its **high specificity for glucose** and ease of use (dipstick method). - It is the preferred test for **monitoring diabetes** and screening for hyperglycemia. *Benedict's test* - **Benedict's test** is a general chemical test for **all reducing sugars** (glucose, fructose, galactose, lactose, maltose), not specifically glucose. - It works by reducing copper sulfate (Cu²⁺) to copper oxide (Cu⁺) in an alkaline solution, forming a colored precipitate (green, yellow, orange, or brick-red depending on sugar concentration). - While it can detect glucose, it **lacks specificity** and can give false positives with other reducing substances (vitamin C, certain drugs), making it less suitable for routine clinical testing. *Fehling's solution* - **Fehling's solution** is also a general chemical test for **reducing sugars** based on copper reduction, similar to Benedict's test. - It consists of two solutions mixed before use and detects various reducing sugars, not just glucose. - It is **not commonly used in clinical urine analysis** due to lack of specificity and the need for heating and mixing two solutions, making it impractical compared to the simple glucose-oxidase dipstick. *None of the above* - This option is incorrect because the **glucose-oxidase test** is indeed the most commonly used test for detecting glucose in urine in modern clinical practice.
Question 322: Fructose intolerance is due to deficiency of which enzyme?
- A. Aldolase B (Correct Answer)
- B. Aldolase A
- C. Fructokinase
- D. Triokinase
Explanation: ***Aldolase B*** - **Hereditary fructose intolerance** is a genetic disorder caused by a deficiency in the enzyme **aldolase B**. - This deficiency leads to an accumulation of **fructose-1-phosphate** in the liver, kidneys, and small intestine, causing **hypoglycemia**, **vomiting**, and **liver damage** upon exposure to fructose. *Fructokinase* - A deficiency in **fructokinase** causes **essential fructosuria**, a benign metabolic disorder. - This condition is asymptomatic because **fructose** simply accumulates in the blood and urine without causing significant clinical problems. *Triokinase* - **Triokinase**, also known as **glycerol kinase**, is involved in glycerol metabolism, converting glycerol to **glycerol-3-phosphate**. - Its deficiency is not directly linked to fructose intolerance and typically presents with **hyperglycerolemia**. *Aldolase A* - **Aldolase A** is one of the three aldolase isoenzymes (A, B, and C) and is primarily involved in **glycolysis**, specifically in the breakdown of **fructose-1,6-bisphosphate**. - A deficiency in aldolase A can lead to **hemolytic anemia** and **myopathy**, not directly fructose intolerance.
Question 323: Which transporter is responsible for the transport of glucose in the pancreas?
- A. GLUT 1
- B. GLUT 2 (Correct Answer)
- C. GLUT 3
- D. GLUT 4
Explanation: ***GLUT 2*** - **GLUT2** is a **low-affinity, high-capacity** glucose transporter primarily found in the **pancreatic beta cells**, liver, small intestine, and kidneys. - In pancreatic beta cells, GLUT2 allows rapid entry of glucose for metabolism, leading to **insulin secretion** in response to elevated blood glucose levels. *GLUT 1* - **GLUT1** is a **ubiquitous glucose transporter** found in most tissues, including red blood cells and the blood-brain barrier. - It has a high affinity for glucose, ensuring **basal glucose uptake** even at low concentrations. *GLUT 3* - **GLUT3** is a **high-affinity glucose transporter** concentrated in **neurons** and the brain. - Its efficient glucose uptake is critical for the constant and high energy demands of the central nervous system. *GLUT 4* - **GLUT4** is an **insulin-dependent glucose transporter** primarily found in **adipose tissue** and **striated muscle (skeletal and cardiac muscle)**. - Insulin stimulates the translocation of GLUT4 to the cell membrane, facilitating glucose uptake from the blood after a meal.
Question 324: Inhibition of glycolysis by increased supply of O2 is called ?
- A. Pasteur effect (Correct Answer)
- B. Crabtree phenomenon
- C. Lewis phenomenon
- D. None of the options
Explanation: ***Pasteur effect*** - The **Pasteur effect** describes the phenomenon where the rate of **glycolysis** is inhibited when **oxygen** is available (aerobic conditions). - This inhibition occurs because **oxidative phosphorylation** is more efficient at generating ATP, leading to reduced reliance on glycolysis for energy production. *Crabtree phenomenon* - The **Crabtree phenomenon** is the opposite of the Pasteur effect, where high concentrations of **glucose** inhibit oxygen consumption in the presence of oxygen. - This is primarily observed in some **cancer cells** and yeast, leading to increased glycolysis even under aerobic conditions. *Lewis phenomenon* - The **Lewis phenomenon** (also known as the hunting reaction) refers to the cyclical vasodilation and constriction of peripheral blood vessels in response to **cold exposure**. - It's a physiological response to protect tissues from **frostbite** and is not related to glycolysis or oxygen supply. *None of the options* - This option is incorrect as the phenomenon described, inhibition of glycolysis by increased O2, is a well-established biochemical process known as the **Pasteur effect**.
Question 325: Which of the following is not a phospholipid ?
- A. Lecithin
- B. Plasmalogen
- C. Cardiolipin
- D. Ganglioside (Correct Answer)
Explanation: ***Ganglioside*** - Gangliosides are a type of **glycosphingolipid** because their structure includes a ceramide (a sphingoid base linked to a fatty acid) and a carbohydrate portion with one or more **sialic acid** residues, but no phosphate group. - They are primarily found in **nerve cell membranes** and are crucial for cell-cell recognition and signaling, differentiating them from phospholipids which contain a phosphate group. *Lecithin* - Lecithin, specifically **phosphatidylcholine**, is a common phospholipid characterized by a **phosphate group** and a **choline head group** attached to a diacylglycerol backbone. - It plays vital roles in cell membrane structure and function and is an important emulsifier. *Plasmalogen* - Plasmalogens are a class of phospholipids characterized by a **vinyl ether linkage** at the *sn*-1 position of the glycerol backbone, instead of the typical ester linkage found in other phospholipids. - They retain the defining **phosphate group** that classifies them as phospholipids. *Cardiolipin* - Cardiolipin is a unique phospholipid composed of **two phosphatidic acid moieties** connected by a glycerol molecule, resulting in four fatty acid chains and two phosphate groups. - It is predominantly found in the **inner mitochondrial membrane**, essential for mitochondrial function.
Question 326: Ketone bodies are not used by?
- A. Brain
- B. Muscle
- C. RBC (Correct Answer)
- D. Renal cortex
Explanation: ***RBC*** - Red blood cells **lack mitochondria**, which are essential organelles for the **oxidation of ketone bodies** (acetoacetate and β-hydroxybutyrate) for energy production. - Their primary energy source is **anaerobic glycolysis** of glucose. *Muscle* - **Skeletal and cardiac muscles** readily utilize **ketone bodies** as an alternative fuel source, especially during prolonged fasting or starvation. - This helps to conserve glucose for other tissues, particularly the brain. *Brain* - The brain can adapt to use **ketone bodies** for energy when glucose supply is limited, such as during prolonged fasting or in cases of uncontrolled diabetes. - This process is crucial for brain function when glucose levels are low. *Renal cortex* - The **renal cortex** is capable of utilizing **ketone bodies** for energy, particularly during starvation. - The kidney is also involved in the **synthesis of glucose** (gluconeogenesis) and the excretion of ketone bodies.
Question 327: Ketone body formation without glycosuria is seen in ?
- A. Diabetes mellitus
- B. Diabetes insipidus
- C. Starvation (Correct Answer)
- D. Obesity
Explanation: ***Starvation*** - During **starvation**, the body depletes its **glycogen stores** and begins to break down **fat for energy**. This process leads to the production of **ketone bodies** (acetoacetate, beta-hydroxybutyrate, and acetone) as an alternative fuel source for the brain and other tissues. - Since there is no underlying problem with **insulin production** or action, blood glucose levels are typically low or normal, and therefore, **glycosuria** (glucose in the urine) is absent. *Diabetes mellitus* - In **uncontrolled diabetes mellitus**, especially Type 1, the body cannot effectively use **glucose** due to lack of insulin, leading to high blood glucose levels (**hyperglycemia**) and subsequently **glycosuria**. - The body then compensates by breaking down **fats**, leading to the formation of **ketone bodies** (**diabetic ketoacidosis**), which results in both **ketonuria** and **glycosuria**. *Diabetes insipidus* - **Diabetes insipidus** is a condition characterized by the inability to conserve water due to insufficient **antidiuretic hormone (ADH)** production or action, leading to excessive urination and thirst. - It does not involve abnormalities in **glucose metabolism** or **ketone body production** and therefore does not typically present with ketonuria or glycosuria. *Obesity* - While **obesity** can lead to **insulin resistance** and is a risk factor for Type 2 Diabetes, it does not directly cause **ketone body formation** in the absence of metabolic derangements such as those seen in uncontrolled diabetes or prolonged starvation. - In most cases of obesity without diabetes, **glucose metabolism** is still adequate enough to prevent significant reliance on **fat breakdown** for energy, meaning there is usually no ketonuria or glycosuria.
Question 328: What is essential for the transfer of fatty acid across the mitochondrial membrane?
- A. Creatinine
- B. Carnitine (Correct Answer)
- C. Biotin
- D. Creatine
Explanation: ***Carnitine*** - **Carnitine** is crucial for transporting **long-chain fatty acids** into the mitochondrial matrix for **beta-oxidation**. - It forms **acylcarnitine** by esterifying with fatty acids, allowing passage through the inner mitochondrial membrane via the **carnitine-acylcarnitine translocase**. *Creatinine* - **Creatinine** is a waste product formed from the breakdown of **creatine phosphate** in muscles and is excreted by the kidneys. - It serves as a marker for **kidney function** and has no role in fatty acid transport. *Biotin* - **Biotin** is a vitamin cofactor essential for **carboxylase enzymes**, including acetyl-CoA carboxylase in **fatty acid synthesis**. - While involved in lipid metabolism, it plays no role in the transport of fatty acids across mitochondrial membranes. *Creatine* - **Creatine** is a nitrogenous organic acid that helps supply energy to cells, primarily muscle, by facilitating the regeneration of **ATP**. - It plays no direct role in the facilitated transport of fatty acids across the mitochondrial membrane.
Question 329: Krabbe's disease is due to deficiency of ?
- A. Sphingomyelinase
- B. Beta galactocerebrosidase (Correct Answer)
- C. Hexosaminidase
- D. Arylsulfatase
Explanation: ***Beta galactocerebrosidase*** - Krabbe's disease is specifically caused by a deficiency of **beta-galactocerebrosidase**, leading to the accumulation of toxic substances in the brain [1]. - This disease predominantly affects the **myelin sheath**, resulting in severe neurological deterioration [1]. *Arylsulfatase* - Deficiency of **arylsulfatase** is associated with **metachromatic leukodystrophy**, not Krabbe's disease. - Symptoms and pathology differ significantly, primarily affecting **sulfatides** rather than galactocerebrosides. *Sphingomyelinase* - A deficiency of **sphingomyelinase** is linked to **Niemann-Pick disease**, characterized by splenomegaly and liver involvement. - This condition does not involve the same neurological deterioration seen in Krabbe's disease. *Hexosaminidase* - Hexosaminidase deficiency is associated with **Tay-Sachs disease**, primarily affecting the **GM2 gangliosides** [2]. - This results in different clinical manifestations, such as **cherry-red spots** and progressive neurodegeneration, rather than the symptoms of Krabbe's disease [2]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Central Nervous System, pp. 1304-1305. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, p. 161.
Question 330: Which of the following lipoproteins is most strongly associated with an increased risk of cardiovascular diseases and is commonly referred to as "bad cholesterol"?
- A. VLDL
- B. Chylomicron
- C. Lp (a)
- D. LDL (Correct Answer)
Explanation: ***LDL*** - **Low-density lipoprotein (LDL)** is commonly referred to as "bad" cholesterol because elevated levels are the **primary driver** of atherosclerotic plaque buildup in arterial walls. - LDL particles transport cholesterol from the liver to peripheral tissues; when present in excess, they infiltrate the arterial intima and undergo oxidative modification, triggering inflammatory responses that lead to atherosclerosis. - **Clinical significance:** LDL cholesterol is the primary target of lipid-lowering therapy in cardiovascular disease prevention. *VLDL* - **Very low-density lipoprotein (VLDL)** primarily transports endogenously synthesized **triglycerides** from the liver to peripheral tissues. - While elevated VLDL levels do contribute to cardiovascular risk (particularly through conversion to small, dense LDL particles), it is not the primary lipoprotein targeted in cardiovascular risk assessment. *Chylomicron* - **Chylomicrons** transport **dietary lipids** (triglycerides and cholesterol) from the intestines to tissues after meals. - They are rapidly cleared from circulation (half-life of 5-10 minutes) and are typically not present during fasting, making their contribution to chronic atherosclerotic plaque formation minimal. *Lp(a)* - **Lipoprotein(a) [Lp(a)]** is structurally similar to LDL but contains an additional apolipoprotein(a) molecule, which has homology to plasminogen and may interfere with fibrinolysis. - While Lp(a) is an independent cardiovascular risk factor, it is less commonly measured in routine clinical practice, and **LDL remains the cornerstone lipoprotein** for cardiovascular risk stratification and management.