Carbohydrate Metabolism Practice Questions

Q: A 30-year-old patient presents with intractable vomiting and inability to eat or drink for the past 3 days. Their blood glucose level remains normal. Which of the following is most important for the maintenance of blood glucose in this patient?

Liver. **Explanation:** The maintenance of blood glucose during fasting or starvation is primarily the responsibility of the **Liver**. In this patient, who has been unable to eat for 3 days, the body has transitioned from the post-absorptive state to the early starvation phase. **Why the Liver is Correct:** The liver is the central organ for glucose homeostasis. It maintains blood glucose through two key pathways: 1. **Glycogenolysis:** The breakdown of stored glycogen (exhausted within 12–24 hours of fasting). 2. **Gluconeogenesis:** The synthesis of glucose from non-carbohydrate precursors (lactate, glycerol, and glucogenic amino acids). By day 3 of starvation, gluconeogenesis in the liver is the **primary source** of blood glucose to support glucose-dependent tissues like the brain and RBCs. The liver uniquely possesses the enzyme **Glucose-6-Phosphatase**, allowing it to release free glucose into the bloodstream. **Why Other Options are Incorrect:** * **Heart:** The heart is a consumer of energy, not a producer. It primarily utilizes fatty acids and ketone bodies during starvation to spare glucose for the brain. * **Skeletal Muscle:** While muscle stores significant glycogen, it lacks **Glucose-6-Phosphatase**. Therefore, muscle glycogen can only be used locally for energy and cannot be released as glucose into the blood. * **Lysosome:** These are organelles involved in cellular degradation (autophagy) and do not play a direct role in systemic blood glucose regulation. **NEET-PG High-Yield Pearls:** * **Key Enzyme:** Glucose-6-Phosphatase is the "marker enzyme" for gluconeogenesis, found in the liver and kidney cortex, but absent in muscle. * **Timeline:** Liver glycogen is depleted in ~18 hours; thereafter, gluconeogenesis becomes the sole source of blood glucose. * **Kidney Role:** In prolonged starvation (>5–6 days), the kidney cortex also contributes significantly (up to 40%) to gluconeogenesis.

Q: Glucuronic acid and Iduronic acid are:

Epimers. ### Explanation **Correct Answer: D. Epimers** **Underlying Concept:** Epimers are stereoisomers that differ in configuration around only **one** specific carbon atom (other than the anomeric carbon). Glucuronic acid and Iduronic acid are both uronic acids derived from glucose. They are **C-5 epimers**. * In **D-Glucuronic acid**, the carboxyl group (-COOH) at C-5 is positioned above the ring (in the D-configuration). * In **L-Iduronic acid**, the configuration at C-5 is inverted. **Why other options are incorrect:** * **Anomers (A):** These differ only at the hemiacetal/hemiketal carbon (C-1 for glucose). Glucuronic and Iduronic acids differ at C-5. * **Enantiomers (B):** These are non-superimposable mirror images where every chiral center is inverted (e.g., D-Glucose and L-Glucose). These two acids are not mirror images. * **Functional Isomers (C):** These have the same molecular formula but different functional groups (e.g., Glucose and Fructose). Both molecules here are uronic acids. **Clinical Pearls for NEET-PG:** 1. **GAG Composition:** Iduronic acid is a major component of Glycosaminoglycans (GAGs) like **Heparin** and **Dermatan sulfate**. 2. **Epimerization:** In the body, D-glucuronic acid residues in heparin chains are converted to L-iduronic acid by the enzyme **uronyl C-5 epimerase**. 3. **Other High-Yield Epimers:** * **Glucose and Galactose:** C-4 epimers. * **Glucose and Mannose:** C-2 epimers. * **Ribulose and Xylulose:** C-3 epimers. 4. **Glucuronic Acid Function:** It is essential for the **conjugation of bilirubin** and the detoxification of xenobiotics in the liver.

Q: McArdle's disease is due to deficiency of which enzyme?

Myophosphorylase. **Explanation:** McArdle’s disease, also known as **GSD Type V**, is an autosomal recessive disorder characterized by the deficiency of **Myophosphorylase** (muscle glycogen phosphorylase). This enzyme is responsible for the rate-limiting step of glycogenolysis in skeletal muscle, breaking down glycogen into glucose-1-phosphate. Without it, muscles cannot mobilize glucose during strenuous exercise, leading to exercise intolerance, muscle cramps, and myoglobinuria. **Analysis of Options:** * **A. Myophosphorylase (Correct):** This is the muscle-specific isoform of glycogen phosphorylase. Its absence prevents the breakdown of muscle glycogen. * **B. Liver phosphorylase:** Deficiency of this enzyme leads to **Hers disease (GSD Type VI)**, which primarily presents with hepatomegaly and mild hypoglycemia, rather than muscle symptoms. * **C. Glucose-6-phosphatase:** Deficiency leads to **Von Gierke’s disease (GSD Type I)**, the most common GSD, characterized by severe fasting hypoglycemia, lactic acidosis, and "doll-like" facies. * **D. Acid maltase (α-1,4-glucosidase):** Deficiency leads to **Pompe disease (GSD Type II)**, which affects the heart and muscles due to lysosomal glycogen accumulation. **High-Yield Clinical Pearls for NEET-PG:** 1. **Ischemic Forearm Exercise Test:** Patients with McArdle’s show a **failure of blood lactate to rise** after exercise (since glycogen cannot be converted to lactate), while ammonia levels rise significantly. 2. **Second Wind Phenomenon:** Patients often experience a sudden improvement in exercise tolerance after 10–15 minutes as the body switches to using free fatty acids and blood glucose for energy. 3. **Burgundy-colored urine:** Post-exercise myoglobinuria can lead to acute renal failure.

Q: Which of the following carbohydrates is not digested in humans?

Lactulose. **Explanation:** The correct answer is **Lactulose**. **1. Why Lactulose is the correct answer:** Lactulose is a synthetic disaccharide composed of **Galactose and Fructose**. Humans lack the specific intestinal enzyme (disaccharidase) required to hydrolyze the bond between these two sugars. Consequently, it remains undigested in the small intestine and passes into the colon. There, it is fermented by colonic bacteria into lactic acid and acetic acid, making it an effective osmotic laxative. **2. Why the other options are incorrect:** * **Maltose:** A disaccharide of Glucose + Glucose. It is digested by the enzyme **Maltase** found in the brush border of the small intestine. * **Sucrose:** Common table sugar (Glucose + Fructose). It is digested by the enzyme **Sucrase**. * **Lactose:** Milk sugar (Galactose + Glucose). It is digested by the enzyme **Lactase**. A deficiency in this enzyme leads to clinical lactose intolerance. **3. High-Yield Clinical Pearls for NEET-PG:** * **Hepatic Encephalopathy:** Lactulose is the drug of choice. It acidifies the gut lumen ($NH_3 \rightarrow NH_4^+$), trapping ammonia as non-absorbable ammonium ions (Ion Trapping), thereby reducing blood ammonia levels. * **Non-digestible Polysaccharide:** While Lactulose is a non-digestible *disaccharide*, **Cellulose** is the most common non-digestible *polysaccharide* in humans due to the absence of cellulase (which breaks $\beta$-1,4 glycosidic bonds). * **Diagnostic Use:** Lactulose is used in "Hydrogen Breath Tests" to diagnose Small Intestinal Bacterial Overgrowth (SIBO).

Q: A 6-month-old infant presents with bilateral cataracts. What metabolic disorder should be suspected if a urine test is performed?

Galactosemia. **Explanation:** **Galactosemia** is the most common metabolic cause of bilateral congenital cataracts. The underlying mechanism involves the deficiency of enzymes (most commonly **Galactose-1-phosphate uridyltransferase**), leading to the accumulation of galactose in the blood. In the lens, the enzyme **aldose reductase** converts excess galactose into **dulcitol (galactitol)**. Dulcitol is osmotically active; it draws water into the lens fibers, causing swelling, denaturation of proteins, and the characteristic **"Oil Droplet" cataract**. Early diagnosis via urine testing for reducing sugars (non-glucose) is vital, as the cataract may be reversible in early stages with a lactose-free diet. **Analysis of Incorrect Options:** * **Homocystinuria:** Characterized by **ectopia lentis** (downward and inward subluxation) rather than cataracts. It is caused by a deficiency of cystathionine beta-synthase. * **Hyperlysinemia:** A rare metabolic disorder associated with ectopia lentis and muscular hypotonia, but not typically presenting with neonatal cataracts. * **Sulfite Oxidase Deficiency:** A rare disorder presenting with seizures, developmental delay, and **ectopia lentis** due to disruption of sulfur-containing amino acid metabolism. **NEET-PG High-Yield Pearls:** * **Oil Droplet Cataract:** Pathognomonic for Galactosemia. * **Sunflower Cataract:** Seen in Wilson’s Disease (Copper deposition). * **Snowflake Cataract:** Seen in Diabetes Mellitus. * **Christmas Tree Cataract:** Seen in Myotonic Dystrophy. * **Zonular (Lamellar) Cataract:** The most common type of congenital cataract; often associated with Vitamin D deficiency or maternal infections.

Q: Cloudy cornea is a feature of which of the following diseases?

All of the above. **Explanation:** The question pertains to **Mucopolysaccharidoses (MPS)**, a group of lysosomal storage disorders characterized by the deficiency of enzymes required to break down glycosaminoglycans (GAGs). The accumulation of these GAGs in various tissues leads to multisystemic manifestations [1]. **1. Why "All of the above" is correct:** Cloudy cornea (corneal clouding) occurs due to the progressive deposition of dermatan sulfate and keratan sulfate within the corneal stroma. * **Hurler Syndrome (MPS IH):** The prototype of MPS, characterized by severe corneal clouding due to alpha-L-iduronidase deficiency [1]. * **Morquio Syndrome (MPS IV):** Unique for its severe skeletal dysplasia, it also presents with fine, diffuse corneal opacities. * **Maroteaux-Lamy Syndrome (MPS VI):** Presents with physical features similar to Hurler but with normal intellect; corneal clouding is a prominent clinical feature. **2. Analysis of Options:** While all three listed conditions (MPS I, IV, and VI) feature corneal clouding, it is crucial to differentiate them from **Hunter Syndrome (MPS II)**. Hunter syndrome is X-linked recessive and is classically distinguished by the **absence of corneal clouding**. **3. High-Yield NEET-PG Pearls:** * **The "Clear Cornea" Rule:** In the context of MPS, if you see "Clear Cornea," think **Hunter Syndrome** (MPS II) or **Sanfilippo Syndrome** (MPS III). * **Enzyme Deficiencies:** * Hurler: Alpha-L-iduronidase [1]. * Hunter: Iduronate sulfatase (X-linked). * Morquio: Galactose-6-sulfatase. * **Clinical Triad for MPS:** Coarse facial features (gargoylism), hepatosplenomegaly, and skeletal deformities (dysostosis multiplex) [1]. * **Urine Test:** Screening is done via the **Toluidine Blue Spot Test**, which detects elevated urinary GAGs. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 163-164.

Q: Glucagon will decrease which of the following?

Glycogenesis.. **Explanation:** Glucagon is a catabolic hormone secreted by the alpha cells of the pancreas in response to low blood glucose levels (hypoglycemia). Its primary goal is to increase blood glucose to maintain homeostasis. **Why Glycogenesis is the Correct Answer:** Glucagon acts via the **cAMP-Protein Kinase A (PKA) pathway**. PKA phosphorylates **Glycogen Synthase**, the rate-limiting enzyme of glycogenesis, converting it into its inactive form (*Glycogen Synthase b*). By inhibiting the synthesis of glycogen, glucagon ensures that glucose is available for release into the bloodstream rather than being stored. **Analysis of Incorrect Options:** * **A. Glycogenolysis:** Glucagon **increases** glycogenolysis. It activates *Glycogen Phosphorylase* via phosphorylation, leading to the rapid breakdown of liver glycogen into glucose. * **B. Gluconeogenesis:** Glucagon **increases** gluconeogenesis. It induces key enzymes like *PEPCK* and *Fructose-1,6-bisphosphatase* while inhibiting *Phosphofructokinase-1 (PFK-1)* via a decrease in Fructose-2,6-bisphosphate levels. * **D. Blood Glucose:** The ultimate physiological effect of glucagon is to **increase** blood glucose levels. **NEET-PG High-Yield Pearls:** * **Target Organ:** Glucagon acts primarily on the **liver**. Unlike epinephrine, glucagon has **no effect** on muscle glycogen because muscle cells lack glucagon receptors. * **Key Enzyme Regulation:** Glucagon promotes **phosphorylation** of enzymes. In carbohydrate metabolism, phosphorylation generally **activates catabolic** enzymes (e.g., Phosphorylase) and **inactivates anabolic** enzymes (e.g., Glycogen Synthase). * **I/G Ratio:** The Insulin/Glucagon ratio dictates the metabolic state; a low ratio (high glucagon) favors mobilization of fuels.

Q: The pathway shown in the figure is seen in which of the following organ(s)?

All of the above. ***All of the above*** - The **HMP shunt (Pentose Phosphate Pathway)** occurs in liver, adipose tissue, and adrenal cortex as all require high **NADPH** for anabolic reactions. - **G6PD** is the key rate-limiting enzyme that generates **NADPH** essential for **fatty acid synthesis** and **steroid synthesis**. *Liver* - The liver has high **HMP shunt activity** to produce **NADPH** for **fatty acid synthesis** and **cholesterol biosynthesis**. - However, this pathway is not exclusive to liver and occurs in other tissues requiring **NADPH**. *Adipose tissue* - Adipose tissue utilizes the **HMP shunt** to generate **NADPH** for **de novo fatty acid synthesis**. - But this pathway is also present in other organs with similar **anabolic requirements**. *Adrenal cortex* - The adrenal cortex uses **HMP shunt-derived NADPH** for **steroid hormone synthesis** (cortisol, aldosterone). - This pathway is not limited to adrenal cortex and functions in other **NADPH-requiring tissues** like **RBCs**, **mammary gland**, and **testes**.

Question 1

A 30-year-old patient presents with intractable vomiting and inability to eat or drink for the past 3 days. Their blood glucose level remains normal. Which of the following is most important for the maintenance of blood glucose in this patient?

Accepted Answer

Liver

Answer

Heart

Answer

Skeletal muscle

Answer

Lysosome

Question 2

Glucuronic acid and Iduronic acid are:

Accepted Answer

Epimers

Answer

Anomers

Answer

Enantiomers

Answer

Functional isomers

Question 3

McArdle's disease is due to deficiency of which enzyme?

Accepted Answer

Myophosphorylase

Answer

Liver phosphorylase

Answer

Glucose-6-phosphatase

Answer

Acid maltase

Question 4

A female infant appeared normal at birth but developed signs of liver disease and muscular weakness at 3 months. She had periods of hypoglycemia, particularly on awakening. Examination revealed hepatomegaly. Laboratory analyses following fasting revealed ketoacidosis, blood pH 7.25, and elevations in both alanine transaminase (ALT) and aspartate transaminase (AST). Administration of glucagon following a carbohydrate meal elicited a normal rise in blood glucose, but glucose levels did not rise when glucagon was administered following an overnight fast. Liver biopsy revealed an increase in the glycogen content. To prevent the frequent episodes of hypoglycemia, which of the following dietary supplements would be most appropriate for this patient?

Accepted Answer

Uncooked cornstarch

Answer

Casein (milk protein)

Answer

Fructose

Answer

Lactose

Question 5

Which of the following carbohydrates is not digested in humans?

Accepted Answer

Lactulose

Answer

Maltose

Answer

Sucrose

Answer

Lactose

Question 6

A 6-month-old infant presents with bilateral cataracts. What metabolic disorder should be suspected if a urine test is performed?

Accepted Answer

Galactosemia

Answer

Homocystinuria

Answer

Hyperlysinemia

Answer

Sulfite oxidase deficiency

Question 7

A 7-month-old infant, recently started on top feeds, presented with diarrhea, vomiting, nausea, abdominal pain, and distension, leading to poor feeding and poor weight gain. On examination, hepatosplenomegaly was observed. Lab findings include decreased blood sugar, increased serum bilirubin, and increased uric acid. A reducing substance was found in the urine during the episode of hypoglycemia. Which of the following statements is FALSE about this condition?

Accepted Answer

The enzyme involved catalyzes the hydrolysis of fructose-1,6-bisphosphate into triose phosphate and glyceraldehyde phosphate.

Answer

The enzyme involved catalyzes the conversion of fructose to fructose-1-phosphate.

Answer

The enzyme involved catalyzes the conversion of galactose-1-phosphate to glucose-1-phosphate.

Answer

The enzyme involved catalyzes the conversion of galactose to galactose-1-phosphate.

Question 8

Cloudy cornea is a feature of which of the following diseases?

Accepted Answer

All of the above

Answer

Hurler's disease

Answer

Morquio's disease

Answer

Maroteaux-Lamy disease

Question 9

Glucagon will decrease which of the following?

Accepted Answer

Glycogenesis.

Answer

Glycogenolysis.

Answer

Gluconeogenesis.

Answer

Blood glucose.

Question 10

The pathway shown in the figure is seen in which of the following organ(s)?

Accepted Answer

All of the above

Answer

Liver

Answer

Adipose tissue

Answer

Adrenal cortex

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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