Biochemical Techniques Practice Questions

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

Grey top tube. **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.

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

Detecting occult blood in stool. **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.

Q: Which enzyme is considered a marker enzyme of mitochondria?

Glutamic dehydrogenase. **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.

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

Reducing sugars. **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.

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

GOD POD method. **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.

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

All of the above.. **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.

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

1%. ### 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

Question 1

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

Accepted Answer

Grey top tube

Answer

Plain tube

Answer

EDTA tube

Answer

Heparin tube

Question 2

The Guaiac test is used for which of the following?

Accepted Answer

Detecting occult blood in stool

Answer

Pentosuria

Answer

Fructosuria

Answer

Pancreatitis

Question 3

Which enzyme is considered a marker enzyme of mitochondria?

Accepted Answer

Glutamic dehydrogenase

Answer

Na+ - K+ ATPase

Answer

Lactate dehydrogenase

Answer

No specific enzyme

Question 4

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

Accepted Answer

Reducing sugars

Answer

Bile salts

Answer

Bile pigments

Answer

Ketone bodies

Question 5

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

Accepted Answer

Trace & metal elements

Answer

Hb-derivatives

Answer

Immunoglobulins

Answer

Organic compounds

Question 6

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

Accepted Answer

Tyrosine

Answer

Phenylalanine

Answer

Cystine

Answer

Tryptophan

Question 7

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

Accepted Answer

Agarose gel electrophoresis

Answer

Gel electrophoresis

Answer

Paper chromatography

Answer

High pressure liquid chromatography

Question 8

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

Accepted Answer

GOD POD method

Answer

O-Toluidine method

Answer

Folin-Wu method

Answer

Hexokinase method

Question 9

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

Accepted Answer

All of the above.

Answer

It is carried out by thermostable DNA polymerase.

Answer

It results in exponential amplification of DNA.

Answer

It is a specific method for amplifying target DNA sequences.

Question 10

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

Accepted Answer

1%

Answer

100%

Answer

10%

Answer

0.10%

Biochemical Techniques — MCQs

Biochemical Techniques — MCQs

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