Carbohydrate Metabolism Practice Questions

Q: Gluconeogenesis occurs in all except:

Palmitate. **Explanation:** The core concept tested here is the **irreversibility of the Pyruvate Dehydrogenase (PDH) complex**. Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors. To form glucose, a substrate must be capable of being converted into Pyruvate or an intermediate of the TCA cycle (like Oxaloacetate). **Why Palmitate is the correct answer:** Palmitate is a 16-carbon saturated fatty acid. Through beta-oxidation, even-chain fatty acids are broken down into **Acetyl-CoA**. In humans, Acetyl-CoA cannot be converted back into Pyruvate because the PDH reaction is irreversible. Furthermore, for every two carbons of Acetyl-CoA that enter the TCA cycle, two carbons are lost as $CO_2$. Therefore, there is no net synthesis of glucose from even-chain fatty acids like palmitate. **Why the other options are incorrect:** * **Glycerol:** Derived from triacylglycerol breakdown, it is phosphorylated to glycerol-3-phosphate and converted to **Dihydroxyacetone phosphate (DHAP)**, a direct intermediate of glycolysis/gluconeogenesis. * **Amino acids:** Glucogenic amino acids (e.g., Alanine) are deaminated to form Pyruvate or TCA cycle intermediates (alpha-ketoglutarate, succinyl-CoA), which can then enter the gluconeogenic pathway. * **Lactic acid:** Via the **Cori Cycle**, lactate produced by anaerobic glycolysis in muscles/RBCs is transported to the liver and converted back to Pyruvate by Lactate Dehydrogenase (LDH) to enter gluconeogenesis. **High-Yield Clinical Pearls for NEET-PG:** 1. **Odd-chain fatty acids** ARE gluconeogenic because their final breakdown product is **Propionyl-CoA**, which converts to Succinyl-CoA. 2. **Leucine and Lysine** are the only purely ketogenic amino acids (cannot form glucose). 3. The major site of gluconeogenesis is the **Liver**, followed by the **Kidney** (especially during prolonged starvation). 4. The rate-limiting enzyme of gluconeogenesis is **Fructose-1,6-bisphosphatase**.

Q: Cataract in diabetic patients is due to the accumulation of sorbitol in the lens. Which enzyme is responsible for this conversion?

NADPH+ dependent aldose reductase. ### Explanation **1. Why Option B is Correct:** The formation of cataracts in diabetic patients is explained by the **Polyol Pathway** (Sorbitol Pathway). In states of hyperglycemia, the enzyme **Aldose Reductase** reduces excess glucose into **Sorbitol** (a sugar alcohol or polyol). This reaction requires **NADPH** as a cofactor. Sorbitol is osmotically active; it draws water into the lens, leading to swelling, crystalline precipitation, and eventual opacity (cataract). While some tissues can convert sorbitol to fructose via sorbitol dehydrogenase, the lens has very low levels of this enzyme, leading to toxic sorbitol accumulation. **2. Why Other Options are Incorrect:** * **Option A (Hexokinase):** This enzyme phosphorylates glucose to Glucose-6-Phosphate in most tissues. It has a low Km (high affinity) for glucose and is involved in glycolysis, not the polyol pathway. * **Option C (Glucokinase):** This is the liver and pancreatic B-cell specific isoenzyme of hexokinase. It has a high Km and functions primarily when glucose levels are high to initiate glycogen synthesis or insulin release. * **Option D (Phosphofructokinase-1):** This is the rate-limiting enzyme of **Glycolysis**. It converts Fructose-6-Phosphate to Fructose-1,6-Bisphosphate and does not interact with free glucose or sorbitol. **3. High-Yield Clinical Pearls for NEET-PG:** * **Tissues involved:** "LUKE" – **L**ens, **U**rethra (Kidney), **K**idney, and **E**pinerium (Peripheral nerves) are susceptible to sorbitol damage because they lack **Sorbitol Dehydrogenase**. * **Cofactor Switch:** Aldose reductase uses **NADPH** (consuming it and causing oxidative stress), while Sorbitol Dehydrogenase uses **NAD+**. * **Galactosemia Connection:** Aldose reductase also converts Galactose to **Galactitol** (Dulcitol), which causes "Oil-drop cataracts" in infants with galactosemia.

Q: Diabetic cataract occurs due to the accumulation of which substance?

Sorbitol. **Explanation:** The development of diabetic cataracts is primarily attributed to the **Polyol Pathway** (Sorbitol pathway). In states of chronic hyperglycemia, the hexokinase enzyme becomes saturated, and excess glucose is shunted into the polyol pathway. 1. **Mechanism (Why Sorbitol is correct):** The enzyme **Aldose Reductase** reduces glucose into **Sorbitol** using NADPH as a cofactor. In tissues like the lens, retina, and peripheral nerves, the subsequent enzyme (Sorbitol Dehydrogenase) is either absent or has very low activity. Consequently, sorbitol accumulates intracellularly. Because sorbitol is a sugar alcohol and is **osmotically active**, it draws water into the lens fibers, leading to swelling, lens opacification, and eventually, a cataract. **Analysis of Incorrect Options:** * **A. Glucose:** While high glucose triggers the process, it is the metabolic byproduct (sorbitol) that causes the osmotic damage, not the glucose molecule itself. * **B. Fructose:** Sorbitol is normally converted to fructose by sorbitol dehydrogenase. Fructose does not accumulate significantly in the lens and is not the primary osmotic agent in this context. * **C. Galactitol:** This is the osmotic byproduct of **galactose** (via aldose reductase). It causes cataracts in patients with **Galactosemia**, not Diabetes Mellitus. **High-Yield NEET-PG Pearls:** * **"Sorbitol is Slow":** Tissues with low Sorbitol Dehydrogenase (Lens, Retina, Kidney, Schwann cells) are most prone to diabetic complications (**L**ens, **R**etina, **K**idney = **LRK**). * **Enzyme involved:** Aldose Reductase (requires NADPH). * **Cataract Type:** Diabetic cataracts are typically described as **"Snowflake cataracts."** * **Galactosemia Connection:** Galactitol accumulation causes cataracts much faster than sorbitol.

Q: Which enzyme is deficient in RBCs?

Pyruvate kinase. **Explanation:** **Pyruvate Kinase (PK)** is the correct answer because it is the most common enzyme deficiency in the glycolytic pathway involving Red Blood Cells (RBCs). Mature RBCs lack mitochondria and depend entirely on **anaerobic glycolysis** for ATP production. A deficiency in PK leads to decreased ATP, causing failure of the Na+/K+ ATPase pumps. This results in loss of intracellular potassium and water, leading to cell dehydration, "echinocyte" formation, and premature destruction in the spleen (**Hereditary Non-spherocytic Hemolytic Anemia**). **Analysis of Incorrect Options:** * **Hexokinase (A) & Phosphofructokinase (B):** While deficiencies in these enzymes can occur, they are extremely rare compared to Pyruvate Kinase deficiency. Both are essential "rate-limiting" steps of glycolysis present in RBCs. * **Glycerol Kinase (D):** This enzyme is **absent** in RBCs (and adipose tissue). It is primarily found in the liver and kidneys. Its absence in RBCs means they cannot utilize glycerol for energy; however, the question context refers to a clinically significant pathological deficiency in the existing glycolytic machinery. **Clinical Pearls for NEET-PG:** * **Inheritance:** PK deficiency is typically **Autosomal Recessive**. * **Biochemical marker:** It leads to an accumulation of **2,3-BPG**, which shifts the oxygen-dissociation curve to the **right**, helping in oxygen unloading to tissues (compensatory mechanism). * **Blood Smear:** Characterized by **Echinocytes** (Burr cells/spiculated cells). * **Second most common cause** of enzyme-deficient hemolytic anemia (after G6PD deficiency).

Q: Seliwanoff's test is positive in which of the following?

Fructose. **Explanation:** Seliwanoff’s test is a biochemical color reaction used to distinguish between **aldoses** (sugars with an aldehyde group) and **ketoses** (sugars with a ketone group). **Why Fructose is Correct:** Fructose is a **ketohexose**. The test involves heating the sugar with Seliwanoff’s reagent, which contains resorcinol and concentrated hydrochloric acid (HCl). The acid dehydrates ketoses much faster than aldoses to form **5-hydroxymethylfurfural**. This compound then reacts with resorcinol to produce a characteristic **cherry-red (fiery red) precipitate**. Because fructose is a ketose, it gives a rapid positive result. **Why Other Options are Incorrect:** * **Glucose, Galactose, and Mannose:** These are all **aldoses**. While aldoses can eventually react to produce a faint pink color if heated for a prolonged period (due to slow conversion to ketoses), they do not produce the rapid cherry-red color characteristic of a positive Seliwanoff’s test. **High-Yield NEET-PG Clinical Pearls:** * **Specific for Ketoses:** Seliwanoff’s test is the gold standard for identifying fructose and sucrose (since sucrose hydrolyzes into glucose and fructose). * **Bial’s Test:** Often confused with Seliwanoff’s, Bial’s test is used to detect **Pentoses** (like ribose), yielding a blue-green color. * **Clinical Correlation:** Fructose metabolism is clinically significant in **Hereditary Fructose Intolerance** (deficiency of Aldolase B) and **Essential Fructosuria** (deficiency of Fructokinase). * **Semen Analysis:** Seliwanoff’s test is used to detect fructose in semen; its absence indicates an obstruction or absence of the seminal vesicles.

Q: Which glycosaminoglycan is affected in Sanfilippo syndromes?

Heparan sulfate. ### Explanation **Sanfilippo Syndrome (Mucopolysaccharidosis Type III)** is a lysosomal storage disorder characterized by the body's inability to break down **Heparan sulfate**. This occurs due to a deficiency in one of four enzymes required for its degradation (Types A, B, C, and D). #### Why Heparan Sulfate is Correct: Heparan sulfate is a glycosaminoglycan (GAG) primarily found in the extracellular matrix and on cell surfaces, particularly in the CNS. In Sanfilippo syndrome, the accumulation of partially degraded heparan sulfate leads to severe progressive **neurodegeneration**, which is the clinical hallmark of this specific MPS type. #### Why Other Options are Incorrect: * **Dermatan sulfate:** This GAG accumulates in **Hurler (MPS I)** and **Hunter (MPS II)** syndromes. Its accumulation is typically associated with coarse facial features, hepatosplenomegaly, and valvular heart disease. * **Keratan sulfate:** This is the primary GAG affected in **Morquio syndrome (MPS IV)**. Clinical presentation involves severe skeletal dysplasia (dysostosis multiplex) without significant intellectual disability. * **Chondroitin sulfate:** While found in many tissues, it is specifically elevated alongside keratan sulfate in Morquio syndrome. It is not the primary diagnostic marker for Sanfilippo. #### NEET-PG High-Yield Pearls: * **Clinical Triad of Sanfilippo:** Severe developmental delay/regression, hyperactivity/behavioral issues, and relatively mild physical (somatic) features compared to Hurler syndrome. * **Enzyme Deficiency:** The most common and severe form is **Type A** (Heparan N-sulfatase deficiency). * **Inheritance:** All Mucopolysaccharidoses are **Autosomal Recessive**, *except* Hunter Syndrome (MPS II), which is **X-linked Recessive**. * **Corneal Clouding:** Notably **absent** in Sanfilippo and Hunter syndromes (a common exam differentiator).

Question 1

Gluconeogenesis occurs in all except:

Accepted Answer

Palmitate

Answer

Glycerol

Answer

Amino acid

Answer

Lactic acid

Question 2

Cataract in diabetic patients is due to the accumulation of sorbitol in the lens. Which enzyme is responsible for this conversion?

Accepted Answer

NADPH+ dependent aldose reductase

Answer

Hexokinase

Answer

Glucokinase

Answer

Phosphofructokinase

Question 3

A patient with type 1 diabetes self-injected insulin prior to their evening meal, but then was distracted and forgot to eat. A few hours later, the individual fainted, and after the paramedics arrived, they did a STAT blood glucose level and found it to be 45 mg/dL. The blood glucose level was so low because which one of the following tissues assimilated most of it under these conditions?

Accepted Answer

Adipose tissue

Answer

Brain

Answer

Liver

Answer

Red blood cells

Question 4

Which of the following statements regarding enzymes involved in liver dephosphorylation and phosphorylation is TRUE?

Accepted Answer

Phosphorylase is always activated by cAMP-dependent Protein Kinase.

Answer

The enzyme is always activated.

Answer

Catecholamines do not play a role in this process.

Answer

Phosphorylation of glycogen synthase increases its activity.

Question 5

Diabetic cataract occurs due to the accumulation of which substance?

Accepted Answer

Sorbitol

Answer

Glucose

Answer

Fructose

Answer

Galactitol

Question 6

Which enzyme is deficient in RBCs?

Accepted Answer

Pyruvate kinase

Answer

Hexokinase

Answer

Phosphofructokinase

Answer

Glycerol kinase

Question 7

Anaerobic glycolysis occurs in all of the following tissues except:

Accepted Answer

Brain

Answer

RBCs

Answer

Lens

Answer

Testis

Question 8

Seliwanoff's test is positive in which of the following?

Accepted Answer

Fructose

Answer

Glucose

Answer

Galactose

Answer

Mannose

Question 9

A 3-month-old baby refuses breast milk since the 2nd day of birth, accepts glucose-water, develops vomiting and severe jaundice by the 5th day. Benedict's test was positive for urine and blood glucose was low. What is the most likely cause due to deficiency of?

Accepted Answer

Galactose-1-phosphate uridyl transferase

Answer

Beta galactosidase

Answer

Glucose-6-phosphatase

Answer

Galactokinase

Question 10

Which glycosaminoglycan is affected in Sanfilippo syndromes?

Accepted Answer

Heparan sulfate

Answer

Dermatan sulfate

Answer

Keratan sulfate

Answer

Chondroitin sulfate

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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