Pulsed-Field Gel Electrophoresis — MCQs
Techniques used for protein expression proteomics study include:
Which of the following bacteria can be identified using phage typing?
Which PCR technique is best suited for identifying a syndrome with multiple causative agents?
Which of the following separates proteins solely on the basis of their molecular size?
Pulsed gel electrophoresis is used for:
What is the technique for accurate quantification of gene expression?
A child presented with bloody stools and abdominal pain. Which enrichment medium should be used for processing the fecal sample?
In the context of clinical biochemistry, which isoenzyme of lactate dehydrogenase (LDH) has the maximum electrophoretic mobility?
A patient presents with bloody diarrhea after eating undercooked meat. Which bacterial pathogen is most likely?
A person returns to Delhi from Bangladesh after 2 days and has diarrhea. Stool examination shows RBC's in stool. The likely organism causing is ?
Pulsed-Field Gel Electrophoresis Indian Medical PG Practice Questions and MCQs
Question 1: Techniques used for protein expression proteomics study include:
- A. PolyAcrylamide Gel Electrophoresis (PAGE)
- B. Gene Expression Analysis (indirectly related to proteomics)
- C. Mass Spectrometry
- D. All of the options (Correct Answer)
Explanation: ***All of the options*** - All listed techniques—**Polyacrylamide Gel Electrophoresis (PAGE)**, **Gene Expression Analysis**, and **Mass Spectrometry**—are used in protein expression proteomics studies, either directly or indirectly, to analyze and quantify proteins. - The integration of these various techniques provides a comprehensive approach to understanding protein expression profiles. *PolyAcrylamide Gel Electrophoresis (PAGE)* - **PAGE** (including 1D and 2D-PAGE) is a fundamental technique for separating proteins based on their **molecular weight** and **isoelectric point**, which is crucial for visualizing and quantifying expressed proteins. - It often serves as an initial separation step before more detailed analysis, such as **mass spectrometry**. *Gene Expression Analysis (indirectly related to proteomics)* - Although **gene expression analysis** (e.g., using **RT-PCR** or **microarrays**) measures mRNA levels, it is indirectly related to proteomics because mRNA levels often **correlate with protein levels**. - It provides insights into the **transcriptional regulation** that influences protein expression, complementing direct protein analysis. *Mass Spectrometry* - **Mass spectrometry** is a powerful and widely used technique in proteomics for **identifying, quantifying, and characterizing proteins** and peptides by measuring their **mass-to-charge ratio**. - It can be used for both **discovery proteomics** (identifying novel proteins) and **targeted proteomics** (quantifying specific proteins).
Question 2: Which of the following bacteria can be identified using phage typing?
- A. Salmonella (Correct Answer)
- B. Streptococcus
- C. Shigella
- D. Pseudomonas
Explanation: ***Salmonella*** - **Phage typing** is the most classical and widely used method for subtyping *Salmonella* species based on their susceptibility to specific bacteriophages. - This technique is extensively used in **epidemiological tracking** of outbreaks, particularly for *Salmonella typhi* and *S. enteritidis*, allowing differentiation of strains within the same species. - **Most commonly associated organism** with phage typing in clinical microbiology and medical education. *Streptococcus* - While phage typing was historically used for *Streptococcus pyogenes* (Group A Streptococcus), it has largely been replaced by **molecular typing methods** and is not the primary association when phage typing is mentioned. - *Streptococcus* species are typically identified through **Gram staining**, **catalase tests**, **hemolysis patterns** on blood agar, and **serological methods** (Lancefield grouping). *Shigella* - **Phage typing** is not routinely used for *Shigella* identification or subtyping in clinical practice. - **Serotyping** and **molecular methods** like pulse-field gel electrophoresis (PFGE) or whole genome sequencing are the standard approaches for *Shigella* differentiation. *Pseudomonas* - While **phage typing** has been used for *Pseudomonas aeruginosa* in research and outbreak investigations, it is not the primary or most commonly cited example of phage typing in medical education. - Routine identification relies on **culture characteristics** (pigment production, oxidase positive), **biochemical tests**, and **molecular methods**.
Question 3: Which PCR technique is best suited for identifying a syndrome with multiple causative agents?
- A. RT-PCR
- B. Multiplex PCR (Correct Answer)
- C. Nested PCR
- D. Conventional PCR
Explanation: ***Multiplex PCR*** - **Multiplex PCR** allows for the simultaneous amplification of **multiple DNA targets** in a single reaction, making it ideal for identifying syndromes with numerous potential causative agents. - This method uses **multiple primer pairs** in one reaction tube, each designed to amplify a specific target sequence, thus efficiently detecting various pathogens or genetic markers. *RT-PCR* - **Reverse Transcription PCR (RT-PCR)** is used to detect **RNA targets** by first converting RNA into cDNA, which is then amplified. - While useful for RNA viruses or gene expression studies, it is not primarily designed for simultaneous detection of multiple diverse causative agents in the same way as multiplex PCR. *Nested PCR* - **Nested PCR** uses two sets of primers in sequential reactions to **increase sensitivity and specificity** by reducing non-specific binding. - This technique is generally employed to detect very low copies of a specific target or to overcome issues with non-specific amplification, rather than for identifying multiple different agents concurrently. *Conventional PCR* - **Conventional PCR** amplifies a **single specific DNA target** using one pair of primers per reaction. [1] - It requires separate reactions for each potential causative agent, making it inefficient and labor-intensive when testing for a syndrome with multiple etiologies. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 56-57.
Question 4: Which of the following separates proteins solely on the basis of their molecular size?
- A. Isoelectric focusing
- B. Chromatography on a diethylaminoethyl (DEAE) cellulose column
- C. Gel filtration chromatography (Correct Answer)
- D. Chromatography on a carboxymethyl (CM) cellulose column
Explanation: ***Gel filtration chromatography*** - Also known as **size-exclusion chromatography**, this method separates proteins based on their **hydrodynamic volume** (molecular size and shape). - Larger proteins pass through the column more quickly because they are excluded from the pores of the stationary phase, while smaller proteins enter the pores and have a longer, more tortuous path. *Isoelectric focusing* - This technique separates proteins based on their **isoelectric point (pI)**, which is the pH at which the protein has no net electrical charge. - Proteins migrate through a pH gradient until they reach the point where their net charge is zero. *Chromatography on a diethylaminoethyl (DEAE) cellulose column* - **DEAE cellulose** is an **anion-exchange resin**, meaning it binds **negatively charged** proteins. - Separation is based on the **net charge** of the protein at a given pH. *Chromatography on a carboxymethyl (CM) cellulose column* - **CM cellulose** is a **cation-exchange resin**, meaning it binds **positively charged** proteins. - Separation is based on the **net charge** of the protein at a given pH.
Question 5: Pulsed gel electrophoresis is used for:
- A. RNA
- B. Protein
- C. Ribosome
- D. DNA (Correct Answer)
Explanation: ***DNA*** - **Pulsed-field gel electrophoresis (PFGE)** is a specialized technique used to separate **large DNA molecules** that are difficult to resolve using conventional gel electrophoresis. - It involves periodically changing the direction of the electric field, which forces large DNA molecules to reorient themselves, allowing for better separation based on size up to **10 Mb**. *RNA* - While conventional gel electrophoresis (e.g., agarose or polyacrylamide gels) can be used to separate **RNA molecules**, PFGE is not typically employed for RNA. - RNA molecules are generally much smaller than the large DNA fragments for which PFGE is designed, and their secondary structures can interfere with pulsed-field separation. *Protein* - **Proteins** are separated using different types of electrophoresis, such as **SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis)**, which separates proteins primarily by size. - PFGE is specifically designed for nucleic acids, particularly very large DNA, and is not suitable for protein separation. *Ribosome* - **Ribosomes** are large macromolecular complexes composed of ribosomal RNA and proteins. - Techniques like **sucrose gradient centrifugation** or specialized gel electrophoresis (e.g., non-denaturing agarose gels for intact ribosomes) are used to separate ribosomes, not PFGE.
Question 6: What is the technique for accurate quantification of gene expression?
- A. PCR
- B. Real-Time Reverse Transcriptase PCR (Correct Answer)
- C. Reverse Transcriptase PCR
- D. Northern blot
Explanation: ***Real-Time Reverse Transcriptase PCR*** - This technique allows for the **quantification of gene expression** by concurrently reverse-transcribing RNA to cDNA and amplifying it while monitoring the accumulation of DNA in real-time using fluorescent reporters. - The ** threshold cycle (Ct) value** is inversely proportional to the initial amount of target mRNA, enabling precise quantification. *Northern blot* - This method is used to detect **RNA sequences** and can provide semi-quantitative data about gene expression levels based on band intensity. - However, it is generally **less sensitive** and provides less precise quantification compared to real-time PCR. *PCR* - **Standard PCR** amplifies DNA, but it is not directly used for gene expression quantification as it starts with DNA templates. - While it can be used to detect the presence of a gene, it does not quantify its expression without further modifications or additional steps like reverse transcription and real-time monitoring. *Reverse Transcriptase PCR* - This technique involves **reverse transcribing RNA into cDNA** and then performing standard PCR to amplify the cDNA. - While it confirms the presence of mRNA and allows for cDNA amplification, it is a **qualitative or semi-quantitative** method for expression, as the endpoint detection does not accurately reflect initial mRNA concentration due to plateau effects.
Question 7: A child presented with bloody stools and abdominal pain. Which enrichment medium should be used for processing the fecal sample?
- A. Blood agar
- B. Selenite F broth (Correct Answer)
- C. Alkaline peptone water
- D. Muller Hinton Broth
Explanation: ***Selenite F broth*** - This **enrichment medium** is specifically designed to isolate **Salmonella** and some species of **Shigella**, which are common causes of bloody stools and abdominal pain in children. - It inhibits the growth of commensal gut flora, allowing pathogenic bacteria to proliferate and be subsequently identified on selective media. *Blood agar* - Blood agar is a **general-purpose enrichment medium** that supports the growth of a wide range of bacteria but does not selectively enrich for specific pathogens. - It would be ineffective in outcompeting the normal fecal flora to isolate rarer enteric pathogens causing the symptoms. *Alkaline peptone water* - This medium is primarily used for the enrichment of **Vibrio cholerae** species, which typically cause watery diarrhea, not bloody stools. - While it helps in the isolation of *Vibrio* species, it is not suitable for the suspected pathogens in this clinical scenario. *Muller Hinton Broth* - Muller-Hinton media are primarily used for **antimicrobial susceptibility testing** (antibiotic sensitivity testing) and are not designed for the primary isolation or enrichment of specific pathogens from clinical samples. - It would not provide a selective advantage for the recovery of organisms causing bloody diarrhea from a fecal sample.
Question 8: In the context of clinical biochemistry, which isoenzyme of lactate dehydrogenase (LDH) has the maximum electrophoretic mobility?
- A. LDH-1 (Correct Answer)
- B. LDH-5
- C. LDH-2
- D. LDH-3
Explanation: ***LDH-1*** - **LDH-1** is composed of four **H subunits** (HHHH) and is the most **anionic** form of lactate dehydrogenase. - Due to its high negative charge, it migrates fastest towards the **anode** during electrophoresis. *LDH-5* - **LDH-5** is composed of four **M subunits** (MMMM) and is the least **anionic** form. - It has the **lowest electrophoretic mobility**, migrating slowest towards the anode or even towards the cathode. *LDH-2* - **LDH-2** consists of three **H subunits** and one **M subunit** (HHHM). - It has the second-highest electrophoretic mobility, positioned after LDH-1. *LDH-3* - **LDH-3** is composed of two **H subunits** and two **M subunits** (HHMM). - Its electrophoretic mobility is intermediate, falling between LDH-2 and LDH-4.
Question 9: A patient presents with bloody diarrhea after eating undercooked meat. Which bacterial pathogen is most likely?
- A. Salmonella typhi
- B. Shigella dysenteriae
- C. Vibrio cholerae
- D. Escherichia coli O157:H7 (Correct Answer)
Explanation: ***Escherichia coli O157:H7*** - **Enterohemorrhagic E. coli (EHEC)** is the classic cause of **bloody diarrhea** following consumption of **undercooked ground beef** or hamburgers, producing **Shiga toxins** that cause **hemorrhagic colitis**. - Can lead to serious complications like **hemolytic uremic syndrome (HUS)**, particularly in children and elderly patients. *Salmonella typhi* - **Salmonella typhi** specifically causes **typhoid fever** with sustained fever, headache, and rose spots, not acute bloody diarrhea from undercooked meat. - While non-typhoidal Salmonella can cause gastroenteritis, it typically produces **non-bloody diarrhea** and is more associated with poultry and eggs. *Shigella dysenteriae* - Although it causes **bloody diarrhea** and severe **dysentery**, transmission is primarily **person-to-person** through the fecal-oral route or contaminated water. - Not typically associated with **undercooked meat consumption** but rather with poor sanitation and contaminated produce. *Vibrio cholerae* - Causes **cholera** with characteristic profuse, **watery diarrhea** ("rice-water stools") leading to severe dehydration, not bloody diarrhea. - Transmission occurs through contaminated **water and seafood**, not undercooked meat.
Question 10: A person returns to Delhi from Bangladesh after 2 days and has diarrhea. Stool examination shows RBC's in stool. The likely organism causing is ?
- A. Enteropathogenic E. Coli
- B. Salmonella typhi
- C. Enterotoxigenic E. Coli
- D. Shigella dysenteriae (Correct Answer)
Explanation: ***Shigella dysenteriae*** - This organism causes **bacillary dysentery**, characterized by fever, abdominal cramps, and frequent, small-volume stools containing **blood and mucus (RBCs)**, which fits the clinical picture of a traveler experiencing diarrhea with RBCs in stool. - The rapid onset (within 2 days of return) and the presence of **RBCs in stool** are highly suggestive of an invasive bacterial pathogen like *Shigella*. *Enteropathogenic E. Coli* - **EPEC** typically causes **watery diarrhea**, particularly in infants, by disrupting intestinal microvilli. - It does **not usually cause bloody stools** or significant red blood cells in the stool. *Salmonella typhi* - While *Salmonella typhi* causes **typhoid fever**, its initial presentation is typically with fever, headache, and constipation, followed by a **pea-soup diarrhea** in later stages, which is usually not bloody. - The rapid onset of bloody diarrhea within 2 days is less characteristic of typhoid fever, which has a longer incubation period. *Enterotoxigenic E. Coli* - **ETEC** is a common cause of **traveler's diarrhea**, but it produces toxins that lead to **watery, non-bloody diarrhea**. - The presence of **RBCs in the stool** makes ETEC an unlikely cause in this scenario.
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