Energy Production and Metabolism — MCQs

Energy Production and Metabolism Indian Medical PG Practice Questions and MCQs

Question 231: Which of the following is false about starvation ketoacidosis?

A. Smell of acetone in breath
B. Metabolic acidosis
C. Benedict's test +ve (Correct Answer)
D. Rothera's test +ve

Explanation: ***Benedict's test +ve (FALSE)*** - **Benedict's test** detects the presence of **reducing sugars** (glucose) in the urine. - In starvation ketoacidosis, there is **no significant glucose in the urine** because blood glucose levels are low to normal. - The body is in a state of **prolonged fasting** with depleted glycogen stores, utilizing **fats for energy** instead of carbohydrates. - Unlike diabetic ketoacidosis where glucosuria occurs due to hyperglycemia, starvation ketoacidosis typically presents with **hypoglycemia or normoglycemia**. - Therefore, Benedict's test would be **negative**, making this statement FALSE. *Smell of acetone in breath (TRUE)* - During starvation, the body breaks down fats into **ketone bodies** (beta-hydroxybutyrate, acetoacetate, and acetone). - **Acetone** is volatile and exhaled through the lungs, producing a characteristic **fruity or sweet smell** on the breath. - This is a classic clinical feature of ketoacidosis. *Metabolic acidosis (TRUE)* - The accumulation of **beta-hydroxybutyrate** and **acetoacetate** (both acidic ketone bodies) in the blood leads to decreased pH. - This results in **high anion gap metabolic acidosis** as the excess acids consume the body's **bicarbonate buffer system**. - The anion gap increases due to unmeasured anions (ketone bodies). *Rothera's test +ve (TRUE)* - **Rothera's test** specifically detects **ketone bodies**, particularly **acetoacetate**, in urine. - In starvation ketoacidosis, there is significant production and excretion of ketone bodies. - This causes a **positive Rothera's test**, confirming ketonuria.

Question 232: Which of the following deficiencies is most directly associated with the development of Wernicke's encephalopathy?

A. Thiamine deficiency (Correct Answer)
B. Niacin deficiency
C. Cobalamin deficiency
D. Folate deficiency

Explanation: ***Thiamine deficiency*** - **Wernicke's encephalopathy** is a neurological emergency caused by a severe deficiency of **thiamine (vitamin B1)**. - Thiamine is crucial for **glucose metabolism** in the brain, and its deficiency leads to damage in specific brain regions, including the **mammillary bodies** and **thalamus**. *Niacin deficiency* - **Niacin (vitamin B3)** deficiency leads to **pellagra**, characterized by the "3 Ds": **dermatitis**, **diarrhea**, and **dementia**. - While it can cause neurological symptoms, they are distinct from the acute presentation of Wernicke's encephalopathy. *Cobalamin deficiency* - **Cobalamin (vitamin B12)** deficiency can cause **megaloblastic anemia** and neurological symptoms such as **peripheral neuropathy**, **ataxia**, and **cognitive impairment**. - However, it does not directly cause Wernicke's encephalopathy, which has a more acute and characteristic triad of symptoms. *Folate deficiency* - **Folate (vitamin B9)** deficiency primarily causes **megaloblastic anemia** and can be associated with **neural tube defects** in newborns. - While it can contribute to neurological issues, it is not the direct cause of Wernicke's encephalopathy.

Question 233: Mechanism of cyanide poisoning is by inhibiting:

A. Mitochondrial DNA synthesis
B. ATP production
C. Electron transport chain
D. Cytochrome oxidase (Correct Answer)

Explanation: ***Cytochrome oxidase*** - Cyanide poisoning works by **irreversibly binding** to the ferric ion (Fe3+) in **cytochrome c oxidase** (Complex IV) of the electron transport chain. - This binding prevents the enzyme from carrying electrons to oxygen, thereby **halting cellular respiration** and ATP production. *Mitochondrial DNA synthesis* - While mitochondria are affected, cyanide does not primarily disrupt **DNA synthesis** in these organelles. - Its main target is the process of energy generation, not genetic replication. *ATP production* - Although cyanide poisoning ultimately leads to a **cessation of ATP production**, this is the *consequence* of its action, not the primary mechanism. - The direct mechanism involves inhibiting a key enzyme in the electron transport chain. *Electron transport chain* - Cyanide does indeed inhibit the **electron transport chain**, but this option is too broad. - The most specific mechanism targets a particular complex within the chain, which is **cytochrome oxidase**.

Question 234: Patient with Type I diabetes mellitus, with complaints of polyuria. Which of the following will occur normally in his body?

A. Increased protein synthesis
B. Glycogenesis in muscle
C. Decreased cholesterol synthesis
D. Increased conversion of fatty acid to acetyl CoA (Correct Answer)

Explanation: ***Increased conversion of fatty acid to acetyl CoA*** - In response to **insulin deficiency** and **hyperglycemia** in Type 1 diabetes, the body shifts from carbohydrate to fat metabolism. - This leads to increased **lipolysis**, releasing fatty acids that are then converted to **acetyl CoA** in the liver for energy or ketone body production. *Incorrect: Increased protein synthesis* - **Insulin** is an **anabolic hormone** that promotes protein synthesis; its deficiency in Type 1 diabetes leads to decreased, not increased, protein synthesis. - Instead, there's often increased **protein catabolism** to provide substrates for gluconeogenesis. *Incorrect: Glycogenesis in muscle* - **Insulin** is required for the uptake of glucose into muscle cells and its subsequent conversion to **glycogen (glycogenesis)**. - In Type 1 diabetes, the lack of insulin significantly impairs muscle glucose uptake and glycogenesis. *Incorrect: Decreased cholesterol synthesis* - In uncontrolled Type 1 diabetes, there is actually **increased cholesterol synthesis**, not decreased. - The increased availability of **acetyl CoA** (from enhanced fatty acid oxidation) provides substrate for cholesterol synthesis via the **HMG-CoA reductase pathway**. - This contributes to the **dyslipidemia** commonly seen in poorly controlled diabetes, including elevated LDL cholesterol and total cholesterol levels.

Question 235: In the citric acid cycle, which enzyme directly produces GTP?

A. Citrate synthase
B. Aconitase
C. Succinate thiokinase (Correct Answer)
D. Isocitrate dehydrogenase

Explanation: ***Succinate thiokinase*** - This enzyme (also known as **succinyl-CoA synthetase**) catalyzes the reversible conversion of **succinyl-CoA to succinate**, coupled with the phosphorylation of GDP to **GTP**. - This is an example of **substrate-level phosphorylation**, directly generating a high-energy phosphate compound. *Citrate synthase* - This enzyme catalyzes the **first committed step** of the citric acid cycle: the condensation of **acetyl-CoA and oxaloacetate** to form citrate. - It does not produce GTP; rather, it uses **acetyl-CoA and water** in its reaction. *Aconitase* - Aconitase is responsible for the **isomerization of citrate to isocitrate** via an intermediate, cis-aconitate. - This reaction involves the **removal and re-addition of water** and does not generate GTP. *Isocitrate dehydrogenase* - This enzyme catalyzes the **first oxidative decarboxylation step** in the citric acid cycle, converting **isocitrate to $\alpha$-ketoglutarate**. - It produces **NADH and CO2**, but not GTP.

Question 236: Which enzyme in the citric acid cycle is directly inhibited by high levels of ATP?

A. Aconitase
B. Malate dehydrogenase
C. Isocitrate dehydrogenase (Correct Answer)
D. Succinate dehydrogenase

Explanation: ***Isocitrate dehydrogenase*** - **Isocitrate dehydrogenase** is a key regulatory enzyme in the **citric acid cycle** that is allosterically inhibited by high levels of **ATP** and **NADH**. - This inhibition signals abundant cellular energy, slowing down the cycle to prevent overproduction of ATP when energy demands are low. *Aconitase* - **Aconitase** catalyzes the **reversible isomerization of citrate to isocitrate**; it is not directly regulated by ATP levels. - Its activity is sensitive to **iron-sulfur cluster** integrity and can be inhibited by fluoroacetate derivatives. *Malate dehydrogenase* - **Malate dehydrogenase** catalyzes the **reversible oxidation of malate to oxaloacetate**, generating NADH. - Its activity is primarily regulated by the **NADH/NAD+ ratio**, not directly by ATP levels. *Succinate dehydrogenase* - **Succinate dehydrogenase** (Complex II of the electron transport chain) oxidizes **succinate to fumarate** and is part of both the citric acid cycle and oxidative phosphorylation. - It is primarily inhibited by **oxaloacetate** and **malonate**, and is not a major regulatory point for ATP feedback inhibition in the citric acid cycle.

Question 237: Which of the following is NOT true regarding the role of NAD+?

A. Acts as an electron carrier
B. Functions as an antioxidant (Correct Answer)
C. Participates in glycolysis
D. Involved in TCA cycle

Explanation: ***Functions as an antioxidant*** - **NAD+** primarily functions as an **electron carrier** in redox reactions, not as an antioxidant that directly neutralizes reactive oxygen species. - While it plays a role in maintaining cellular redox balance, its direct function is not scavenging free radicals like **glutathione** or **vitamins C and E**. *Acts as an electron carrier* - **NAD+** is a crucial coenzyme that accepts electrons and protons during metabolic reactions, converting into **NADH**. - **NADH** then donates these electrons to the **electron transport chain** to generate **ATP**. *Participates in glycolysis* - In glycolysis, **NAD+** is reduced to **NADH** during the oxidation of **glyceraldehyde-3-phosphate** to **1,3-bisphosphoglycerate**. - This step is vital for producing **ATP** and regenerating **NAD+** for continued glycolytic flux. *Involved in TCA cycle* - **NAD+** is reduced to **NADH** at several steps in the **TCA cycle**, including the conversion of **isocitrate to α-ketoglutarate**, **α-ketoglutarate to succinyl CoA**, and **malate to oxaloacetate**. - These **NADH** molecules are then funneled into the **electron transport chain** for oxidative phosphorylation.

Question 238: What are the products of the isocitrate to α-ketoglutarate conversion in the TCA cycle?

A. GTP, CO2
B. NADPH, H2O
C. FADH2, ATP
D. NADH, CO2 (Correct Answer)

Explanation: ***NADH, CO2*** - The conversion of **isocitrate to α-ketoglutarate** is an oxidative decarboxylation step catalyzed by **isocitrate dehydrogenase**. - This reaction produces **NADH** (from NAD+) and **carbon dioxide (CO2)**, as a carbon atom is lost. *GTP, CO2* - **GTP** is produced during the conversion of **succinyl-CoA to succinate** in a substrate-level phosphorylation step, not during the isocitrate to α-ketoglutarate conversion. - While CO2 is produced in the latter, GTP is not. *NADPH, H2O* - **NADPH** is primarily generated in the **pentose phosphate pathway** and is used for reductive biosynthesis, not directly produced in the TCA cycle. - **H2O** is consumed or produced in other steps of the TCA cycle but not as a direct product of this specific reaction. *FADH2, ATP* - **FADH2** is produced during the conversion of **succinate to fumarate** by succinate dehydrogenase. - **ATP** (or GTP which can be converted to ATP) is produced in the succinyl-CoA to succinate step, not at the isocitrate dehydrogenase step.

Question 239: Congenital lactic acidosis is due to the defect of:

A. Transaldolase
B. Pyruvate dehydrogenase (Correct Answer)
C. Alpha-ketoglutarate dehydrogenase
D. Branched chain alpha-ketoacid dehydrogenase

Explanation: ***Pyruvate dehydrogenase*** - A defect in **pyruvate dehydrogenase (PDH)** is the most common cause of **congenital lactic acidosis** - PDH is a crucial enzyme complex that converts **pyruvate to acetyl-CoA**, linking glycolysis to the citric acid cycle - When PDH is deficient, **pyruvate accumulates** and is shunted to **lactate** via lactate dehydrogenase, causing persistent elevation of blood lactate levels - Clinical features include **neurological dysfunction, developmental delay, and metabolic acidosis** from birth or early infancy *Transaldolase* - **Transaldolase** is an enzyme in the **pentose phosphate pathway** - Its deficiency primarily affects **NADPH production and ribose-5-phosphate synthesis**, not lactate metabolism - Transaldolase deficiency causes hepatosplenomegaly and liver dysfunction, but is **not a direct cause of congenital lactic acidosis** *Alpha-ketoglutarate dehydrogenase* - **Alpha-ketoglutarate dehydrogenase** is part of the **citric acid cycle (TCA cycle)** - Its deficiency would impair energy production and lead to accumulation of **alpha-ketoglutarate**, not lactate - Defects cause **neurological dysfunction** but do not primarily present with **lactic acidosis** *Branched chain alpha-ketoacid dehydrogenase* - **Branched chain alpha-ketoacid dehydrogenase (BCKDH)** metabolizes **branched-chain amino acids** (leucine, isoleucine, valine) - Deficiency causes **maple syrup urine disease (MSUD)**, characterized by accumulation of **branched-chain keto acids** and their corresponding amino acids - Presents with characteristic maple syrup odor in urine, neurological symptoms, but **not lactic acidosis**

Question 240: A patient with chronic granulomatous disease is most likely to have a defect in which enzyme?

A. Superoxide dismutase
B. Catalase
C. Myeloperoxidase
D. NADPH oxidase (Correct Answer)

Explanation: ***NADPH oxidase*** - **Chronic granulomatous disease (CGD)** is characterized by a defect in **NADPH oxidase**, an enzyme critical for the formation of **superoxide radicals**. - Without a functional **NADPH oxidase**, phagocytes cannot mount a **respiratory burst** to kill certain bacteria and fungi, leading to recurrent infections and granuloma formation. *Superoxide dismutase* - This enzyme converts **superoxide** into **hydrogen peroxide** and oxygen, an essential step in detoxifying reactive oxygen species. - A defect here would lead to an accumulation of superoxide, but is not the primary cause of the susceptibility to specific infections seen in CGD. *Catalase* - **Catalase** breaks down **hydrogen peroxide** into water and oxygen, protecting cells from oxidative damage. - While important for reducing oxidative stress, its deficiency is not responsible for the impaired microbial killing in CGD, rather, it's involved in the *breakdown* of products generated by NADPH oxidase. *Myeloperoxidase* - **Myeloperoxidase (MPO)** combines **hydrogen peroxide** with chloride ions to produce **hypochlorous acid (bleach)**, a potent microbicidal agent. - Although crucial for killing, MPO can only function if NADPH oxidase first produces sufficient hydrogen peroxide; thus, its deficiency presents differently than CGD.

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