Biochemical Techniques — MCQs

Biochemical Techniques Indian Medical PG Practice Questions and MCQs

Question 31: Which ketone body cannot be detected by Rothera's test?

A. Acetoacetate
B. Acetone
C. Beta-hydroxybutyrate (Correct Answer)
D. All the above can be detected

Explanation: ### Explanation **1. Why Beta-hydroxybutyrate is the Correct Answer:** Rothera’s test (Nitroprusside test) relies on a chemical reaction between **sodium nitroprusside** and a **keto group** in an alkaline medium to produce a purple/permanganate-colored ring. * **Beta-hydroxybutyrate** lacks a true keto (C=O) group; it contains a hydroxyl (-OH) group instead. * Because it is technically a hydroxy acid rather than a ketone, it does not react with the nitroprusside reagent. This is a high-yield distinction because beta-hydroxybutyrate is often the predominant ketone body in severe diabetic ketoacidosis (DKA). **2. Analysis of Incorrect Options:** * **Option A (Acetoacetate):** This is the primary ketone body detected by Rothera’s test. It reacts strongly and rapidly with the reagent. * **Option B (Acetone):** Acetone also contains a keto group and gives a positive result, though the reaction is significantly less sensitive (about 10–20 times less) than with acetoacetate. * **Option D:** Incorrect, as the test is specific only to molecules with reactive keto groups. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Sensitivity:** Rothera’s test is most sensitive to **Acetoacetate**, followed by Acetone. * **The "Negative" Trap:** In early or severe DKA, the redox state shifts to favor beta-hydroxybutyrate. A patient may have life-threatening ketosis, but the urine Rothera’s test may appear weakly positive or even negative until the patient is treated and beta-hydroxybutyrate is oxidized back to acetoacetate. * **Gerhardt’s Ferric Chloride Test:** Another test for ketones, but it only detects acetoacetate (not acetone or beta-hydroxybutyrate). * **Reagent Composition:** Rothera’s powder contains Sodium Nitroprusside (active reagent), Ammonium Sulfate (saturation), and Liquor Ammonia (alkalinity).

Question 32: Arrange the electrophoretic pattern of serum lipoproteins in sequence, starting from the point of origin.

A. Chylomicron, Beta lipoprotein, Pre-beta lipoprotein, Alpha lipoprotein (Correct Answer)
B. Pre-beta lipoprotein, Beta lipoprotein, Alpha lipoprotein, Chylomicron
C. Beta lipoprotein, Alpha lipoprotein, Pre-beta lipoprotein, Chylomicron
D. Pre-beta lipoprotein, Alpha lipoprotein, Chylomicron, Beta lipoprotein

Explanation: ### Explanation The separation of lipoproteins by electrophoresis is based on their **net surface charge**, which is determined by their protein (apoprotein) content. In an alkaline medium (pH 8.6), lipoproteins carry a negative charge and migrate toward the anode (+). **1. Why Option A is Correct:** The migration distance from the origin (cathode) depends on the protein-to-lipid ratio. * **Chylomicrons (Origin):** These are the largest and least dense, containing ~99% lipid and very little protein. They have negligible charge and remain at the **point of application (origin)**. * **Beta-lipoproteins (LDL):** These have more protein than chylomicrons but less than VLDL/HDL, migrating just past the origin. * **Pre-beta lipoproteins (VLDL):** Despite being less dense than LDL, VLDL contains **Apo C-II**, which provides a higher negative charge, causing it to migrate faster than LDL (Beta). * **Alpha-lipoproteins (HDL):** These have the highest protein content (~50%), giving them the strongest negative charge and the **fastest migration** toward the anode. **2. Why Other Options are Wrong:** * **Options B, C, and D** are incorrect because they misplace the Chylomicrons (which must be at the origin) or fail to recognize that HDL (Alpha) is always the fastest-moving fraction due to its high protein density. A common point of confusion is the position of LDL and VLDL; remember that **Pre-beta (VLDL) moves faster than Beta (LDL)**. **3. Clinical Pearls & High-Yield Facts:** * **Mnemonic:** "**C**an **B**e **P**re-**A**lpha" (Chylomicron, Beta, Pre-beta, Alpha). * **Density vs. Electrophoresis:** While density (ultracentrifugation) follows the order **HDL > LDL > VLDL > Chylomicron**, electrophoretic mobility follows **HDL > VLDL > LDL > Chylomicron**. * **Type I Hyperlipoproteinemia:** Characterized by a heavy band at the origin (Chylomicrons). * **Type II Hyperlipoproteinemia:** Shows a prominent Beta-band (LDL).

Question 33: Which vacutainer is used for electrolyte estimation?

A. Na Citrate
B. EDTA
C. Fluoride
D. Lithium heparin (Correct Answer)

Explanation: **Explanation:** The correct answer is **Lithium Heparin (Green top)**. For electrolyte estimation, plasma is often preferred over serum to avoid the "pseudohyperkalemia" caused by potassium release from platelets during the clotting process. Heparin acts as an anticoagulant by activating **Antithrombin III**, which inactivates Thrombin and Factor Xa. Lithium heparin is specifically used because lithium does not interfere with the measurement of common electrolytes like Sodium ($Na^+$), Potassium ($K^+$), or Chloride ($Cl^-$). **Analysis of Incorrect Options:** * **Na Citrate (Light Blue top):** Used for coagulation studies (PT/APTT). It contains sodium, which would falsely elevate sodium levels and interfere with electrolyte panels. * **EDTA (Purple top):** Used for Hematology (CBC). EDTA is a chelating agent often prepared as a Potassium salt ($K_2$ or $K_3$ EDTA). Using this tube for electrolytes would cause a **critically false elevation of Potassium** and a false decrease in Calcium/Magnesium due to chelation. * **Fluoride (Grey top):** Used for blood glucose estimation. It contains Sodium Fluoride (an antiglycolytic agent) and Potassium Oxalate. The presence of these salts makes it unsuitable for electrolyte measurement. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Draw:** To prevent cross-contamination of additives, the standard sequence is: Blood Culture → Citrate → Serum (Red/SSGT) → Heparin → EDTA → Fluoride. * **Potassium Sensitivity:** EDTA contamination is the most common cause of "spurious hyperkalemia" in lab reports. * **Gold Standard:** While Lithium Heparin is used for rapid (STAT) plasma electrolytes, **Serum Separator Tubes (SST/Yellow top)** are also commonly used in routine clinical practice once the blood has clotted.

Question 34: The Biuret test is primarily used to detect the presence of which class of biomolecules?

A. Carbohydrates
B. Proteins (Correct Answer)
C. Lipids
D. All of the above

Explanation: **Explanation:** The **Biuret test** is a chemical assay used to detect the presence of **peptide bonds**, which are the fundamental linkages in **proteins**. **Why Proteins are the Correct Answer:** The test relies on the reaction between cupric ions ($Cu^{2+}$) and the nitrogen atoms of peptide bonds in an alkaline medium. When at least two peptide bonds are present (as in dipeptides, polypeptides, and proteins), they form a coordination complex with the copper ions, resulting in a characteristic **violet or purple color** change. The intensity of the color is directly proportional to the number of peptide bonds present, making it useful for both qualitative detection and quantitative estimation (spectrophotometry). **Why Other Options are Incorrect:** * **Carbohydrates:** These are detected using tests like **Benedict’s** (for reducing sugars), **Molisch’s** (general), or **Iodine** (for starch). They lack peptide bonds. * **Lipids:** These are typically identified using the **Sudan dye test** or the **Saponification test**. Lipids are composed of fatty acids and glycerol, not amino acid chains. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Minimum Requirement:** The Biuret test requires at least **two peptide bonds**. Therefore, free amino acids (except histidine) and the dipeptide glycylglycine give a negative result. * **Interference:** High concentrations of ammonium salts can interfere with the reaction. * **Clinical Use:** It is the standard method for measuring **Total Serum Protein** levels in clinical biochemistry labs. * **Biuret Compound:** The test is named after the compound 'biuret' ($NH_2-CO-NH-CO-NH_2$), which is formed by heating urea and also gives a positive result.

Question 35: What is the probe used in a Western blot?

A. Antibody (Correct Answer)
B. mRNA
C. DNA
D. tRNA

Explanation: **Explanation:** Western blotting (also known as protein immunoblotting) is a core laboratory technique used to detect specific **proteins** in a given sample of tissue homogenate or extract. 1. **Why Option A is correct:** The fundamental principle of Western blotting is the **antigen-antibody interaction**. After proteins are separated by size via SDS-PAGE and transferred to a membrane (nitrocellulose or PVDF), a specific **antibody** (the probe) is added. This antibody binds specifically to its target protein (the antigen), allowing for visualization and quantification. 2. **Why other options are incorrect:** * **Option B (mRNA):** mRNA is the target in **Northern blotting**. * **Option C (DNA):** DNA is the target in **Southern blotting**. * **Option D (tRNA):** tRNA is not typically the primary target of standard blotting techniques. **High-Yield NEET-PG Clinical Pearls:** * **Mnemonic (SNOW DROP):** * **S**outhern — **D**NA * **N**orthern — **R**NA * **O** — **O** (Placeholder) * **W**estern — **P**rotein * **Clinical Application:** Western blot was historically the "gold standard" confirmatory test for **HIV** (detecting antibodies against viral proteins like gp120, gp41, and p24), though it has largely been replaced by 4th generation immunoassays and NAAT. * **Southwestern Blot:** A specific variation used to identify **DNA-binding proteins** (e.g., transcription factors like c-Jun or c-Fos) using labeled double-stranded DNA probes.

Question 36: Which of the following techniques is used to identify an important amino acid residue involved in enzyme catalysis?

A. Electrophoresis
B. Chromatography
C. Mass spectrometry
D. Site-directed mutagenesis (Correct Answer)

Explanation: **Explanation:** **1. Why Site-Directed Mutagenesis is Correct:** Site-directed mutagenesis (SDM) is a molecular biology technique used to make specific, intentional changes to the DNA sequence of a gene. In enzymology, this is the gold standard for identifying **catalytic residues**. By substituting a specific amino acid (e.g., changing a Serine to an Alanine) at the suspected active site, researchers can observe the resulting change in enzyme activity. If the mutation leads to a complete loss of catalysis, it confirms that the specific amino acid residue is essential for the enzyme's function. **2. Why Other Options are Incorrect:** * **Electrophoresis (A):** This technique separates proteins or nucleic acids based on their **charge and size** (e.g., SDS-PAGE). It is used for purity analysis or molecular weight determination, not for identifying specific functional residues. * **Chromatography (B):** This is a **purification technique** used to separate mixtures based on properties like solubility, size (Gel filtration), or affinity. It does not provide information on which specific amino acid is involved in the catalytic mechanism. * **Mass Spectrometry (C):** While excellent for determining the **primary sequence** (proteomics) or molecular mass of a protein, it identifies what amino acids are present but cannot inherently determine which one is "important" for the catalytic step without functional testing. **High-Yield Clinical Pearls for NEET-PG:** * **Structure-Function Relationship:** SDM is the primary tool used to study the "Structure-Function" relationship of proteins. * **PCR-based:** Modern SDM typically utilizes PCR with primers containing the desired mutation. * **Enzyme Engineering:** This technique is also used to create "designer enzymes" with increased thermal stability or altered substrate specificity for industrial and medical use.

Question 37: The Biuret test is a confirmatory test for which type of biomolecule?

A. Protein (Correct Answer)
B. Fat
C. Carbohydrate
D. None of the above

Explanation: **Explanation:** The **Biuret test** is a chemical assay used to detect the presence of **peptide bonds**, making it a specific test for **proteins**. **Why Protein is Correct:** The test relies on the reaction between cupric ions ($Cu^{2+}$) and peptide bonds in an alkaline medium. When proteins are present, the $Cu^{2+}$ ions form a coordination complex with the nitrogen atoms of the peptide linkages. This results in a characteristic color change from **light blue to violet or purple**. For a positive result, the molecule must contain at least **two peptide bonds** (tripeptides and larger). Note: Free amino acids (except histidine) do not give a positive Biuret test. **Why Other Options are Incorrect:** * **Fat (Lipids):** Lipids are identified using tests like the **Sudan III/IV dye test** (solubility in organic solvents) or the Saponification test. They lack peptide bonds. * **Carbohydrates:** These are detected using **Molisch’s test** (general), **Benedict’s/Fehling’s test** (reducing sugars), or the **Iodine test** (starch). They consist of glycosidic linkages, not peptide bonds. **High-Yield Clinical Pearls for NEET-PG:** * **Sensitivity:** The intensity of the purple color is directly proportional to the number of peptide bonds (protein concentration), allowing for **quantitative estimation** via spectrophotometry (at 540 nm). * **Exception:** Although it is called the "Biuret" test, the compound **biuret** (formed by heating urea) is not a protein but gives a positive result because it contains two peptide bonds. * **Clinical Use:** It is the standard method used in clinical laboratories to measure **Total Serum Protein** levels.

Question 38: SYBR Green Dye is used for?

A. HPLC
B. Immunofluorescence
C. PCR (Correct Answer)
D. ELISA

Explanation: **Explanation:** **SYBR Green I** is an asymmetrical cyanine dye used primarily in **Real-Time PCR (qPCR)** for the quantification of DNA. **Why PCR is the Correct Answer:** SYBR Green is a **fluorescent DNA-binding dye** that specifically binds to the minor groove of **double-stranded DNA (dsDNA)**. When it binds to dsDNA, its fluorescence intensity increases up to 1000-fold. During the extension phase of PCR, as more dsDNA is synthesized, the fluorescence signal increases proportionally. This allows for the real-time monitoring of DNA amplification without the need for sequence-specific probes (like TaqMan). **Analysis of Incorrect Options:** * **A. HPLC (High-Performance Liquid Chromatography):** This is a technique used to separate, identify, and quantify components in a mixture based on their chemical properties (e.g., HbA1c estimation). It does not typically utilize DNA-intercalating dyes. * **B. Immunofluorescence:** This technique uses antibodies tagged with fluorophores (like FITC) to detect specific antigens in tissues or cells, not general DNA quantification. * **D. ELISA (Enzyme-Linked Immunosorbent Assay):** This relies on antigen-antibody interactions and an enzyme-substrate reaction (producing a color change) to detect proteins or hormones, not DNA binding dyes. **High-Yield Clinical Pearls for NEET-PG:** * **Non-Specificity:** Unlike TaqMan probes, SYBR Green binds to *any* dsDNA, including non-specific products like **primer-dimers**. * **Melt Curve Analysis:** To ensure specificity when using SYBR Green, a "Melt Curve" is performed after PCR. A single peak indicates a pure, specific product. * **Other DNA Dyes:** Ethidium Bromide (EtBr) is used in traditional Gel Electrophoresis but is more toxic (mutagenic) than SYBR Green.

Question 39: What method is used to detect an antigen-antibody reaction?

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

Explanation: **Explanation:** The correct answer is **ELISA (Enzyme-Linked Immunosorbent Assay)**. ELISA is a plate-based assay technique designed for detecting and quantifying substances such as peptides, proteins, antibodies, and hormones. It relies on the principle of **antigen-antibody (Ag-Ab) interaction**. In this method, an enzyme-linked conjugate reacts with a substrate to produce a color change (chromogenic reaction), indicating the presence and concentration of the target analyte. **Analysis of Incorrect Options:** * **Southern Blot:** Used for the detection of specific **DNA** sequences. It involves DNA digestion, electrophoresis, and hybridization with a DNA probe. (Mnemonic: **S**outhern = **D**NA). * **Northern Blot:** Used for the detection of specific **RNA** (mRNA) sequences to study gene expression. (Mnemonic: **N**orthern = **R**NA). * **Western Blot:** Used to detect specific **proteins**. While it also uses antibodies, it is primarily a technique for protein identification based on molecular weight after gel electrophoresis. In the context of this question, ELISA is the primary, high-throughput method specifically defined by the Ag-Ab reaction for screening. **High-Yield NEET-PG Pearls:** * **SNOW DROP Mnemonic:** **S**outhern-**D**NA, **N**orthern-**R**NA, **O**-O, **W**estern-**P**rotein. * **ELISA Applications:** It is the standard **screening test** for HIV (detecting p24 antigen or anti-HIV antibodies). * **Western Blot Application:** Historically used as the **confirmatory test** for HIV (detecting antibodies against gp120, gp41, and p24). * **Southwestern Blot:** A hybrid technique used to detect **DNA-binding proteins** (e.g., transcription factors).

Question 40: What is the reduction potential of potassium?

A. -2.93 V (Correct Answer)
B. -0.44 V
C. -0.34 V
D. 0.54 V

Explanation: **Explanation:** The **standard reduction potential ($E_0$)** measures the tendency of a chemical species to be reduced (gain electrons). In biochemistry and clinical chemistry, this concept is fundamental to understanding the electron transport chain and the functioning of ion-selective electrodes used in blood gas analysis. **Why Option A is Correct:** Potassium (K) is an alkali metal with a single valence electron. It is highly electropositive and has a powerful tendency to lose that electron (oxidation) rather than gain one. Therefore, its reduction potential is highly negative. The standard reduction potential for the reaction $K^+ + e^- \rightarrow K$ is **-2.93 V**. This reflects its position near the top of the electrochemical series as a potent reducing agent. **Analysis of Incorrect Options:** * **Option B (-0.44 V):** This is the reduction potential for **Iron ($Fe^{2+}$)**. In clinical biochemistry, iron's ability to cycle between oxidation states ($Fe^{2+}/Fe^{3+}$) is crucial for oxygen transport in hemoglobin. * **Option C (-0.34 V):** This value is close to the reduction potential of **Copper ($Cu/Cu^{2+}$ is +0.34 V)**, though the sign is reversed here. Copper is a vital cofactor in enzymes like Cytochrome c Oxidase. * **Option D (0.54 V):** This is the reduction potential for the **Iodine/Iodide** system. Iodine is essential for thyroid hormone synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Ion-Selective Electrodes (ISE):** Most automated biochemistry analyzers measure serum electrolytes ($Na^+$, $K^+$, $Cl^-$) based on the principles of potentiometry and reduction potentials. * **The "Most Negative" Rule:** In the electrochemical series, Lithium has the most negative reduction potential (~ -3.04 V), followed closely by Potassium. * **Biological Significance:** While these standard potentials are measured under non-biological conditions, the relative values determine the direction of electron flow in metabolic redox reactions.

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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