Isoenzymes and Clinical Significance — MCQs
Which isoenzyme of LDH is seen in the heart?
Which of the following reagents would be most useful in determining the N-terminal amino acid of a polypeptide?
Which isoform of LDH is raised in Anemia ?
What are isoenzymes?
A patient came to the hospital with severe abdominal pain, and lipase levels were elevated. On imaging, a stone is found in the common bile duct (CBD). Which enzyme is most likely elevated in this condition?
In which of the following conditions is serum alkaline phosphatase typically not elevated?
An 8 year old boy complains of increasing muscle weakness. On examination, his calves are bulky and show muscle tightening. His serum creatine kinase levels are increasing with age. Which of the following is the most likely diagnosis?
Creatine kinase is elevated in MI after
Enzyme that can be traced in semen sample of 8-10 weeks is:
Which of the following biochemical markers is NOT elevated in a child presenting with jaundice, icterus, pruritus, and clay-colored stools?
Isoenzymes and Clinical Significance Indian Medical PG Practice Questions and MCQs
Question 1: Which isoenzyme of LDH is seen in the heart?
- A. LDH 1 (Correct Answer)
- B. LDH 2
- C. LDH 3
- D. LDH 4
Explanation: ***LDH 1*** - **LDH 1** is the predominant isoenzyme found in the **heart muscle**, particularly in the ventricles. Its elevation is a key indicator of cardiac tissue damage. - It is also found in **red blood cells** and the **kidneys**. *LDH 2* - **LDH 2** is also present in the **heart**, but it is typically the second most abundant isoenzyme after LDH 1 in cardiac tissue. - It is predominantly found in the **reticuloendothelial system**, including macrophages and lymphoid tissues. *LDH 3* - **LDH 3** is primarily associated with **pulmonary tissue**, the **spleen**, and **lymph nodes**. - Its elevation often suggests conditions affecting these organs, such as pulmonary embolism or pneumonia. *LDH 4* - **LDH 4** is found in the **kidney**, **placenta**, and **pancreas**, with some presence in skeletal muscle. - **LDH 5** (not LDH 4) is the primary isoenzyme in the **liver** and **skeletal muscle**.
Question 2: Which of the following reagents would be most useful in determining the N-terminal amino acid of a polypeptide?
- A. Trypsin
- B. Carboxypeptidase
- C. Phenylisothiocyanate (Correct Answer)
- D. Cyanogen bromide
Explanation: ***Phenylisothiocyanate*** - **Phenylisothiocyanate** (PITC), also known as Edman's reagent, is used in the **Edman degradation** method to identify the N-terminal amino acid. - It sequentially cleaves the **N-terminal amino acid** without hydrolyzing the rest of the peptide chain, allowing for identification by chromatography. *Trypsin* - **Trypsin** is a protease that cleaves peptide bonds at the carboxyl side of **lysine** and **arginine** residues. - It is used for peptide fragmentation, not for determining the N-terminal amino acid. *Carboxypeptidase* - **Carboxypeptidases** are exopeptidases that cleave amino acids from the **C-terminal end** of a polypeptide chain. - They are used to identify the C-terminal amino acid, not the N-terminal. *Cyanogen bromide* - **Cyanogen bromide (CNBr)** is a chemical reagent that specifically cleaves peptide bonds on the C-terminal side of **methionine** residues. - It is used for specific peptide fragmentation and not for N-terminal sequencing.
Question 3: Which isoform of LDH is raised in Anemia ?
- A. LDH 5
- B. LDH 4
- C. LDH 1
- D. LDH 2 (Correct Answer)
Explanation: ***Correct: LDH 2*** - **LDH 2 (H3M1)** is highly abundant in **red blood cells (RBCs)**, second only to LDH 1. - In **hemolytic anemia**, there is increased destruction of red blood cells, leading to the release of intracellular LDH isoforms into the bloodstream. - Both **LDH 1 and LDH 2 are significantly elevated** in hemolytic anemia, as RBCs contain predominantly these two isoforms. - The elevation of LDH 2 is particularly notable and diagnostically useful in anemia, especially hemolytic conditions. *Incorrect: LDH 1* - **LDH 1 (H4)** is the most abundant isoform in **heart muscle** and is also present in **red blood cells**. - While LDH 1 is indeed elevated in hemolytic anemia, this option is not the best answer in this context. - LDH 1 elevation is classically associated with **myocardial infarction**. *Incorrect: LDH 4* - **LDH 4 (HM3)** is present in various tissues, including **liver, skeletal muscle, and kidneys**, but in lower concentrations. - Not the primary isoform associated with anemia. *Incorrect: LDH 5* - **LDH 5 (M4)** is predominantly found in the **liver and skeletal muscles**. - Elevation of LDH 5 typically indicates **liver damage, muscle injury, or malignancies**, not primarily anemia.
Question 4: What are isoenzymes?
- A. Physically same forms of different enzymes
- B. Forms of same enzyme that catalyze different reactions
- C. Forms of different enzyme that catalyze same reactions
- D. Physically distinct forms of the same enzyme (Correct Answer)
Explanation: ***Physically distinct forms of the same enzyme*** - Isoenzymes are **multiple forms of an enzyme** that catalyze the **same reaction** but differ in their **physical or biochemical properties**, such as electrophoretic mobility, optimal pH, or kinetic parameters. - These differences usually arise from **genetic variations** (different genes encoding isoforms) or **post-translational modifications** (e.g., phosphorylation, glycosylation). *Physically same forms of different enzymes* - This statement is incorrect as isoenzymes are forms of the **same enzyme**, not different enzymes. - While different enzymes can catalyze similar reactions in certain pathways, they are not referred to as isoenzymes if they are structurally identical. *Forms of same enzyme that catalyze different reactions* - This describes enzymes with **broad substrate specificity** or those that act on different substrates but are not necessarily isoenzymes. - Isoenzymes specifically catalyze the **same chemical reaction**, but they may do so with different efficiencies or under different regulatory controls. *Forms of different enzyme that catalyze same reactions* - This describes a scenario where different enzymes might exhibit **catalytic promiscuity** or broad specificity, but not isoenzymes. - Isoenzymes are always derived from the **same parent enzyme** and catalyze the identical reaction.
Question 5: A patient came to the hospital with severe abdominal pain, and lipase levels were elevated. On imaging, a stone is found in the common bile duct (CBD). Which enzyme is most likely elevated in this condition?
- A. ALT
- B. GGT
- C. LDH
- D. AST
- E. ALP (Correct Answer)
Explanation: ***ALP (Alkaline Phosphatase)*** - **ALP** is the **most characteristic enzyme elevation** in **biliary obstruction** from a CBD stone. - ALP is found in high concentrations in the **bile duct epithelium** and hepatocytes adjacent to bile ducts, and rises dramatically with **cholestasis** and **obstructive jaundice**. - In CBD stone obstruction, ALP typically rises **3-10 times normal**, making it the hallmark biochemical marker of this condition. - While lipase is elevated due to associated pancreatitis, **ALP elevation specifically indicates the biliary obstruction**. *GGT (Gamma-Glutamyl Transferase)* - **GGT** is also elevated in **cholestasis** and **bile duct obstruction**. - GGT often rises in parallel with ALP and helps confirm the hepatobiliary origin of ALP elevation (vs. bone source). - However, **ALP is more specific** and typically shows greater magnitude of elevation in acute CBD obstruction, making it the **most likely** elevated enzyme in this clinical context. *ALT (Alanine Aminotransferase)* - **ALT** may be **mildly to moderately elevated** if there is secondary hepatocellular injury from biliary obstruction. - However, ALT primarily indicates **hepatocyte damage** rather than cholestasis, and its elevation is typically **less pronounced** than ALP in obstructive biliary disease. - The pattern in CBD obstruction is **cholestatic** (high ALP) rather than **hepatocellular** (high ALT). *AST (Aspartate Aminotransferase)* - **AST** can be elevated in various conditions including liver, heart, and muscle damage. - Like ALT, it may show mild elevation in biliary obstruction but is **not the primary marker**. - AST is less specific than ALP for diagnosing CBD stone obstruction. *LDH (Lactate Dehydrogenase)* - **LDH** is a **non-specific marker** of tissue damage found in multiple organs. - While it may be elevated, it provides little diagnostic value when specific markers like **ALP and lipase** are available. - LDH does not help differentiate biliary obstruction from other causes of abdominal pain.
Question 6: In which of the following conditions is serum alkaline phosphatase typically not elevated?
- A. Primary biliary cirrhosis
- B. Hyperparathyroidism
- C. Hepatitis
- D. Multiple myeloma (Correct Answer)
Explanation: In which of the following conditions is serum alkaline phosphatase typically not elevated? ***Hyperparathyroidism*** - In hyperparathyroidism, serum alkaline phosphatase levels can be normal or only mildly elevated, depending on bone metabolism changes. - This condition primarily elevates **serum calcium** and affects the bones rather than significantly affecting alkaline phosphatase levels [1]. *Primary biliary cirrhosis* - This condition is associated with **cholestasis**, leading to significantly elevated alkaline phosphatase levels due to liver involvement [1]. - Typically, patients also have other markers such as **AMA (anti-mitochondrial antibodies)** indicating liver-specific pathology. *Hepatitis* - Hepatitis can cause a rise in serum alkaline phosphatase levels, especially if there is associated cholestasis or liver injury. - It often accompanies elevated transaminases, indicating liver inflammation. *Multiple myeloma* - Multiple myeloma may show elevated alkaline phosphatase levels due to bone lesions and increased osteoclastic activity [2]. - The condition is primarily characterized by **monoclonal protein** detection and associated with various bone complications. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Liver And Biliary System Disease, pp. 392-393.
Question 7: An 8 year old boy complains of increasing muscle weakness. On examination, his calves are bulky and show muscle tightening. His serum creatine kinase levels are increasing with age. Which of the following is the most likely diagnosis?
- A. Congenital myopathy
- B. Dystrophin deficiency (Correct Answer)
- C. Myelin deficiency
- D. Hereditary sensorimotor neuropathy
- E. Becker muscular dystrophy
Explanation: ***Dystrophin deficiency*** - The presentation of an 8-year-old boy with **increasing muscle weakness**, **bulky calves** (pseudohypertrophy), and **elevated creatine kinase** is highly characteristic of Duchenne muscular dystrophy, which is caused by a dystrophin deficiency. - The muscle tightening and progressive increase in **creatine kinase** over time further support this diagnosis, as muscle breakdown leads to enzyme release. - Duchenne typically presents between **3-5 years** with progressive weakness, wheelchair dependence by early teens, and very high CK levels (10-100x normal). *Becker muscular dystrophy* - While also caused by **dystrophin deficiency** (partial rather than complete), Becker muscular dystrophy has a **later onset** (usually age 10-15 years) and **milder, slower progression**. - The age of 8 years with significant symptoms and the rapid progression described are more consistent with **Duchenne** than Becker. *Congenital myopathy* - While hereditary muscle disorders, congenital myopathies typically present at birth or in early infancy with **generalized hypotonia** and **muscle weakness**, often static or slowly progressive. - They usually do not exhibit the prominent calf pseudohypertrophy seen in Duchenne muscular dystrophy. *Myelin deficiency* - Myelin deficiency disorders primarily affect the **nervous system**, leading to issues with nerve signal transmission rather than direct muscle weakness and pseudohypertrophy. - Symptoms would typically include **neurological deficits** such as sensory loss, ataxia, or spasticity. *Hereditary sensorimotor neuropathy* - These neuropathies (e.g., Charcot-Marie-Tooth disease) involve both **sensory and motor nerves**, leading to muscle weakness, atrophy, and sensory loss, often in the distal limbs. - They do not typically cause **calf pseudohypertrophy** or progressively high creatine kinase levels as seen in primary muscle diseases.
Question 8: Creatine kinase is elevated in MI after
- A. 4-8 hours
- B. >24 hours
- C. 12-24 hours
- D. 2-4 hours (Correct Answer)
Explanation: ***2-4 hours*** - **Creatine kinase (CK)** levels typically begin to rise within **2-4 hours** after the onset of myocardial infarction. - This early elevation makes CK an effective, though non-specific, marker for **acute MI** in the initial stages [1]. *4-8 hours* - While CK levels may continue to rise during this period, the initial measurable elevation usually occurs earlier, within **2-4 hours**. - A significant elevation at 4-8 hours would indicate that the myocardial event occurred at least several hours prior. *12-24 hours* - Creatine kinase levels typically peak much earlier, between **12-24 hours**, rather than just beginning to elevate at this time. - By this time, other more specific markers like **troponins** would also be significantly elevated and are often preferred for diagnosis [1], [2]. *>24 hours* - Beyond 24 hours, CK levels usually start to decline, making it less useful for the initial detection of an acute MI that began many hours earlier. - For events occurring over 24 hours ago, a positive CK would indicate that the event had happened, but it's not the first time it would be elevated.
Question 9: Enzyme that can be traced in semen sample of 8-10 weeks is:
- A. CPK enzyme
- B. LDH
- C. ALP test
- D. Acid phosphatase test (Correct Answer)
Explanation: ***Acid phosphatase test*** - The **acid phosphatase (AP) test** is a crucial forensic test for identifying seminal fluid, even in aged or degraded samples. - While detectable for months, it remains a reliable indicator in semen samples for at least **8-10 weeks** due to its relative stability. *CPK enzyme* - **Creatine phosphokinase (CPK)** is primarily associated with muscle and brain tissue damage, not a specific marker for semen. - It is not routinely traced in semen samples for forensic analysis due to its low specificity. *LDH* - **Lactate dehydrogenase (LDH)** is an enzyme found in various tissues throughout the body, reflecting general cellular damage or metabolism. - It lacks the specificity to be a reliable forensic marker for the presence of semen. *ALP test* - **Alkaline phosphatase (ALP)** is commonly used in clinical settings to assess liver and bone health. - It is not a principal enzyme marker used for the forensic identification of seminal fluid due to its widespread distribution in the body.
Question 10: Which of the following biochemical markers is NOT elevated in a child presenting with jaundice, icterus, pruritus, and clay-colored stools?
- A. Gamma glutamyl transpeptidase
- B. Alkaline phosphatase
- C. 5' nucleotidase
- D. Glutamate dehydrogenase (Correct Answer)
Explanation: ***Glutamate dehydrogenase*** - **Glutamate dehydrogenase (GLDH)** is an enzyme primarily found in the mitochondria of hepatocytes and is a marker of **hepatocellular necrosis**. - In a presentation dominated by **cholestasis** (jaundice, pruritus, clay-colored stools), GLDH is typically not elevated; rather, enzymes indicative of bile duct obstruction would be. *Alkaline phosphatase* - **Alkaline phosphatase (ALP)** is an enzyme found in the bile duct epithelium and is significantly elevated in **cholestatic conditions**, such as bile duct obstruction. - Its presence in high levels is a strong indicator of an issue with bile flow. *Gamma glutamyl transpeptidase* - **Gamma-glutamyl transpeptidase (GGT)** is a microsomal enzyme found in liver cells and bile duct epithelium, with very high sensitivity for **cholestatic liver disease**. - Its elevation along with ALP helps confirm a cholestatic pattern of liver injury. *5' nucleotidase* - **5' nucleotidase (5'-NT)** is an enzyme found in the cell membranes of hepatocytes and bile duct cells. - It is considered a more **specific marker for cholestasis** than ALP, as it is not elevated in bone diseases or pregnancy.
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