Biochemical Techniques — MCQs

Biochemical Techniques Indian Medical PG Practice Questions and MCQs

Question 171: Which of the following anticoagulants used in estimating blood glucose prevents glycolysis?

A. Oxalate
B. Citrate
C. NaF (Correct Answer)
D. Heparin

Explanation: **Explanation** The correct answer is **Sodium Fluoride (NaF)**. **Mechanism of Action** In blood samples collected for glucose estimation, RBCs and WBCs continue to consume glucose via glycolysis (at a rate of ~5–7% per hour). To prevent this, **Sodium Fluoride (NaF)** is used. It acts as an **antiglycolytic agent** by inhibiting the enzyme **Enolase** in the glycolytic pathway. It does this by forming a complex with magnesium and phosphate, thereby depriving the enzyme of its essential cofactor, magnesium ($Mg^{2+}$). **Analysis of Options** * **NaF (Correct):** Specifically inhibits Enolase to preserve glucose levels. It is usually paired with Potassium Oxalate (which acts as the anticoagulant). * **Oxalate (A):** Acts as an anticoagulant by precipitating calcium as calcium oxalate. While often present in the "grey-top" vial with NaF, it has no antiglycolytic properties. * **Citrate (B):** Used primarily for coagulation studies (PT/APTT) and ESR (Westergren method). It works by chelating calcium but does not stop glycolysis. * **Heparin (C):** An indirect thrombin inhibitor used for arterial blood gases (ABG) and pH levels. It does not inhibit glycolytic enzymes. **High-Yield Clinical Pearls for NEET-PG** * **Grey-top Vacutainer:** Contains NaF (antiglycolytic) and Potassium Oxalate (anticoagulant) in a **1:3 ratio**. * **Enzyme Inhibition:** NaF is a classic example of **competitive/non-competitive inhibition** (specifically, it competes with the substrate for the magnesium binding site). * **Urease Interaction:** NaF also inhibits the enzyme **Urease**. Therefore, blood collected in fluoride vials cannot be used for urea estimation if the urease method is employed. * **Iodoacetate:** Another antiglycolytic agent that inhibits **Glyceraldehyde-3-phosphate dehydrogenase**, though it is less commonly used than NaF.

Question 172: A positive D-xylose test indicates all of the following except?

A. Pancreatic insufficiency (Correct Answer)
B. Small intestinal mucosal disease
C. Impaired carbohydrate absorption in the small intestine
D. Malabsorption

Explanation: **Explanation:** The **D-xylose absorption test** is a clinical tool used to differentiate between **malabsorption caused by mucosal disease** and **malabsorption caused by pancreatic insufficiency.** **Why Pancreatic Insufficiency is the correct answer:** D-xylose is a pentose sugar that is absorbed directly by the proximal small intestinal mucosa via passive diffusion (and some facilitated transport). Unlike complex carbohydrates, it **does not require pancreatic enzymes** (like amylase) or bile salts for digestion. Therefore, in patients with pancreatic insufficiency, D-xylose absorption remains **normal**. A "positive" test (low urinary or blood levels of D-xylose) indicates a problem with the intestinal wall itself, not the pancreas. **Analysis of other options:** * **Small intestinal mucosal disease (B):** Conditions like Celiac disease or Tropical sprue damage the villi, leading to decreased surface area and a positive (abnormal) test. * **Impaired carbohydrate absorption (C):** Since D-xylose is a marker for the intestine's ability to absorb monosaccharides, a low result directly confirms impaired absorption. * **Malabsorption (D):** D-xylose is a classic screening test for general malabsorption syndromes originating in the small bowel. **High-Yield Clinical Pearls for NEET-PG:** * **Procedure:** Patient is given 25g of D-xylose; a 5-hour urine collection <4g or a 1-hour serum level <20mg/dL is considered abnormal. * **False Positives:** Low D-xylose levels can occur despite a healthy mucosa in cases of **Renal dysfunction** (impaired excretion), **SIBO** (bacteria metabolize the sugar), or **delayed gastric emptying**. * **Gold Standard:** While D-xylose tests for mucosal integrity, the **Small Bowel Biopsy** remains the gold standard for diagnosing specific mucosal diseases like Celiac.

Question 173: What is the primary purpose of flow cytometry?

A. To detect and quantify specific cell populations based on surface or intracellular markers (Correct Answer)
B. To measure blood flow velocity in the brain
C. To determine the net oxygen supply to tissues
D. To assess the amount of oxygen bound to hemoglobin

Explanation: **Explanation:** **Flow cytometry** is a sophisticated laser-based technology used to analyze the physical and chemical characteristics of particles or cells in a fluid suspension. 1. **Why Option A is Correct:** The core principle involves passing cells in a single file through a laser beam. As cells pass, they scatter light and emit fluorescence from tagged antibodies. This allows for the simultaneous measurement of **cell size** (Forward Scatter), **granularity/complexity** (Side Scatter), and the presence of specific **surface markers** (e.g., CD4, CD8) or **intracellular proteins** (e.g., cytokines, DNA content). It is the gold standard for immunophenotyping. 2. **Why Other Options are Incorrect:** * **Option B:** Measuring blood flow velocity in the brain is typically done using **Transcranial Doppler (TCD)** ultrasound. * **Option C & D:** Assessing oxygen supply and hemoglobin saturation involves **Pulse Oximetry** or **Arterial Blood Gas (ABG)** analysis, which measure physiological parameters rather than cellular markers. 3. **High-Yield Clinical Pearls for NEET-PG:** * **CD4/CD8 Counts:** Flow cytometry is the primary tool for monitoring disease progression in **HIV/AIDS**. * **Leukemia/Lymphoma:** It is essential for "lineage assignment" to differentiate between AML, ALL, and various lymphomas. * **PNH (Paroxysmal Nocturnal Hemoglobinuria):** Used to detect the absence of **CD55 and CD59** on RBCs/WBCs (the current gold standard for diagnosis). * **Reticulocyte Count:** Automated flow cytometry provides a more accurate reticulocyte count than manual microscopy. * **DNA Ploidy:** Used in oncology to determine the DNA index and cell cycle phase (S-phase fraction).

Question 174: The name 'Cytochrome P450' is derived from which characteristic?

A. A molecular weight of 450 Daltons
B. Production by 450 genes
C. Absorption of light at 450 nanometres (Correct Answer)
D. Presence of 450 isoforms

Explanation: ### Explanation **Correct Option: C. Absorption of light at 450 nanometres** The name **Cytochrome P450 (CYP450)** is derived from its unique spectral properties. These enzymes are hemoproteins that contain a heme group. When the iron in the heme is in the reduced state ($Fe^{2+}$) and binds to **carbon monoxide (CO)**, the resulting complex exhibits a characteristic absorption maximum (peak) at a wavelength of **450 nanometres**. The "P" stands for "pigment," and "450" refers to this specific wavelength. --- ### Analysis of Incorrect Options: * **A. Molecular weight of 450 Daltons:** This is incorrect. The molecular weight of a typical CYP450 protein is much higher, ranging between **45,000 to 55,000 Daltons** (45–55 kDa). * **B. Production by 450 genes:** Humans possess approximately **57 functional genes** and 58 pseudogenes encoding these enzymes, not 450. * **D. Presence of 450 isoforms:** While there are many isoforms categorized into families (e.g., CYP1, CYP2, CYP3), the number is nowhere near 450 in humans. --- ### NEET-PG High-Yield Pearls: * **Location:** Primarily found in the **Smooth Endoplasmic Reticulum (Microsomes)** and the inner mitochondrial membrane. * **Function:** They are monooxygenases involved in **Phase I metabolism** (Hydroxylation) of drugs and endogenous compounds (steroids, cholesterol). * **Most Abundant Isoform:** **CYP3A4** is the most common isoform in the liver, responsible for metabolizing nearly 50% of all clinical drugs. * **Inducers vs. Inhibitors:** * *Inducers:* Rifampicin, Phenytoin, Phenobarbitone (increase drug metabolism). * *Inhibitors:* Ketoconazole, Erythromycin, Grapefruit juice (decrease drug metabolism, leading to toxicity). * **Key Reaction:** $RH + O_2 + NADPH + H^+ \rightarrow ROH + H_2O + NADP^+$

Question 175: Which of the following methods cannot be used to separate proteins?

A. Electrophoresis
B. Ultracentrifugation
C. Gas chromatography
D. None of the above (Correct Answer)

Explanation: **Explanation** The correct answer is **None of the above** because all the listed techniques—Electrophoresis, Ultracentrifugation, and Gas Chromatography—are valid methods for the separation and analysis of proteins. 1. **Electrophoresis (Option A):** This is the most common laboratory technique for protein separation. It utilizes an electric field to move proteins through a matrix (like polyacrylamide gel) based on their **charge-to-mass ratio**. SDS-PAGE is the gold standard for separating proteins by molecular weight. 2. **Ultracentrifugation (Option B):** This method separates proteins based on their **sedimentation coefficient**, which is influenced by size, shape, and density. High-speed centrifugal force allows for the separation of subcellular fractions and large protein complexes. 3. **Gas Chromatography (Option C):** While less common for intact large proteins due to their non-volatility, GC is a standard method for separating **amino acids** (the building blocks of proteins) or small volatile peptides after derivatization. In specialized proteomic workflows, GC-MS is used for detailed protein composition analysis. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Isoelectric Focusing (IEF):** Separates proteins based on their **isoelectric point (pI)**, where the net charge is zero. * **2D-Electrophoresis:** Combines IEF (1st dimension) and SDS-PAGE (2nd dimension) for high-resolution separation. * **Salting Out:** A chemical method using Ammonium Sulfate to precipitate proteins based on solubility. * **Molecular Sieve (Gel Filtration):** Separates proteins strictly by **size**; larger proteins elute first as they are excluded from the pores of the beads.

Question 176: The ability of amino acids/proteins to behave like zwitterions forms the basis for separating them using which of the following techniques?

A. Gel filtration chromatography
B. Ion exchange chromatography
C. Isoelectric focusing (Correct Answer)
D. Mass spectrometry

Explanation: ### Explanation **Correct Answer: C. Isoelectric focusing** **1. Why Isoelectric Focusing is correct:** Amino acids and proteins are **amphoteric** molecules, meaning they can act as both acids and bases. At a specific pH known as the **isoelectric point (pI)**, the molecule exists as a **zwitterion**—a dipolar ion with a net charge of zero. Isoelectric focusing (IEF) utilizes this property by creating a stable pH gradient within an electrophoresis gel. When an electric field is applied, proteins migrate through the gradient until they reach the pH region that matches their pI. At this point, they become zwitterions (net charge = 0), lose their electrophoretic mobility, and stop moving, forming sharp "focused" bands. **2. Why other options are incorrect:** * **A. Gel filtration chromatography:** Separates molecules based on **size and molecular weight** (molecular sieving), not charge or zwitterionic properties. * **B. Ion exchange chromatography:** Separates molecules based on their **net surface charge** at a fixed pH. While it utilizes charge, it does not rely on the zwitterionic state (zero charge) to achieve separation; rather, it relies on the attraction to charged resin beads. * **D. Mass spectrometry:** Separates molecules based on their **mass-to-charge (m/z) ratio** in a vacuum, primarily used for identifying protein sequences and post-translational modifications. **3. High-Yield Clinical Pearls for NEET-PG:** * **pI Calculation:** For a simple amino acid, $pI = (pK_1 + pK_2) / 2$. * **Zwitterion Property:** At pI, proteins show **minimum solubility** and **minimum buffering capacity**. * **2D Electrophoresis:** This high-resolution technique combines **Isoelectric Focusing** (1st dimension, separates by pI) and **SDS-PAGE** (2nd dimension, separates by size). * **Clinical Application:** IEF is the gold standard for detecting **oligoclonal bands** in cerebrospinal fluid (CSF) for the diagnosis of Multiple Sclerosis.

Question 177: Which of the following methods is not antibody-dependent?

A. ELISA
B. Chromatin Immunoprecipitation assay
C. Flow cytometry
D. SDS-PAGE (Correct Answer)

Explanation: **Explanation:** The correct answer is **SDS-PAGE** (Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis). **Why SDS-PAGE is the correct answer:** SDS-PAGE is a technique used to separate proteins based solely on their **molecular weight**. The process involves denaturing proteins with SDS (an anionic detergent) which imparts a uniform negative charge to the proteins. They are then separated by size as they migrate through a polyacrylamide gel toward the anode. This process is purely physical and chemical; it does **not** require the specificity of an antibody-antigen interaction. **Analysis of Incorrect Options:** * **ELISA (Enzyme-Linked Immunosorbent Assay):** This is the gold standard for antibody-dependent assays. It utilizes specific antibodies (primary and secondary) to detect and quantify antigens or antibodies in a sample. * **Chromatin Immunoprecipitation (ChIP) assay:** As the name suggests ("Immuno"), this technique uses specific antibodies to isolate DNA-binding proteins (like transcription factors or histones) along with the DNA sequences they are bound to. * **Flow Cytometry:** This technique frequently uses **fluorescently labeled antibodies** to identify specific cell surface markers (CD markers) or intracellular proteins to characterize cell populations. **High-Yield Clinical Pearls for NEET-PG:** * **Western Blot:** Often confused with SDS-PAGE. While SDS-PAGE separates the proteins, the subsequent "blotting" and detection phase **is** antibody-dependent. * **Southern Blot:** Detects DNA (Mnemonic: **S**outhern-**D**NA). * **Northern Blot:** Detects RNA (Mnemonic: **N**orthern-**R**NA). * **Western Blot:** Detects Protein (Mnemonic: **W**estern-**P**rotein). * **SDS-PAGE Utility:** It is the first step in Western Blotting and is used to determine protein purity and estimate molecular weight.

Question 178: Western Blot detects which of the following molecules?

A. DNA
B. RNA
C. Protein (Correct Answer)
D. mRNA

Explanation: **Explanation:** **Western Blot** is a core molecular biology technique used to detect specific **proteins** in a given sample of tissue homogenate or extract. The process involves three key steps: 1. **Separation:** Proteins are separated based on molecular weight using gel electrophoresis (usually SDS-PAGE). 2. **Transfer:** The separated proteins are transferred (blotted) onto a membrane (nitrocellulose or PVDF). 3. **Detection:** The membrane is incubated with specific labeled antibodies that bind to the target protein, allowing for its visualization. **Analysis of Incorrect Options:** * **Option A (DNA):** DNA is detected using the **Southern Blot**. This is used for identifying specific DNA sequences or gene mutations. * **Option B & D (RNA/mRNA):** RNA (including mRNA) is detected using the **Northern Blot**. This technique measures gene expression by quantifying the amount of RNA in a sample. **NEET-PG High-Yield Pearls:** To remember these techniques easily, use the mnemonic **"SNOW DROP"**: * **S**outhern = **D**NA * **N**orthern = **R**NA * **O** (ignore) = **O** (ignore) * **W**estern = **P**rotein **Clinical Correlation:** * **HIV Diagnosis:** 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 NAT. * **South-Western Blot:** A hybrid technique used to detect **DNA-binding proteins** (e.g., transcription factors).

Question 179: All of the following are used in a PCR reaction, EXCEPT?

A. Buffer
B. ddNTPs (Correct Answer)
C. Oligonucleotide Primer pair
D. Template DNA

Explanation: **Explanation:** The **Polymerase Chain Reaction (PCR)** is an *in vitro* method for the exponential amplification of specific DNA sequences. The correct answer is **ddNTPs (dideoxynucleoside triphosphates)** because they are not used in standard PCR; instead, they are the hallmark of **Sanger Sequencing (Chain Termination Method).** * **Why ddNTPs are the exception:** ddNTPs lack a **3'-OH group** on the sugar moiety. In DNA synthesis, the 3'-OH is essential for forming a phosphodiester bond with the next incoming nucleotide. If a ddNTP is incorporated, DNA polymerization terminates immediately. Standard PCR requires continuous elongation, thus it uses **dNTPs** (deoxynucleotides), not ddNTPs. **Analysis of other options:** * **Buffer (Option A):** Essential to maintain the optimal pH and ionic strength (usually containing $MgCl_2$) for the stability and activity of the DNA polymerase. * **Oligonucleotide Primer pair (Option C):** PCR requires two synthetic, single-stranded DNA primers that are complementary to the sequences flanking the target region to provide a starting 3'-OH group. * **Template DNA (Option D):** The sample DNA containing the specific target sequence that needs to be amplified. **High-Yield Clinical Pearls for NEET-PG:** * **Taq Polymerase:** Derived from *Thermus aquaticus*, it is heat-stable, allowing it to function at high temperatures ($72^\circ C$) during the extension phase. * **Steps of PCR:** 1. Denaturation ($94-96^\circ C$), 2. Annealing ($50-65^\circ C$), 3. Extension ($72^\circ C$). * **RT-PCR:** Uses Reverse Transcriptase to convert RNA into cDNA before amplification (Gold standard for diagnosing COVID-19). * **Real-Time PCR (qPCR):** Allows for the quantification of DNA in real-time using fluorescent dyes (e.g., SYBR Green).

Question 180: Which of the following statements is true regarding fluorescence?

A. Spontaneous illumination in the dark.
B. Release of light of a longer wavelength after absorbing light of a shorter wavelength. (Correct Answer)
C. Release of light of a shorter wavelength after absorbing light of a longer wavelength.
D. Continuous emission of lights of different wavelengths.

Explanation: ### Explanation **Core Concept: Stokes’ Law** Fluorescence is a luminescence phenomenon where a molecule (fluorophore) absorbs a photon of high energy (shorter wavelength) and subsequently emits a photon of lower energy (**longer wavelength**). This occurs because some energy is lost as heat (vibrational relaxation) during the brief interval between absorption and emission. This shift in wavelength is known as the **Stokes Shift**. **Analysis of Options:** * **Option B (Correct):** According to the formula $E = hc/\lambda$, energy is inversely proportional to wavelength. Since the emitted light has less energy than the absorbed light, the emitted wavelength must be longer. * **Option A:** This describes **Bioluminescence** (e.g., fireflies) or **Chemiluminescence**, where light is produced by a chemical reaction without prior light absorption. * **Option C:** This is physically impossible under standard conditions as it would violate the Law of Conservation of Energy (emitting more energy than absorbed). * **Option D:** This describes a continuous spectrum. Fluorescence typically involves specific excitation and emission peaks characteristic of the molecule. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Stokes Shift:** The distance between the maximum absorption and maximum emission wavelengths. * **Quenching:** The decrease in fluorescence intensity due to molecular interactions (e.g., pH changes, heavy metals). * **Clinical Applications:** * **FACS (Fluorescence-Activated Cell Sorting):** Used for CD4/CD8 counts in HIV. * **Immunofluorescence:** Used in diagnosing autoimmune bullous diseases (e.g., Pemphigus) and nephropathies. * **FISH (Fluorescence In Situ Hybridization):** Used for detecting chromosomal abnormalities like Trisomy 21 or the Philadelphia chromosome.

Practice by Chapter

Spectrophotometry and Colorimetry

Practice Questions

Chromatography Techniques

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Electrophoresis and Applications

Practice Questions

Centrifugation and Ultracentrifugation

Practice Questions

Radioisotope Techniques

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Enzyme-Linked Immunosorbent Assay (ELISA)

Practice Questions

Polymerase Chain Reaction (PCR)

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Blotting Techniques: Southern, Northern, Western

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Mass Spectrometry in Biochemistry

Practice Questions

Recombinant DNA Technology

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DNA Sequencing

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Proteomics and Metabolomics

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