Carbohydrate Metabolism Practice Questions

Q: Insulin-dependent glucose transport occurs in all of the following tissues except:

Liver. **Explanation:** The core concept behind this question is the tissue-specific distribution of **Glucose Transporters (GLUT)**. Glucose uptake into cells occurs via facilitated diffusion, mediated by different GLUT isoforms. **Why Liver is the correct answer:** The liver expresses **GLUT-2**, which is an **insulin-independent** transporter. GLUT-2 has a high $K_m$ (low affinity) for glucose, allowing the liver to sense and uptake glucose proportionally to blood glucose levels (e.g., after a meal) without requiring insulin signaling to move the transporter to the cell membrane. This ensures the liver can perform essential functions like gluconeogenesis or glycogenolysis even when insulin levels are low. **Why the other options are incorrect:** * **Skeletal Muscle & Heart (Options A & D):** These tissues primarily utilize **GLUT-4**, which is the only **insulin-dependent** glucose transporter. In the resting state, GLUT-4 is sequestered in intracellular vesicles. Insulin binding to its receptor triggers the translocation of these vesicles to the plasma membrane to allow glucose entry. * **Adipose Tissue (Option B):** Like muscle, adipocytes also rely on **GLUT-4** for glucose uptake to provide the glycerol backbone (glycerol-3-phosphate) required for triglyceride synthesis. **NEET-PG High-Yield Pearls:** * **GLUT-1:** Found in RBCs and the Blood-Brain Barrier (basal uptake). * **GLUT-2:** Found in Liver, Pancreatic beta cells, Kidney, and Small Intestine (Bidirectional). * **GLUT-3:** Found in Neurons (highest affinity/lowest $K_m$). * **GLUT-4:** Found in Skeletal muscle, Heart, and Adipose tissue (**Insulin-dependent**). * **GLUT-5:** Specifically for **Fructose** transport (found in small intestine and spermatozoa). * **SGLT-1/2:** Active transport (Sodium-dependent) found in the intestine and renal tubules.

Q: All of the following are considered Glycogen Storage Disorders EXCEPT?

Scheie's disease. **Explanation:** The correct answer is **Scheie’s disease** because it is a **Mucopolysaccharidosis (MPS Type I-S)**, not a Glycogen Storage Disorder (GSD). It is caused by a deficiency of the lysosomal enzyme **α-L-iduronidase**, leading to the accumulation of dermatan sulfate and heparan sulfate. Clinically, it is the mildest form of MPS I, characterized by corneal clouding, stiff joints, and a normal lifespan, unlike the more severe Hurler syndrome. **Analysis of Incorrect Options (GSDs):** * **Anderson’s disease (GSD Type IV):** Caused by a deficiency of the **Branching enzyme**. It leads to the accumulation of abnormal glycogen with long outer chains (amylopectin-like), resulting in early-onset liver cirrhosis and heart failure. * **Her’s disease (GSD Type VI):** Caused by a deficiency of **Liver Phosphorylase**. It typically presents with hepatomegaly and mild hypoglycemia, as the liver cannot break down glycogen into glucose-1-phosphate. * **Tarui’s disease (GSD Type VII):** Caused by a deficiency of **Muscle Phosphofructokinase (PFK)**. It presents similarly to McArdle’s disease (Type V) with exercise-induced muscle cramps and myoglobinuria, but may also feature hemolytic anemia. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for GSDs:** "**V**on Gierke, **P**ompe, **C**ori, **A**nderson, **M**cArdle, **H**ers, **T**arui" (**V**ery **P**oor **C**arbohydrate **A**dabolism **M**akes **H**umans **T**ired). * **Pompe Disease (Type II):** The only GSD that is also a **Lysosomal Storage Disorder** (Acid Maltase deficiency). * **Von Gierke (Type I):** Most common GSD; presents with severe hypoglycemia, lactic acidosis, and hyperuricemia.

Q: In the fasting state, which one of the following is utilized by the liver for gluconeogenesis?

Glycerol. **Explanation:** **1. Why Glycerol is Correct:** Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors. During fasting, adipose tissue undergoes lipolysis, breaking down triacylglycerols (TAGs) into free fatty acids and **glycerol**. While fatty acids cannot be converted to glucose, glycerol is transported to the liver. Here, it is phosphorylated by **glycerol kinase** to glycerol-3-phosphate and then oxidized to **dihydroxyacetone phosphate (DHAP)**, a direct intermediate of the gluconeogenic pathway. **2. Why the Other Options are Incorrect:** * **Even-chain fatty acids:** These are oxidized to Acetyl-CoA. In humans, there is no metabolic pathway to convert Acetyl-CoA into pyruvate or oxaloacetate (the Pyruvate Dehydrogenase reaction is irreversible). Thus, even-chain fatty acids cannot serve as a substrate for gluconeogenesis. * **Liver glycogen:** While liver glycogen is broken down to maintain blood glucose during fasting, this process is called **glycogenolysis**, not gluconeogenesis. Gluconeogenesis specifically refers to the de novo synthesis of glucose from non-carbohydrate sources. * **Ketone bodies:** These are alternative fuels produced from Acetyl-CoA during prolonged fasting, but they cannot be reversed back into glucose. **High-Yield Clinical Pearls for NEET-PG:** * **Key Substrates:** The major gluconeogenic precursors are **Lactate** (Cori Cycle), **Glucogenic Amino Acids** (mainly Alanine), and **Glycerol**. * **Odd-chain fatty acids:** Unlike even-chain, these *can* be gluconeogenic because their terminal metabolism yields **Propionyl-CoA**, which enters the TCA cycle as Succinyl-CoA. * **Enzyme Localization:** Glycerol kinase is primarily present in the **liver and kidneys**, which is why adipose tissue itself cannot reuse glycerol.

Q: What is the mechanism by which pyruvate is transported from the cytosol to the mitochondria?

Proton symporter. ### Explanation **Mechanism of Pyruvate Transport** Pyruvate is generated in the cytosol via glycolysis. However, its oxidative decarboxylation into Acetyl-CoA (by the Pyruvate Dehydrogenase Complex) occurs within the mitochondrial matrix. To cross the inner mitochondrial membrane (IMM), which is impermeable to polar molecules, pyruvate utilizes a specific transport protein called the **Mitochondrial Pyruvate Carrier (MPC)**. The transport is a **Proton (H⁺) Symporter** mechanism. Pyruvate is co-transported with a proton into the matrix, driven by the electrochemical gradient (proton motive force) generated by the electron transport chain. This ensures that pyruvate moves from the cytosol into the mitochondria effectively to fuel the TCA cycle. **Analysis of Incorrect Options:** * **A. Chloride antiporter:** Chloride antiporters (like the Band 3 protein/Bicarbonate-Chloride exchanger) are primarily involved in gas exchange in RBCs (Chloride shift), not organic acid transport in mitochondria. * **C. ATP-dependent antiporter:** While some mitochondrial transporters (like the Adenine Nucleotide Translocase) exchange molecules, pyruvate transport is driven by the proton gradient, not direct ATP hydrolysis. * **D. Facilitated uniporter:** Pyruvate does not move down its own concentration gradient alone; it requires the symport of a proton to overcome the electrochemical barrier of the IMM. **High-Yield Clinical Pearls for NEET-PG:** * **MPC Inhibition:** Thiazolidinediones (TZDs), used in Type 2 Diabetes, have been shown to modulate the Mitochondrial Pyruvate Carrier. * **Lactic Acidosis:** If pyruvate transport or the PDH complex is defective, pyruvate is converted to lactate in the cytosol, leading to metabolic acidosis. * **Outer vs. Inner Membrane:** Pyruvate crosses the *outer* mitochondrial membrane freely through voltage-dependent anion channels (VDACs/porins), but requires the *MPC symporter* for the *inner* membrane.

Q: A 14-year-old female on strenuous exercise presented with muscle pains and voiding red-colored urine. What is the most likely diagnosis?

Glycolytic pathway defect. ### Explanation The clinical presentation of **exercise-induced muscle pain** and **myoglobinuria** (red-colored urine) in a teenager points toward a metabolic myopathy. **1. Why Glycolytic Pathway Defect is Correct:** The most likely diagnosis is **McArdle Disease (GSD Type V)**, a defect in muscle glycogen phosphorylase, or a similar glycolytic defect (e.g., Tarui disease). In these conditions, the muscle cannot break down glycogen into glucose-6-phosphate to fuel **anaerobic glycolysis**. During **strenuous/intense exercise**, the body relies heavily on rapid ATP production via glycolysis. A block here leads to an energy crisis, muscle fiber breakdown (rhabdomyolysis), and subsequent myoglobinuria. A classic sign is the "second wind" phenomenon. **2. Why Other Options are Incorrect:** * **Carnitine Acyltransferase (CPT II) Deficiency:** While this also causes exercise-induced myoglobinuria, it typically occurs after **prolonged, low-intensity endurance exercise** or fasting, as it affects fatty acid oxidation rather than immediate glycolytic energy. * **Mitochondrial Deficiency Myopathy:** These usually present with progressive muscle weakness, exercise intolerance, and **lactic acidosis**, but myoglobinuria is less common. * **Hypokalemic Periodic Paralysis:** This presents as episodes of flaccid muscle paralysis, often triggered by high-carbohydrate meals or rest after exercise, but it does **not** cause myoglobinuria or muscle pain. **Clinical Pearls for NEET-PG:** * **Ischemic Forearm Exercise Test:** Failure of blood lactate to rise after exercise is diagnostic for McArdle disease. * **Myoglobinuria vs. Hematuria:** In myoglobinuria, the urine dipstick is positive for blood, but microscopy shows **no RBCs**. * **Key Enzyme:** Muscle Glycogen Phosphorylase (absent in McArdle).

Q: Which is the major site of expression of GLUT5?

Sperm. **Explanation:** The correct answer is **Sperm**. GLUT5 is a unique member of the glucose transporter family because it functions primarily as a **fructose transporter** rather than a glucose transporter. 1. **Why Sperm is correct:** Mature spermatozoa utilize fructose as their primary energy source for motility. Fructose is secreted in high concentrations by the seminal vesicles. GLUT5 is highly expressed on the plasma membrane of sperm cells to facilitate the uptake of this specific sugar. It is also found in high concentrations in the **jejunum (small intestine)**, where it mediates the absorption of dietary fructose. 2. **Why other options are incorrect:** * **Renal tubules:** While GLUT2 and GLUT1 are the primary transporters for glucose reabsorption in the kidneys, GLUT5 has minimal expression here. * **Brain:** The brain primarily relies on **GLUT1** (blood-brain barrier) and **GLUT3** (neurons) for a steady supply of glucose. * **Placenta:** The placenta predominantly expresses **GLUT1** to ensure the continuous transfer of glucose from maternal to fetal circulation. **High-Yield Clinical Pearls for NEET-PG:** * **GLUT1:** Found in RBCs, Blood-Brain Barrier, and Placenta (Basal glucose uptake). * **GLUT2:** Bidirectional transporter found in **Liver, Pancreas (B-cells), and Kidney**. It has a high $K_m$ (low affinity). * **GLUT3:** Highest affinity for glucose; found in **Neurons**. * **GLUT4:** The only **insulin-dependent** transporter; found in **Skeletal muscle and Adipose tissue**. * **GLUT5:** Specific for **Fructose**; located in the Small Intestine and Sperm.

Q: Severe thiamine deficiency is associated with which of the following?

Decreased RBC transketolase activity. **Explanation:** **1. Why Option A is Correct:** Thiamine (Vitamin B1) acts as a coenzyme in its active form, **Thiamine Pyrophosphate (TPP)**, for several key enzymes. One of these is **Transketolase**, a crucial enzyme in the Hexose Monophosphate (HMP) Shunt. Since transketolase is present in red blood cells (RBCs) and its activity is strictly dependent on TPP, measuring **RBC Transketolase activity** is the most reliable diagnostic biochemical marker for thiamine deficiency. In severe deficiency, this activity decreases significantly; if the activity increases by >25% upon adding TPP in vitro (TPP effect), it confirms the deficiency. **2. Why the Other Options are Incorrect:** * **Option B (Increased clotting time):** Clotting time is related to coagulation factors and Vitamin K deficiency, not Thiamine. * **Option C (Decreased RBC transaminase activity):** Transaminases (ALT/AST) require **Pyridoxal Phosphate (Vitamin B6)** as a cofactor, not Thiamine. * **Option D (Increased xanthic acid excretion):** Increased excretion of **Xanthurenic acid** in urine following a tryptophan load is a classic marker for **Vitamin B6 (Pyridoxine) deficiency**, as B6 is required for the kynurenine pathway. **3. High-Yield Clinical Pearls for NEET-PG:** * **Key TPP-dependent enzymes:** 1. Pyruvate Dehydrogenase (Link reaction) 2. alpha-Ketoglutarate Dehydrogenase (TCA cycle) 3. Branched-chain alpha-ketoacid dehydrogenase (Maple Syrup Urine Disease) 4. Transketolase (HMP Shunt) * **Clinical Presentation:** Deficiency leads to **Beriberi** (Dry: neurological; Wet: high-output heart failure) and **Wernicke-Korsakoff Syndrome** (triad of ataxia, ophthalmoplegia, and confusion), commonly seen in chronic alcoholics. * **Biochemical Tip:** Always measure transketolase activity *before* administering glucose to a suspected thiamine-deficient patient to avoid precipitating Wernicke’s encephalopathy.

Q: Which glycolytic enzyme(s) are inhibited by fluoride?

Enolase. **Explanation:** **Enolase** is the correct answer because it is specifically inhibited by fluoride ions. In the glycolytic pathway, Enolase catalyzes the dehydration of 2-phosphoglycerate to phosphoenolpyruvate (PEP). This enzyme requires magnesium ions ($Mg^{2+}$) as a cofactor. Fluoride binds with phosphate and magnesium to form a **magnesium-fluorophosphate complex**, which competitively displaces $Mg^{2+}$ from the enzyme’s active site, thereby halting glycolysis. **Analysis of Incorrect Options:** * **Hexokinase (A):** This is the first regulatory step of glycolysis (Glucose to Glucose-6-P). It is inhibited by its product, Glucose-6-phosphate, but not by fluoride. * **Aldolase (B):** This enzyme cleaves Fructose-1,6-bisphosphate into DHAP and Glyceraldehyde-3-phosphate. It is not a target for fluoride inhibition. * **Pyruvate Kinase (D):** This is the final irreversible step of glycolysis. It is regulated by allosteric effectors (like Fructose-1,6-bisphosphate) and covalent modification, but it is not inhibited by fluoride. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Blood Glucose Estimation:** In clinical practice, blood samples for glucose estimation are collected in **grey-topped vials** containing **Sodium Fluoride (NaF)**. * **Preservation:** NaF acts as a glycolytic inhibitor, preventing RBCs from consuming the glucose in the sample, ensuring an accurate measurement of the patient's blood sugar levels. * **Anticoagulant Pair:** NaF is usually paired with **Potassium Oxalate**, which acts as the anticoagulant by chelating calcium. * **Note:** Fluoride inhibition is reversible if more magnesium is added, but in laboratory settings, it effectively "fixes" the glucose concentration for up to 48–72 hours.

Q: The pyruvate dehydrogenase multienzyme complex is responsible for all the following reactions EXCEPT:

Production of a molecule of ATP. **Explanation:** The **Pyruvate Dehydrogenase (PDH) Complex** is a mitochondrial multienzyme system that serves as the bridge between glycolysis and the TCA cycle. It converts pyruvate into Acetyl-CoA through a process known as **oxidative decarboxylation**. **Why Option B is correct:** The PDH complex does **not** produce ATP directly. Instead, it reduces $NAD^+$ to **NADH**. This NADH later enters the Electron Transport Chain (ETC) to generate ATP via oxidative phosphorylation. Direct ATP (or GTP) production in the carbohydrate metabolic pathway occurs via substrate-level phosphorylation in glycolysis and the TCA cycle (Succinate thiokinase reaction), but not in the PDH reaction. **Analysis of Incorrect Options:** * **Option A:** PDH catalyzes the removal of a carboxyl group from pyruvate as $CO_2$. This is the "decarboxylation" step performed by the $E_1$ subunit (Pyruvate dehydrogenase). * **Option C:** The remaining two-carbon unit after decarboxylation is oxidized to form an **acetyl group** (hydroxyethyl group initially). * **Option D:** The acetyl group is eventually transferred to **Coenzyme A (CoA-SH)** to form Acetyl-CoA, which is the final product that enters the TCA cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Coenzymes (The "Tender Loving Care For Nancy" Mnemonic):** PDH requires five cofactors: **T**hiamine pyrophosphate ($B_1$), **L**ipoic acid, **C**oenzyme A ($B_5$), **F**AD ($B_2$), and **N**AD ($B_3$). * **Deficiency:** Thiamine deficiency (Beriberi/Wernicke-Korsakoff) leads to PDH failure, causing lactic acidosis and neurological symptoms. * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to the -SH groups of **Lipoic acid**, leading to similar clinical features as thiamine deficiency.

Question 1

Insulin-dependent glucose transport occurs in all of the following tissues except:

Accepted Answer

Liver

Answer

Heart

Answer

Adipose tissues

Answer

Skeletal muscles

Question 2

All of the following are considered Glycogen Storage Disorders EXCEPT?

Accepted Answer

Scheie's disease

Answer

Anderson's disease

Answer

Her's disease

Answer

Tarui's disease

Question 3

In the fasting state, which one of the following is utilized by the liver for gluconeogenesis?

Accepted Answer

Glycerol

Answer

Even-chain fatty acids

Answer

Liver glycogen

Answer

Ketone bodies

Question 4

What is the mechanism by which pyruvate is transported from the cytosol to the mitochondria?

Accepted Answer

Proton symporter

Answer

Chloride antiporter

Answer

ATP-dependent antiporter

Answer

Facilitated uniporter

Question 5

A 14-year-old female on strenuous exercise presented with muscle pains and voiding red-colored urine. What is the most likely diagnosis?

Accepted Answer

Glycolytic pathway defect

Answer

Carnitine acyltransferase deficiency

Answer

Mitochondrial deficiency myopathy

Answer

Hypokalemic periodic paralysis

Question 6

Which is the major site of expression of GLUT5?

Accepted Answer

Sperm

Answer

Renal tubules

Answer

Brain

Answer

Placenta

Question 7

Severe thiamine deficiency is associated with which of the following?

Accepted Answer

Decreased RBC transketolase activity

Answer

Increased clotting time

Answer

Decreased RBC transaminase activity

Answer

Increased xanthic acid excretion

Question 8

A 10-month-old male child presents with vomiting, lethargy, and severe jaundice, which began when weaning was initiated with fruit juice. Laboratory investigations reveal prolonged clotting time, hypoalbuminemia, and elevated bilirubin and transaminase levels. What is the diagnosis?

Accepted Answer

Hereditary fructose intolerance

Answer

Von Gierke's disease

Answer

Benign fructosuria

Answer

Lactose intolerance

Question 9

Which glycolytic enzyme(s) are inhibited by fluoride?

Accepted Answer

Enolase

Answer

Hexokinase

Answer

Aldolase

Answer

Pyruvate Kinase

Question 10

The pyruvate dehydrogenase multienzyme complex is responsible for all the following reactions EXCEPT:

Accepted Answer

Production of a molecule of ATP

Answer

A decarboxylation reaction

Answer

Formation of an acetyl group from pyruvate

Answer

Combination of the acetyl group with a cofactor

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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