Biochemical Techniques — MCQs

Biochemical Techniques Indian Medical PG Practice Questions and MCQs

Question 141: The total body radical burden can be estimated by which of the following methods?

A. FOX Assay
B. Estimation of Dialdehydes
C. Measurement of Pentane and Methane in exhaled air
D. All of the above (Correct Answer)

Explanation: **Explanation:** The question focuses on the assessment of **Oxidative Stress**, which occurs when there is an imbalance between the production of reactive oxygen species (ROS) and antioxidant defenses. Since free radicals are highly unstable and short-lived, we estimate the "radical burden" by measuring stable end-products of **Lipid Peroxidation**. 1. **FOX Assay (Ferrous Oxidation-Xylenol Orange):** This is a sensitive biochemical method used to measure **hydroperoxides**. It relies on the principle that hydroperoxides oxidize ferrous (Fe²⁺) to ferric (Fe³⁺) ions, which then form a colored complex with xylenol orange. 2. **Estimation of Dialdehydes:** One of the most common markers of lipid peroxidation is **Malondialdehyde (MDA)**. It is frequently measured using the **TBA (Thiobarbituric Acid) test**. The presence of these dialdehydes directly correlates with the extent of radical-induced damage to polyunsaturated fatty acids (PUFAs). 3. **Measurement of Pentane and Ethane in Exhaled Air:** During the peroxidation of omega-3 and omega-6 fatty acids, volatile hydrocarbons like **ethane and pentane** are produced. These gases are excreted via the lungs, and their measurement in exhaled breath serves as a non-invasive "window" into systemic lipid peroxidation. *(Note: The option mentions Methane, but in the context of lipid peroxidation, Ethane/Pentane are the classic markers; however, in many standardized exams, this is grouped as volatile hydrocarbon analysis).* **Clinical Pearls for NEET-PG:** * **Gold Standard for Lipid Peroxidation:** Measurement of **Isoprostanes** (specifically F2-isoprostanes) in urine or plasma is currently considered the most reliable marker. * **MDA** is the most commonly used clinical marker despite being less specific than isoprostanes. * **Antioxidant Enzymes:** Remember that Superoxide Dismutase (SOD), Catalase, and Glutathione Peroxidase are the body's primary enzymatic defenses against radical burden.

Question 142: Molecular size of proteins is assessed by which of the following methods?

A. Sedimentation
B. Absorption mass spectroscopy (Correct Answer)
C. Lyophilization
D. Salting out

Explanation: ### Explanation **Correct Option: B. Absorption Mass Spectroscopy** Mass Spectrometry (MS) is the gold standard for determining the **molecular weight (size)** and structural composition of proteins. It works by ionizing chemical species and sorting the ions based on their **mass-to-charge (m/z) ratio**. Techniques like MALDI-TOF (Matrix-Assisted Laser Desorption/Ionization - Time of Flight) are specifically designed to measure the mass of large biomolecules like proteins with extreme precision. **Analysis of Incorrect Options:** * **A. Sedimentation:** While ultracentrifugation can provide an estimate of molecular weight via the Svedberg unit (sedimentation coefficient), it primarily measures the rate at which particles settle. It is more commonly used for separating organelles or determining the density of lipoproteins rather than precise molecular sizing of individual proteins. * **C. Lyophilization:** Also known as freeze-drying, this is a **preservation technique**. It removes water from a protein sample to increase its shelf life and stability; it is not an analytical method for size assessment. * **D. Salting Out:** This is a **purification technique** based on solubility. By adding high concentrations of salts (like ammonium sulfate), proteins precipitate out of solution. It does not provide information regarding the molecular size. **High-Yield Clinical Pearls for NEET-PG:** * **SDS-PAGE:** Another common method to estimate protein molecular weight based on electrophoretic mobility. * **Gel Filtration Chromatography:** Also known as Size-Exclusion Chromatography; it separates proteins based on size (larger proteins elute first). * **Proteomics:** Mass Spectrometry is the cornerstone of proteomics, used in clinical settings for identifying bacterial strains (MALDI-TOF) and detecting metabolic disorders in newborns (Tandem MS).

Question 143: Which process is used to separate fragments of DNA?

A. Gel electrophoresis (Correct Answer)
B. Paper chromatography
C. High-speed centrifugation
D. Thin-layer chromatography

Explanation: **Explanation:** **Gel Electrophoresis** is the gold-standard technique for separating DNA fragments based on their **size and charge**. DNA molecules are negatively charged (due to the phosphate backbone); when placed in an electric field, they migrate toward the positive electrode (anode). The gel matrix (usually agarose for DNA) acts as a molecular sieve: smaller fragments move faster and further through the pores, while larger fragments move more slowly. **Analysis of Incorrect Options:** * **Paper Chromatography (B):** Primarily used to separate small polar molecules like amino acids or sugars based on their solubility in a stationary vs. mobile phase. * **High-speed Centrifugation (C):** Separates particles based on density and sedimentation rate (e.g., separating organelles or proteins). While "Density Gradient Centrifugation" can separate DNA by density (CsCl gradient), it is not the standard method for separating fragments by length. * **Thin-layer Chromatography (D):** Used for rapid analysis of non-volatile mixtures, such as lipids or drugs, based on adsorption. **High-Yield Facts for NEET-PG:** * **Agarose Gel:** Used for larger DNA fragments (50 bp to several Mb). * **Polyacrylamide Gel (PAGE):** Used for smaller DNA fragments or proteins due to its higher resolving power. * **Visualization:** DNA bands are typically visualized using **Ethidium Bromide (EtBr)**, which fluoresces orange under UV light. * **Pulsed-Field Gel Electrophoresis (PFGE):** A specialized variation used to separate exceptionally large genomic DNA fragments (e.g., for bacterial typing).

Question 144: Flow cytometry analysis is performed for which of the following estimations?

A. Blood glucose estimation
B. Separation of proteins
C. CD4/CD8 estimation in AIDS (Correct Answer)
D. Estimation of LDL

Explanation: **Explanation:** **Flow Cytometry** is a sophisticated laser-based technology used to analyze the physical and chemical characteristics of particles (usually cells) as they flow in a fluid stream through a beam of light. **Why Option C is Correct:** The primary clinical application of flow cytometry is **Immunophenotyping**. In patients with HIV/AIDS, specific surface markers (Clusters of Differentiation) are identified using fluorescent-labeled monoclonal antibodies. Flow cytometry measures the fluorescence intensity to quantify the absolute counts and ratios of **CD4+ (T-helper cells)** and **CD8+ (T-cytotoxic cells)**. This is the gold standard for monitoring disease progression and the efficacy of Antiretroviral Therapy (ART). **Why Other Options are Incorrect:** * **A. Blood glucose estimation:** Typically performed using enzymatic methods like the **Glucose Oxidase-Peroxidase (GOD-POD)** method or hexokinase method. * **B. Separation of proteins:** Achieved through **Electrophoresis** (based on charge/size) or **Chromatography**. * **D. Estimation of LDL:** Calculated using the **Friedewald formula** [LDL = Total Cholesterol – (HDL + VLDL)] or measured via direct homogeneous assays. **High-Yield Clinical Pearls for NEET-PG:** * **Principle:** Flow cytometry works on the principles of **Light Scattering** (Forward scatter for size; Side scatter for granularity/complexity) and **Fluorescence**. * **Other Applications:** Diagnosis of leukemias/lymphomas, DNA ploidy analysis, and detecting **PNH (Paroxysmal Nocturnal Hemoglobinuria)** by looking for the absence of CD55 and CD59. * **FACS:** Fluorescence-Activated Cell Sorting is a specialized type of flow cytometry that not only analyzes but also physically sorts cells into different containers.

Question 145: What is the principle of the Benedict's test for urine reducing substances?

A. Cu2+ ions are reduced to Cu+ ions by enediols (Correct Answer)
B. Cu+ ions are oxidized to Cu2+ ions by enediols
C. Fe2+ ions are reduced to Fe+ ions by enediols
D. Fe+ ions are reoxidized to Fe2+ ions by enediols

Explanation: ### Explanation **1. Why Option A is Correct:** Benedict’s test is a semi-quantitative test used to detect **reducing sugars** (like glucose, fructose, and galactose) in urine. In an alkaline medium, reducing sugars undergo tautomerization to form highly reactive **enediols**. These enediols act as reducing agents; they donate electrons to the **cupric ions (Cu²⁺)** present in the Benedict’s reagent (as copper sulfate), reducing them to **cuprous ions (Cu⁺)**. These cuprous ions then react with hydroxyl ions to form cuprous hydroxide, which upon heating yields a **cuprous oxide (Cu₂O) precipitate**. The color change (from blue to green, yellow, orange, or brick red) depends on the concentration of the reducing sugar. **2. Why Other Options are Incorrect:** * **Option B:** This is the reverse of the actual reaction. Oxidation involves losing electrons, whereas the copper ions must *gain* electrons (reduction) to form the precipitate. * **Options C & D:** Benedict’s reagent specifically utilizes **Copper (Cu)**, not Iron (Fe). Iron-based redox reactions are characteristic of other biochemical processes (like the Fenton reaction or Cytochrome chain), but not standard urine sugar testing. **3. Clinical Pearls for NEET-PG:** * **Specificity:** Benedict’s test is **not specific for glucose**. It gives positive results for other reducing sugars (lactose in pregnancy, fructose in Essential Fructosuria, galactose in Galactosemia) and non-sugar reducers (Vitamin C, Uric acid, Salicylates). * **Glucose Specificity:** To confirm if a positive Benedict’s test is specifically due to glucose, the **Glucose Oxidase method (Dipstick)** is used. * **Composition:** Benedict’s reagent contains **Copper sulfate** (provides Cu²⁺), **Sodium carbonate** (provides alkaline medium), and **Sodium citrate** (prevents spontaneous precipitation of Cu(OH)₂ by acting as a chelating agent).

Question 146: Which of the following correctly defines the relationship between Quality Assurance (QA), Quality Control (QC), Internal Quality Assurance (IQA), and External Quality Assurance (EQA)?

A. QA = QC + IQA + EQA (Correct Answer)
B. QC = QA + IQA - EQA
C. QC = QA + IQA + EQA
D. QA = QC + IQA - EQA

Explanation: ### Explanation In the clinical biochemistry laboratory, ensuring the accuracy and reliability of test results is paramount. This is achieved through a hierarchical framework where **Quality Assurance (QA)** serves as the overarching umbrella. **1. Why Option A is Correct:** Quality Assurance (QA) is a comprehensive system that encompasses all activities aimed at ensuring the final laboratory report is accurate. It is the sum of: * **Quality Control (QC):** The daily operational techniques used to monitor the precision and accuracy of the analytical process (e.g., running control samples). * **Internal Quality Assurance (IQA):** Continuous monitoring of the entire path of workflow within the lab, from specimen collection (pre-analytical) to result interpretation (post-analytical). * **External Quality Assurance (EQA):** Also known as Proficiency Testing, where an external agency sends "blind" samples to the lab to compare its performance against other laboratories. Mathematically and conceptually, **QA = QC + IQA + EQA**. It represents the total management system that integrates daily controls, internal protocols, and external validation. **2. Why Other Options are Wrong:** * **Options B, C, and D** incorrectly swap the hierarchy. QC is a *subset* of QA, not the other way around. Furthermore, QA is an additive process of various safety nets; subtracting any component (as seen in B and D) would create a gap in the quality chain, making the formula logically flawed. **3. Clinical Pearls for NEET-PG:** * **Pre-analytical Phase:** The most common source of laboratory errors (up to 70%), such as wrong patient identification or hemolysis. * **Westgard Rules:** Used in QC to determine if an analytical run is "in control" or needs rejection. * **Levey-Jennings (L-J) Chart:** The standard graphical tool used to plot daily QC values to detect shifts and trends. * **EQA/Proficiency Testing:** Essential for identifying "systemic errors" that internal controls might miss.

Question 147: Sodium fluoride is added to:

A. Prevent glycolysis (Correct Answer)
B. Prevent glucogenolysis
C. Prevent coagulation
D. All

Explanation: **Explanation:** **1. Why Option A is Correct:** Sodium fluoride (NaF) is the agent of choice for blood glucose estimation because it acts as a potent **glycolytic inhibitor**. It works by inhibiting the enzyme **Enolase** in the glycolytic pathway. Specifically, fluoride ions bind with magnesium and phosphate to form a magnesium-fluorophosphate complex, which competes with the substrate for the active site of Enolase. By blocking this step (the conversion of 2-phosphoglycerate to phosphoenolpyruvate), NaF prevents the breakdown of glucose by RBCs and WBCs *in vitro*, ensuring the measured glucose level reflects the patient's actual blood sugar at the time of collection. **2. Why Other Options are Incorrect:** * **Option B (Glycogenolysis):** While NaF inhibits glycolysis, its primary clinical utility in a vacutainer is preventing the metabolic consumption of free glucose. Glycogenolysis (breakdown of glycogen) occurs primarily in the liver and muscle; in a blood sample, the immediate concern is the rapid consumption of existing glucose by glycolysis. * **Option C (Coagulation):** Sodium fluoride has very weak anticoagulant properties. Therefore, it is almost always combined with **Potassium Oxalate**, which acts as the actual anticoagulant by precipitating calcium ions. **3. High-Yield Clinical Pearls for NEET-PG:** * **The Grey Top Vacutainer:** Contains Sodium Fluoride (antiglycolytic) and Potassium Oxalate (anticoagulant) in a **1:3 ratio**. * **Rate of Glycolysis:** At room temperature, blood glucose levels decrease by approximately **5–10 mg/dL per hour** if an inhibitor is not used. * **Enzyme Inhibition:** Remember that fluoride inhibition of Enolase is **reversible** and requires magnesium as a cofactor for the complex to form. * **Interference:** High concentrations of fluoride can inhibit other enzymes like urease; thus, fluoride tubes should not be used for urea estimation using urease methods.

Question 148: Mutations that completely disrupt the function of a gene are used in which of the following techniques?

A. Non-sense mutation
B. Restriction fragment length polymorphism
C. Targeted gene disruption (Correct Answer)
D. Knock-in

Explanation: **Explanation:** **Targeted gene disruption**, commonly known as **Gene Knockout**, is the correct answer. This technique involves the deliberate "silencing" or inactivation of a specific gene to study its function. By using homologous recombination to replace a functional gene with an inactive or mutated DNA sequence, researchers can observe the resulting phenotypic changes. This allows for the determination of the gene's physiological role and the creation of animal models for human genetic diseases. **Analysis of Incorrect Options:** * **Non-sense mutation (A):** This is a type of point mutation where a codon is changed to a premature stop codon (UAG, UAA, UGA). While it disrupts gene function, it is a *type* of mutation, not a laboratory *technique* used to study gene function. * **Restriction Fragment Length Polymorphism (RFLP) (B):** This is a technique used to detect variations in DNA sequences (polymorphisms) based on different patterns of DNA fragments after digestion with restriction enzymes. It is used for mapping and diagnosis, not for disrupting gene function. * **Knock-in (D):** This technique involves inserting a functional gene (often a mutated version or a reporter gene) into a specific locus. Unlike a knockout, which aims to eliminate function, a knock-in aims to replace or add a specific function. **High-Yield Clinical Pearls for NEET-PG:** * **Knockout Mice:** The most common model for studying Mendelian disorders. * **CRISPR-Cas9:** The modern, most efficient tool for targeted gene disruption and editing. * **RNA Interference (RNAi):** Often confused with knockout; RNAi causes gene "knockdown" (temporary/partial suppression) rather than permanent disruption at the DNA level.

Question 149: Restriction fragment length polymorphism uses which of the following techniques?

A. Southern blotting (Correct Answer)
B. Northern blotting
C. Western blotting
D. Eastern blotting

Explanation: ### Explanation **1. Why Southern Blotting is Correct:** Restriction Fragment Length Polymorphism (RFLP) is a technique used to detect variations in homologous DNA sequences. The process involves: * **Digestion:** DNA is cut into fragments using **Restriction Endonucleases**. * **Separation:** Fragments are separated by size via gel electrophoresis. * **Southern Blotting:** The DNA fragments are transferred from the gel to a membrane and hybridized with a labeled **DNA probe**. Since RFLP specifically analyzes **DNA** variations, Southern blotting (the gold standard for DNA detection) is the integral technique used to visualize these fragments. **2. Why Other Options are Incorrect:** * **Northern Blotting:** This technique is used for the detection and quantification of **RNA**. It is used to study gene expression but cannot be used for RFLP as RFLP analyzes genomic DNA. * **Western Blotting:** This is used for the detection of specific **Proteins** using antibodies (e.g., confirmatory test for HIV). * **Eastern Blotting:** This is an extension of western blotting used to detect **post-translational modifications** of proteins (like carbohydrates or lipids). It is not a standard diagnostic tool for genetic polymorphisms. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Mnemonic (SNOW DROP):** * **S**outhern = **D**NA * **N**orthern = **R**NA * **O** = **O** (Nothing) * **W**estern = **P**rotein * **RFLP Applications:** Used in forensic "DNA fingerprinting," paternity testing, and identifying carriers of genetic diseases like Sickle Cell Anemia (where a mutation destroys a restriction site). * **Restriction Endonucleases:** Also known as "molecular scissors," they recognize specific palindromic sequences.

Question 150: A thin, emaciated 25-year-old male presents with purple plaques and nodules on his face and arms, coughing, and shortness of breath. Which one of the following tests would be most efficient for diagnosing the cause of his problems?

A. Southern blot
B. Northern blot
C. Western blot (Correct Answer)
D. Sanger sequencing

Explanation: ### **Explanation** **Clinical Correlation:** The patient presents with classic signs of **Kaposi Sarcoma** (purple plaques/nodules) and respiratory symptoms (suggestive of *Pneumocystis jirovecii* pneumonia). This clinical triad in a young, emaciated male is highly suspicious for **HIV/AIDS**. **Why Western Blot is Correct:** The **Western blot** is a biochemical technique used to detect specific **proteins** in a sample. Historically, it has been the "gold standard" confirmatory test for HIV diagnosis. It detects antibodies against specific viral proteins (such as gp120, gp41, and p24). While modern 4th-generation ELISA/p24 antigen assays have largely replaced it in routine screening, Western blot remains the classic academic answer for confirming the presence of viral proteins/antibodies in this context. **Why Other Options are Incorrect:** * **Southern Blot:** Used for the detection of specific **DNA** sequences. While it can detect viral DNA integration, it is not the standard or most efficient diagnostic tool for HIV. * **Northern Blot:** Used for the detection of **RNA** sequences. It measures gene expression but is not used for routine clinical diagnosis of HIV. * **Sanger Sequencing:** Used to determine the exact **nucleotide sequence** of DNA. It is used for detecting drug resistance mutations in HIV but is too slow and expensive for primary diagnosis. **High-Yield NEET-PG Pearls:** * **Mnemonic for Blots (SNOW DROP):** * **S**outhern = **D**NA * **N**orthern = **R**NA * **O** = **O** (No match) * **W**estern = **P**rotein * **HIV Diagnosis:** Screening is done by **ELISA** (high sensitivity); Confirmation is done by **Western Blot** (high specificity). * **Kaposi Sarcoma:** Caused by **HHV-8** (Human Herpesvirus 8).

Practice by Chapter

Spectrophotometry and Colorimetry

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Chromatography Techniques

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Electrophoresis and Applications

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Centrifugation and Ultracentrifugation

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Radioisotope Techniques

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Enzyme-Linked Immunosorbent Assay (ELISA)

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Polymerase Chain Reaction (PCR)

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Blotting Techniques: Southern, Northern, Western

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Mass Spectrometry in Biochemistry

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Recombinant DNA Technology

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DNA Sequencing

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Proteomics and Metabolomics

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