Biochemical Techniques Practice Questions

Q: Which anticoagulant used in blood glucose estimation prevents glycolysis?

Sodium fluoride. **Explanation:** The correct answer is **Sodium fluoride**. In blood glucose estimation, it is crucial to prevent the consumption of glucose by red blood cells (glycolysis) after the sample is drawn. **Mechanism of Action:** Sodium fluoride acts as a **glycolytic inhibitor**. It specifically inhibits the enzyme **Enolase** in the glycolytic pathway by forming a complex with magnesium and phosphate. Since Enolase is required to convert 2-phosphoglycerate to phosphoenolpyruvate, its inhibition halts the breakdown of glucose. Sodium fluoride is typically used in combination with **Potassium oxalate** (which acts as the anticoagulant by chelating calcium). **Analysis of Incorrect Options:** * **EDTA (Ethylenediaminetetraacetic acid):** Primarily used for Hematology (CBC) and HbA1c. It chelates calcium to prevent clotting but does not inhibit glycolytic enzymes. * **Heparin:** An indirect thrombin inhibitor used for arterial blood gases (ABG) and electrolyte analysis. It does not prevent glucose degradation. * **Sodium citrate:** Used for coagulation studies (PT/aPTT) and ESR (Westergren method). It chelates calcium but has no effect on glycolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Gray-top vacutainer:** Contains Sodium fluoride and Potassium oxalate; used specifically for glucose and lactate estimation. * **Rate of Glycolysis:** At room temperature, glucose levels in a blood sample decrease by approximately **5–7% per hour** if a preservative is not used. * **HbA1c Exception:** For Glycated Hemoglobin (HbA1c) estimation, **EDTA** is the preferred anticoagulant, not fluoride, as the test measures glucose attached to hemoglobin over time rather than free plasma glucose.

Q: Which of the following techniques is used for the detection of variation in DNA sequence and gene expression?

Microarray. **Explanation:** The correct answer is **Microarray**. This technique is uniquely capable of analyzing thousands of genes simultaneously. It utilizes a solid surface (chip) containing microscopic spots of DNA probes. When labeled sample DNA or cDNA binds to these probes, it allows for the detection of **Single Nucleotide Polymorphisms (SNPs)**—representing variations in DNA sequence—and the quantification of mRNA levels, which reflects **gene expression**. **Analysis of Options:** * **A. Western blot:** Used specifically for the detection and quantification of **proteins** using antibody-antigen interactions. It does not analyze DNA or RNA. * **B. Northern blot:** Used for the detection of specific **RNA** sequences to study gene expression. However, it cannot detect DNA sequence variations (mutations/polymorphisms) and typically analyzes one gene at a time. * **C. Southern blot:** Used for the detection of specific **DNA** sequences (e.g., RFLP). While it detects DNA variations, it cannot measure gene expression (mRNA). * **D. Microarray:** The "high-throughput" nature of microarrays allows for the simultaneous assessment of both structural variations (DNA) and functional activity (Expression). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic (SNoW DRoP):** **S**outhern-**D**NA; **N**orthern-**R**NA; **W**estern-**P**rotein. * **Southwestern Blot:** Used to detect **DNA-binding proteins** (e.g., transcription factors like c-Jun or c-Fos). * **Clinical Use of Microarray:** Widely used in oncology (to categorize tumors based on gene expression profiles) and in prenatal testing to detect submicroscopic chromosomal deletions or duplications.

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

Beta-hydroxybutyrate. ### 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).

Q: Which vacutainer is used for electrolyte estimation?

Lithium heparin. **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.

Q: What is the probe used in a Western blot?

Antibody. **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.

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

Site-directed mutagenesis. **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.

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

ELISA. **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).

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.

Question 1

Which anticoagulant used in blood glucose estimation prevents glycolysis?

Accepted Answer

Sodium fluoride

Answer

EDTA

Answer

Heparin

Answer

Sodium citrate

Question 2

Which of the following techniques is used for the detection of variation in DNA sequence and gene expression?

Accepted Answer

Microarray

Answer

Western blot

Answer

Northern blot

Answer

Southern blot

Question 3

Nephelometry is a technique used in the measurement of:

Accepted Answer

Immunoglobulins

Answer

Refraction of light

Answer

Size of renal stones

Answer

Optical density of fluids

Question 4

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

Accepted Answer

Beta-hydroxybutyrate

Answer

Acetoacetate

Answer

Acetone

Answer

All the above can be detected

Question 5

Which vacutainer is used for electrolyte estimation?

Accepted Answer

Lithium heparin

Answer

Na Citrate

Answer

EDTA

Answer

Fluoride

Question 6

What is the probe used in a Western blot?

Accepted Answer

Antibody

Answer

mRNA

Answer

DNA

Answer

tRNA

Question 7

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

Accepted Answer

Site-directed mutagenesis

Answer

Electrophoresis

Answer

Chromatography

Answer

Mass spectrometry

Question 8

Nanotechnology has found tremendous application in the diagnosis of cancers because of all of the following advantages, except?

Accepted Answer

Nanocrystals exhibit a narrow difference between their excitation and emission peak spectra

Answer

Nanocrystals exhibit bright, photostable fluorescence

Answer

Nanocrystals have a narrow spectrum wavelength

Answer

Peak spectrum wavelength is tunable

Question 9

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

Accepted Answer

ELISA

Answer

Southern blot

Answer

Northern blot

Answer

Western blot

Question 10

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

Biochemical Techniques — MCQs

Biochemical Techniques — MCQs

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