Biochemical Techniques — MCQs

Biochemical Techniques Indian Medical PG Practice Questions and MCQs

Question 41: Which of the following tubes contains Sodium fluoride as an anticoagulant?

A. Plain tube
B. EDTA tube
C. Grey top tube (Correct Answer)
D. Heparin tube

Explanation: **Explanation:** The **Grey top tube** is the correct answer because it contains **Sodium fluoride (NaF)**, which acts as a potent **antiglycolytic agent**. In blood samples, RBCs and WBCs continue to consume glucose via glycolysis even after collection (at a rate of ~5–7% per hour). Sodium fluoride inhibits the enzyme **Enolase** in the glycolytic pathway, thereby preserving glucose levels for accurate measurement. It is often combined with Potassium oxalate, which acts as the anticoagulant by precipitating calcium. **Analysis of Incorrect Options:** * **A. Plain tube (Red top):** Contains no anticoagulant or procoagulants (or may have a clot activator). It is used for **serum** collection for chemistry, serology, and immunology. * **B. EDTA tube (Lavender top):** Contains Ethylenediaminetetraacetic acid, which chelates calcium. It is the gold standard for **Hematology (CBC, HbA1c)** because it preserves cell morphology. * **C. Heparin tube (Green top):** Contains Lithium or Sodium Heparin. It works by activating **Antithrombin III**, which neutralizes thrombin. It is used for arterial blood gases (ABG) and certain specialized chemistry tests. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Inhibition:** Sodium fluoride specifically inhibits **Enolase** (requires $Mg^{2+}$); fluoride removes magnesium as a fluorophosphate complex. * **Glucose Preservation:** While NaF inhibits glycolysis, the effect is not immediate (takes 1–2 hours); therefore, immediate centrifugation is still ideal. * **Urea Testing:** Do not use Grey top tubes for Urea estimation via the Urease method, as fluoride inhibits the Urease enzyme, leading to falsely low results. * **Order of Draw:** In a multi-tube collection, the Grey top tube is typically drawn **last** to prevent additive carryover.

Question 42: The Guaiac test is used for which of the following?

A. Pentosuria
B. Fructosuria
C. Detecting occult blood in stool (Correct Answer)
D. Pancreatitis

Explanation: **Explanation:** The **Guaiac test** (also known as the Guaiac Fecal Occult Blood Test or gFOBT) is a diagnostic method used to detect **occult (hidden) blood** in the stool, which is often a sign of gastrointestinal bleeding or colorectal cancer. **Mechanism:** The test relies on the **pseudoperoxidase activity of hemoglobin**. The test paper is impregnated with alpha-guaiaconic acid. When a developer (hydrogen peroxide) is added to the stool sample, the heme portion of hemoglobin acts as a catalyst, oxidizing the guaiac to a blue-colored quinone compound. A positive result is indicated by this rapid color change. **Analysis of Options:** * **A & B (Pentosuria & Fructosuria):** These are metabolic disorders involving reducing sugars. They are typically screened using **Benedict’s test** or **Seliwanoff’s test** (specific for ketoses like fructose), not the Guaiac test. * **D (Pancreatitis):** Acute pancreatitis is diagnosed via clinical presentation and elevated serum **Amylase and Lipase** levels. While stool tests (like Fecal Elastase) are used for chronic pancreatitis, the Guaiac test has no diagnostic role here. **High-Yield Clinical Pearls for NEET-PG:** * **False Positives:** Can be caused by the ingestion of **red meat** (contains animal hemoglobin), peroxidase-rich vegetables (broccoli, cauliflower, horseradish), or NSAIDs. * **False Negatives:** High doses of **Vitamin C (Ascorbic acid)** can cause a false negative by inhibiting the oxidation reaction. * **Modern Alternative:** The **Fecal Immunochemical Test (FIT)** is now preferred over the Guaiac test because it uses antibodies specific to human globin, eliminating the need for dietary restrictions.

Question 43: Which enzyme is considered a marker enzyme of mitochondria?

A. Na+ - K+ ATPase
B. Glutamic dehydrogenase (Correct Answer)
C. Lactate dehydrogenase
D. No specific enzyme

Explanation: **Explanation** Marker enzymes are specific enzymes used to identify and assess the purity of isolated cell organelles during biochemical analysis. **Why Glutamic Dehydrogenase (GDH) is correct:** Glutamic dehydrogenase is a high-yield marker enzyme for the **mitochondrial matrix**. It plays a crucial role in nitrogen metabolism by catalyzing the oxidative deamination of glutamate. While **Succinate Dehydrogenase (SDH)** is the classic marker for the inner mitochondrial membrane, GDH is the definitive marker for the matrix. **Analysis of Incorrect Options:** * **A. Na+ - K+ ATPase:** This is the classic marker enzyme for the **Plasma Membrane**. It maintains the electrochemical gradient across the cell surface. * **C. Lactate Dehydrogenase (LDH):** This is the marker enzyme for the **Cytosol**. It is the terminal enzyme of anaerobic glycolysis. * **D. No specific enzyme:** Incorrect, as almost every organelle has a specific biochemical marker (e.g., Acid phosphatase for Lysosomes, Catalase for Peroxisomes). **High-Yield Clinical Pearls for NEET-PG:** * **Mitochondrial Markers:** * Outer Membrane: Monoamine Oxidase (MAO). * Inner Membrane: Succinate Dehydrogenase (SDH), Cytochrome Oxidase. * Matrix: Glutamic Dehydrogenase (GDH). * **Other Key Markers:** * **Golgi Apparatus:** Galactosyl transferase. * **Endoplasmic Reticulum:** Glucose-6-phosphatase. * **Lysosomes:** Acid phosphatase. * **Nucleus:** DNA Polymerase / RNA Polymerase. * **Clinical Note:** Elevated serum GDH levels can indicate severe liver cell necrosis, as the enzyme is released from the mitochondria into the bloodstream.

Question 44: Benedict test is used for the detection of which substance in urine?

A. Bile salts
B. Bile pigments
C. Reducing sugars (Correct Answer)
D. Ketone bodies

Explanation: **Explanation:** **1. Why Reducing Sugars is Correct:** The Benedict’s test is a semi-quantitative test used to detect **reducing sugars** (such as glucose, fructose, lactose, and maltose) in the urine. The Benedict’s reagent contains **cupric ions ($Cu^{2+}$)** in an alkaline medium. When heated with a reducing sugar, the free aldehyde or ketone group of the sugar reduces the blue cupric ions to insoluble **cuprous oxide ($Cu_2O$)**, which forms a colored precipitate. The color change (Green → Yellow → Orange → Brick Red) indicates the concentration of sugar present. **2. Why Other Options are Incorrect:** * **Bile Salts:** Detected using **Hay’s Sulphur Test**, which relies on the property of bile salts to lower the surface tension of urine. * **Bile Pigments (Bilirubin):** Detected using **Fouchet’s Test**, where barium chloride and Fouchet’s reagent produce a green/blue color (biliverdin). * **Ketone Bodies:** Detected using **Rothera’s Test**, where sodium nitroprusside reacts with acetone or acetoacetate to form a purple/permanganate-colored ring. **3. Clinical Pearls & High-Yield Facts:** * **Specificity:** Benedict’s test is **not specific for glucose**. It can give positive results for other reducing substances like Vitamin C (Ascorbic acid), salicylates, and uric acid. * **Glucose Specificity:** To specifically detect glucose (and rule out other sugars), the **Dipstick method (Glucose Oxidase method)** is used. * **Inborn Errors of Metabolism:** A positive Benedict’s test with a negative Dipstick test suggests the presence of non-glucose reducing sugars like **galactose** (Galactosemia) or **fructose** (Hereditary Fructose Intolerance). * **Sucrose:** It is a **non-reducing sugar** and will give a negative Benedict’s test unless it is first hydrolyzed into glucose and fructose.

Question 45: Atomic absorption Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) is used to measure/detect what?

A. Hb-derivatives
B. Immunoglobulins
C. Organic compounds
D. Trace & metal elements (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Trace & metal elements** **Why it is correct:** Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) is a highly sensitive analytical technique designed specifically for the detection and quantification of **trace elements and metals** (e.g., Lead, Mercury, Arsenic, Copper, Zinc, and Selenium) in biological fluids like blood, serum, or urine. The process involves using an "Inductively Coupled Plasma" (an extremely high-temperature ionized gas) to atomize and ionize the sample. These ions are then separated and measured based on their **mass-to-charge ratio** using a mass spectrometer. It is considered the "gold standard" because it can detect multiple elements simultaneously at extremely low concentrations (parts per trillion). **Why the other options are incorrect:** * **A. Hb-derivatives:** These are typically measured using **Spectrophotometry** or **Co-oximetry**, which rely on the specific light absorption patterns of different hemoglobin forms (e.g., carboxyhemoglobin, methemoglobin). * **B. Immunoglobulins:** These are proteins and are measured using immunological assays such as **ELISA, Nephelometry, or Turbidimetry**, which utilize antigen-antibody complexes. * **C. Organic compounds:** While Mass Spectrometry can be used for organics, it is usually coupled with Gas Chromatography (GC-MS) or Liquid Chromatography (LC-MS). ICP-MS is specifically optimized for inorganic elemental analysis. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** ICP-MS is the preferred method for diagnosing **heavy metal poisoning** (e.g., Lead or Arsenic toxicity). * **Wilson’s Disease:** ICP-MS can be used to accurately measure copper levels in liver biopsy samples or serum. * **Sensitivity:** It is significantly more sensitive than Flame Atomic Absorption Spectroscopy (AAS). * **Key Concept:** Remember: **ICP = Ionization by Plasma; MS = Separation by Mass.** If the question mentions "elements" or "metals," ICP-MS is the top choice.

Question 46: Millon's test is used for the detection of which amino acid?

A. Phenylalanine
B. Cystine
C. Tyrosine (Correct Answer)
D. Tryptophan

Explanation: ### Explanation **Correct Answer: C. Tyrosine** **1. Why Tyrosine is Correct:** Millon’s test is a specific biochemical test used to detect the presence of **Tyrosine**. The reagent used (Millon’s reagent) consists of mercuric nitrate and mercurous nitrate dissolved in concentrated nitric acid. The underlying principle is the **nitration of the phenol group** present in the side chain of Tyrosine. When the reagent is added to a solution containing Tyrosine and heated, a white precipitate forms, which subsequently turns **brick-red** due to the formation of a mercury complex of nitrated Tyrosine. **2. Why Other Options are Incorrect:** * **A. Phenylalanine:** Although it is an aromatic amino acid, it lacks the reactive **hydroxyl (-OH) group** on the benzene ring (phenol group) required to react with Millon’s reagent. It is typically detected using the Xanthoproteic test (though less reactive than Tyrosine). * **B. Cystine:** This is a sulfur-containing amino acid. It is detected using the **Lead Acetate test**, which produces a black precipitate of lead sulfide. * **D. Tryptophan:** This amino acid contains an **indole ring**. It is specifically detected using the **Hopkins-Cole test** (Glyoxylic acid test), which produces a characteristic violet/purple ring. **3. NEET-PG High-Yield Clinical Pearls:** * **Xanthoproteic Test:** Detects aromatic amino acids (Tyrosine, Tryptophan) by reacting with nitric acid to give a yellow color. * **Sakaguchi Test:** Specific for **Arginine** (detects the guanidino group). * **Pauly’s Test:** Specific for **Histidine** and Tyrosine (detects the imidazole and phenol rings). * **Ninhydrin Test:** A general test for all alpha-amino acids, yielding a **Ruhemann’s purple** color (except Proline, which gives a yellow color). * **Clinical Correlation:** Tyrosine is the precursor for catecholamines (Dopamine, Epinephrine, Norepinephrine), Thyroid hormones (T3, T4), and Melanin. Deficiency of the enzyme *Tyrosinase* leads to Albinism.

Question 47: DNA fragments formed by the action of Restriction endonucleases are separated by which technique?

A. Gel electrophoresis
B. Agarose gel electrophoresis (Correct Answer)
C. Paper chromatography
D. High pressure liquid chromatography

Explanation: **Explanation:** **Why Agarose Gel Electrophoresis is the Correct Answer:** DNA fragments are negatively charged due to their phosphate backbone. When subjected to an electric field, they migrate toward the positive electrode (anode). **Agarose gel electrophoresis** is the standard technique used to separate DNA fragments based on their size. The agarose matrix acts as a molecular sieve; smaller DNA fragments move faster and further through the pores than larger ones. Since restriction endonucleases cut DNA into fragments of varying lengths, this technique is essential for visualizing and identifying those specific fragments. **Analysis of Incorrect Options:** * **Gel electrophoresis (Option A):** While technically correct, it is a general term. In the context of NEET-PG, "Agarose gel electrophoresis" is the **more specific and superior answer** for DNA. Polyacrylamide gel electrophoresis (PAGE) is typically reserved for proteins or very small DNA sequences (sequencing). * **Paper chromatography (Option C):** This technique separates substances based on solubility and adsorption. It is used for amino acids and sugars, not for large, charged macromolecules like DNA fragments. * **High-pressure liquid chromatography (Option D):** HPLC is primarily used for the quantitative analysis and purification of small molecules, drugs, and metabolites. It is not the standard method for separating restriction-digested DNA fragments. **High-Yield Clinical Pearls for NEET-PG:** * **Staining:** DNA on an agarose gel is visualized using **Ethidium Bromide (EtBr)**, which fluoresces orange under UV light. * **Southern Blotting:** After electrophoresis, DNA can be transferred to a nitrocellulose membrane for hybridization; this entire process is known as Southern Blotting (Mnemonic: **S**outhern = **D**NA, **N**orthern = **R**NA, **W**estern = **P**rotein). * **Pulsed-field gel electrophoresis (PFGE):** Used for separating exceptionally large DNA fragments (e.g., whole chromosomes).

Question 48: Which of the following methods is MOST sensitive for glucose estimation?

A. GOD POD method (Correct Answer)
B. O-Toluidine method
C. Folin-Wu method
D. Hexokinase method

Explanation: **Explanation:** The estimation of blood glucose is a fundamental biochemical investigation. The **GOD-POD (Glucose Oxidase-Peroxidase) method** is considered highly sensitive and specific because it utilizes a coupled enzymatic reaction. 1. **Glucose Oxidase (GOD)** specifically oxidizes $\beta$-D-glucose to gluconic acid and hydrogen peroxide ($H_2O_2$). 2. **Peroxidase (POD)** then breaks down $H_2O_2$ to release nascent oxygen, which reacts with a chromogen (like 4-aminophenazone) to form a colored quinoneimine complex. The intensity of the color is directly proportional to the glucose concentration. Its high sensitivity stems from the enzymatic specificity for glucose, minimizing interference from other substances. **Analysis of Incorrect Options:** * **Hexokinase Method:** While this is the **"Reference Method"** (Gold Standard) due to its extreme accuracy and precision, in many clinical laboratory contexts, the GOD-POD method is preferred for routine high-sensitivity screening due to cost-effectiveness and ease of automation. * **O-Toluidine Method:** A condensation method where glucose reacts with aromatic amines in glacial acetic acid. It is specific but uses corrosive reagents and is less sensitive than enzymatic methods. * **Folin-Wu Method:** A classic copper reduction method. It is the **least specific** because it measures all "reducing sugars" and non-sugar reducing substances (like glutathione and uric acid), leading to falsely elevated values. **High-Yield Clinical Pearls for NEET-PG:** * **Specimen Collection:** Use **Fluoride vials** (Grey top). Sodium fluoride inhibits **enolase**, preventing glycolysis, while Potassium Oxalate acts as the anticoagulant. * **Gold Standard:** Hexokinase method. * **Most Specific:** GOD-POD (specifically for $\beta$-D-glucose). * **Renal Threshold for Glucose:** Approximately **180 mg/dL**. When blood glucose exceeds this, glucosuria occurs.

Question 49: Which of the following statements is true about polymerase chain reaction (PCR)?

A. It is carried out by thermostable DNA polymerase.
B. It results in exponential amplification of DNA.
C. It is a specific method for amplifying target DNA sequences.
D. All of the above. (Correct Answer)

Explanation: **Explanation:** Polymerase Chain Reaction (PCR) is an *in vitro* enzymatic method used to amplify specific target DNA sequences. It is a cornerstone of molecular biology and clinical diagnostics. 1. **Thermostable DNA Polymerase (Option A):** PCR involves repeated cycles of high-temperature heating (denaturation at ~95°C). Standard DNA polymerases would denature at these temperatures. Therefore, PCR utilizes heat-stable enzymes like **Taq polymerase** (derived from *Thermus aquaticus*), which remains functional throughout the thermal cycling. 2. **Exponential Amplification (Option B):** Each cycle of PCR theoretically doubles the amount of target DNA ($2^n$, where $n$ is the number of cycles). After 30 cycles, a single copy of DNA can be amplified into billions of copies, making it highly sensitive. 3. **Specificity (Option C):** The specificity of PCR is determined by **synthetic oligonucleotide primers**. These primers are designed to be complementary only to the flanking regions of the specific target DNA sequence, ensuring that only the desired segment is amplified. Since all three statements accurately describe the fundamental principles of PCR, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Steps of PCR:** Denaturation (95°C) → Annealing (50-65°C) → Extension (72°C). * **RT-PCR:** Uses Reverse Transcriptase to convert RNA into cDNA before amplification (Gold standard for **COVID-19/SARS-CoV-2** diagnosis). * **Real-Time PCR (qPCR):** Allows for the quantification of DNA as the reaction progresses using fluorescent dyes (e.g., SYBR Green). * **Applications:** Diagnosis of genetic mutations (e.g., Sickle cell anemia), detection of infectious agents (HIV, TB), and forensic DNA profiling.

Question 50: If the optical density is 2, what percentage of light is transmitted?

A. 100%
B. 10%
C. 1% (Correct Answer)
D. 0.10%

Explanation: ### Explanation **Concept and Calculation:** Optical Density (OD), also known as **Absorbance (A)**, is a logarithmic measurement of how much light is absorbed by a solution. It is mathematically defined by the **Beer-Lambert Law** using the formula: $$A = \log_{10} \left(\frac{1}{T}\right) \quad \text{or} \quad A = 2 - \log_{10}(\%T)$$ Where **T** is Transmittance (expressed as a fraction) and **%T** is the percentage of light transmitted. Given **OD = 2**: $$2 = \log_{10} \left(\frac{1}{T}\right)$$ $$10^2 = \frac{1}{T} \implies 100 = \frac{1}{T}$$ $$T = 0.01$$ To find the percentage: $0.01 \times 100 = \mathbf{1\%}$. **Analysis of Options:** * **Option C (1%):** Correct. An OD of 2 means the solution is dense enough to absorb 99% of the incident light, allowing only 1/100th (1%) to pass through. * **Option A (100%):** Incorrect. This occurs when **OD = 0** (no light is absorbed), typical of a "blank" solution. * **Option B (10%):** Incorrect. This corresponds to an **OD of 1** ($\log_{10}(10) = 1$). * **Option D (0.10%):** Incorrect. This corresponds to an **OD of 3** ($\log_{10}(1000) = 3$). **Clinical Pearls for NEET-PG:** 1. **Inverse Relationship:** Absorbance and Transmittance are inversely related; as a solution becomes more concentrated (higher OD), less light is transmitted. 2. **Linearity:** According to Beer’s Law, Absorbance is directly proportional to the concentration of the solute. This is the principle used in clinical chemistry to measure blood glucose, urea, and creatinine. 3. **High-Yield Values:** * OD 0 = 100% Transmission * OD 1 = 10% Transmission * OD 2 = 1% Transmission * OD 3 = 0.1% Transmission

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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