FMGE 2025 — Microbiology

Q: Patient with HIV presenting with bouts of cough and fever. Which medium is appropriate for diagnosis?

Lowenstein-Jensen (LJ) medium. ***Lowenstein-Jensen (LJ) medium*** - This egg-based culture medium is the standard solid medium used globally for the isolation of **Mycobacterium tuberculosis**. - Given the patient's history of **HIV** and symptoms of cough and fever, **Pulmonary Tuberculosis (TB)** is the differential diagnosis most likely requiring this specialized medium. *Tinsdale media* - Tinsdale medium is specifically used for the primary isolation and identification of **Corynebacterium diphtheriae**. - The characteristic presentation of **diphtheria** (pseudomembrane, severe pharyngitis) differs significantly from the suspected chronic respiratory symptoms of TB. *MacConkey agar* - This is a selective and differential medium designed to isolate and differentiate **Gram-negative enteric bacilli** (e.g., *E. coli*, *Salmonella*). - It is completely inadequate for the isolation of the slow-growing, **acid-fast bacilli** characteristic of mycobacteria. *Loeffler serum slope (LSS)* - Loeffler serum slope is an enrichment medium primarily used to promote the growth and demonstration of characteristic morphology in **Corynebacterium diphtheriae**. - While sometimes used for initial inoculum, it is not the definitive medium required for the isolation of **Mycobacterium tuberculosis** due to its composition and intended purpose. [Practice more Microbiology PYQs on OnCourse]

Q: A firefighter was admitted with fever, cough, culture shown in the image reveals gram-negative, oxidase-positive bacilli. Colonies transilluminating while passing under UV light. Choose the correct agent.

Pseudomonas aeruginosa. ***Pseudomonas aeruginosa*** - This organism is a **gram-negative**, **oxidase-positive** bacillus, matching the description. The image shows growth with a characteristic green pigment (**pyocyanin** and **pyoverdin**). - The key identifier is the production of **pyoverdin**, a fluorescent siderophore, which causes the colonies to transilluminate under UV light. *Klebsiella* - *Klebsiella* is **oxidase-negative**, which rules it out based on the biochemical test results provided. - It typically produces large, **mucoid colonies** due to its prominent capsule and is a lactose fermenter, features not described here. *Staphylococcus aureus* - *Staphylococcus aureus* is a **gram-positive coccus**, not a gram-negative bacillus, making it incorrect based on the initial microscopy. - It is also **oxidase-negative** and characteristically forms opaque, often golden-yellow colonies, not greenish fluorescent ones. *Serratia* - *Serratia* is a gram-negative bacillus but is **oxidase-negative**, which is inconsistent with the findings. - Certain species like *Serratia marcescens* are known for producing a red pigment called **prodigiosin**, especially at room temperature, not a green fluorescent pigment. [Practice more Microbiology PYQs on OnCourse]

Q: A firefighter with fever and cough has a culture showing gram-negative, oxidase-positive bacilli with fluorescent colonies under UV light. Which of the following organisms is most likely causing the infection?

Pseudomonas aeruginosa. ***Pseudomonas aeruginosa***- This organism is a non-lactose fermenting, **Gram-negative bacillus** that is definitively **oxidase-positive**.- It characteristically produces the siderophore **pyoverdine**, which is fluorescent under UV light, confirming the most likely etiology.*Klebsiella*- *Klebsiella* species are **oxidase-negative** (**distinguishing factor**) and typically highly encapsulated, often causing consolidation in pneumonia.- They are also strong **lactose fermenters** and do not produce fluorescent pigments.*Staphylococcus aureus*- *S. aureus* is a **Gram-positive coccus** (stains purple) arranged in clusters, not a Gram-negative bacillus (stains pink).- It is non-motile, shows no fluorescence, and is identified by being coagulase and catalase positive.*Serratia*- *Serratia marcescens* is generally **oxidase-negative** (unlike the clinical finding) and belongs to the Enterobacteriaceae family.- Under room temperature, *Serratia* often produces a characteristic **red pigment** (**prodigiosin**), not a fluorescent pigment. [Practice more Microbiology PYQs on OnCourse]

Q: The toughness of bacterial cell walls is due to:

Peptidoglycan. ***Peptidoglycan***- This unique polymer forms a robust, mesh-like structural layer (the **murein layer**) that encases the bacteria, providing essential mechanical strength and rigidity to the cell wall. Its strong cross-links allow it to withstand extremely high internal **osmotic pressure**, preventing the cell from bursting (lysis). *Teichoic acid* - These are polymers found exclusively in **Gram-positive** bacteria, anchored either to the peptidoglycan or the plasma membrane (lipoteichoic acid). - They contribute to the cell wall's overall integrity, and act as antigens, but are not the primary element responsible for the physical **toughness**. *Lipopolysaccharides* - LPS molecules make up the outer leaflet of the **outer membrane** in **Gram-negative** bacteria. - They function primarily as an **endotoxin** (lipid A component) and an O-antigen, contributing to virulence and permeability, not the mechanical load-bearing structure. *Outer membrane* - This membrane is characteristic of **Gram-negative** bacteria and acts as a protective, selective barrier (excluding certain antibiotics and chemicals). - While providing protective resistance, the intrinsic mechanical **toughness** preventing osmotic lysis is conferred by the underlying **peptidoglycan** layer. [Practice more Microbiology PYQs on OnCourse]

Q: A group of students consumed fried rice in a restaurant. Six hours later, they start vomiting. What is the etiological agent?

Bacillus cereus. ***Bacillus cereus***- The short incubation period (1 to 6 hours) and the primary symptom being **vomiting** are highly characteristic of the **emetic-type** food poisoning caused by *B. cereus*. - The emetic form is typically associated with contaminated **cooked rice** (like fried rice) that has been improperly stored, allowing the bacteria to proliferate and produce the heat-stable toxin, **cereulide**. *Clostridium botulinum* - This bacterium causes **botulism**, a severe neuroparalytic syndrome (e.g., descending flaccid paralysis, diplopia), not typically an acute emetic gastroenteritis. - The incubation period for botulism is usually longer (12 to 36 hours), and it is most often linked to improperly canned or vacuum-packed foods. *Escherichia coli* - Diarrheagenic *E. coli* strains typically require a longer incubation period (usually 1 or more days) and cause symptoms dominated by **diarrhea** (either watery or bloody), with vomiting being a less prominent feature. - Common infectious sources are often fecally contaminated water or uncooked produce, rather than cooked and cooled rice. *Shigella dysentriae* - *S. dysentriae* causes **bacillary dysentery**, characterized by a longer incubation period (1 to 7 days) and symptoms including fever, severe abdominal cramping, and **bloody, mucoid stools** (dysentery). - While systemic symptoms might occur, the rapid onset of vomiting alone is not characteristic of Shigellosis. [Practice more Microbiology PYQs on OnCourse]

26 Previous Year Questions with Answers & Explanations

Questions

Patient with HIV presenting with bouts of cough and fever. Which medium is appropriate for diagnosis?

A firefighter was admitted with fever, cough, culture shown in the image reveals gram-negative, oxidase-positive bacilli. Colonies transilluminating while passing under UV light. Choose the correct agent.

A firefighter with fever and cough has a culture showing gram-negative, oxidase-positive bacilli with fluorescent colonies under UV light. Which of the following organisms is most likely causing the infection?

The toughness of bacterial cell walls is due to:

A group of students consumed fried rice in a restaurant. Six hours later, they start vomiting. What is the etiological agent?

A 70-year-old man with a prosthetic valve was admitted with a murmur and blood culture positive for Gram-positive cocci. Choose the most common agent causing prosthetic valve infective endocarditis.

A patient’s laboratory results were positive for malarial antigen, and the Widal test findings were as follows: O ag >1:300, H ag >1:20, A >1:20, B >1:20. What is the most likely diagnosis?

A patient's sample is positive for infectious Hepatitis B. Choose the correct serological marker combination for this condition.

Which of the following is the culture medium used for tuberculosis?

Q10

What is the most common causative organism of prosthetic heart valve endocarditis?

FMGE 2025 - Microbiology FMGE Practice Questions and MCQs

Question 1: Patient with HIV presenting with bouts of cough and fever. Which medium is appropriate for diagnosis?

A. MacConkey agar
B. Loeffler serum slope (LSS)
C. Tinsdale media
D. Lowenstein-Jensen (LJ) medium (Correct Answer)

Explanation: ***Lowenstein-Jensen (LJ) medium*** - This egg-based culture medium is the standard solid medium used globally for the isolation of **Mycobacterium tuberculosis**. - Given the patient's history of **HIV** and symptoms of cough and fever, **Pulmonary Tuberculosis (TB)** is the differential diagnosis most likely requiring this specialized medium. *Tinsdale media* - Tinsdale medium is specifically used for the primary isolation and identification of **Corynebacterium diphtheriae**. - The characteristic presentation of **diphtheria** (pseudomembrane, severe pharyngitis) differs significantly from the suspected chronic respiratory symptoms of TB. *MacConkey agar* - This is a selective and differential medium designed to isolate and differentiate **Gram-negative enteric bacilli** (e.g., *E. coli*, *Salmonella*). - It is completely inadequate for the isolation of the slow-growing, **acid-fast bacilli** characteristic of mycobacteria. *Loeffler serum slope (LSS)* - Loeffler serum slope is an enrichment medium primarily used to promote the growth and demonstration of characteristic morphology in **Corynebacterium diphtheriae**. - While sometimes used for initial inoculum, it is not the definitive medium required for the isolation of **Mycobacterium tuberculosis** due to its composition and intended purpose.

Question 2: A firefighter was admitted with fever, cough, culture shown in the image reveals gram-negative, oxidase-positive bacilli. Colonies transilluminating while passing under UV light. Choose the correct agent.

A. Staphylococcus aureus
B. Pseudomonas aeruginosa (Correct Answer)
C. Serratia
D. Klebsiella

Explanation: ***Pseudomonas aeruginosa*** - This organism is a **gram-negative**, **oxidase-positive** bacillus, matching the description. The image shows growth with a characteristic green pigment (**pyocyanin** and **pyoverdin**). - The key identifier is the production of **pyoverdin**, a fluorescent siderophore, which causes the colonies to transilluminate under UV light. *Klebsiella* - *Klebsiella* is **oxidase-negative**, which rules it out based on the biochemical test results provided. - It typically produces large, **mucoid colonies** due to its prominent capsule and is a lactose fermenter, features not described here. *Staphylococcus aureus* - *Staphylococcus aureus* is a **gram-positive coccus**, not a gram-negative bacillus, making it incorrect based on the initial microscopy. - It is also **oxidase-negative** and characteristically forms opaque, often golden-yellow colonies, not greenish fluorescent ones. *Serratia* - *Serratia* is a gram-negative bacillus but is **oxidase-negative**, which is inconsistent with the findings. - Certain species like *Serratia marcescens* are known for producing a red pigment called **prodigiosin**, especially at room temperature, not a green fluorescent pigment.

Question 3: A firefighter with fever and cough has a culture showing gram-negative, oxidase-positive bacilli with fluorescent colonies under UV light. Which of the following organisms is most likely causing the infection?

A. Staphylococcus aureus
B. Pseudomonas aeruginosa (Correct Answer)
C. Serratia
D. Klebsiella

Explanation: ***Pseudomonas aeruginosa***- This organism is a non-lactose fermenting, **Gram-negative bacillus** that is definitively **oxidase-positive**.- It characteristically produces the siderophore **pyoverdine**, which is fluorescent under UV light, confirming the most likely etiology.*Klebsiella*- *Klebsiella* species are **oxidase-negative** (**distinguishing factor**) and typically highly encapsulated, often causing consolidation in pneumonia.- They are also strong **lactose fermenters** and do not produce fluorescent pigments.*Staphylococcus aureus*- *S. aureus* is a **Gram-positive coccus** (stains purple) arranged in clusters, not a Gram-negative bacillus (stains pink).- It is non-motile, shows no fluorescence, and is identified by being coagulase and catalase positive.*Serratia*- *Serratia marcescens* is generally **oxidase-negative** (unlike the clinical finding) and belongs to the Enterobacteriaceae family.- Under room temperature, *Serratia* often produces a characteristic **red pigment** (**prodigiosin**), not a fluorescent pigment.

Question 4: The toughness of bacterial cell walls is due to:

A. Peptidoglycan (Correct Answer)
B. Teichoic acid
C. Lipopolysaccharides
D. Outer membrane

Explanation: ***Peptidoglycan***- This unique polymer forms a robust, mesh-like structural layer (the **murein layer**) that encases the bacteria, providing essential mechanical strength and rigidity to the cell wall. Its strong cross-links allow it to withstand extremely high internal **osmotic pressure**, preventing the cell from bursting (lysis). *Teichoic acid* - These are polymers found exclusively in **Gram-positive** bacteria, anchored either to the peptidoglycan or the plasma membrane (lipoteichoic acid). - They contribute to the cell wall's overall integrity, and act as antigens, but are not the primary element responsible for the physical **toughness**. *Lipopolysaccharides* - LPS molecules make up the outer leaflet of the **outer membrane** in **Gram-negative** bacteria. - They function primarily as an **endotoxin** (lipid A component) and an O-antigen, contributing to virulence and permeability, not the mechanical load-bearing structure. *Outer membrane* - This membrane is characteristic of **Gram-negative** bacteria and acts as a protective, selective barrier (excluding certain antibiotics and chemicals). - While providing protective resistance, the intrinsic mechanical **toughness** preventing osmotic lysis is conferred by the underlying **peptidoglycan** layer.

Question 5: A group of students consumed fried rice in a restaurant. Six hours later, they start vomiting. What is the etiological agent?

A. Clostridium botulinum
B. Shigella dysentriae
C. Escherichia coli
D. Bacillus cereus (Correct Answer)

Explanation: ***Bacillus cereus***- The short incubation period (1 to 6 hours) and the primary symptom being **vomiting** are highly characteristic of the **emetic-type** food poisoning caused by *B. cereus*. - The emetic form is typically associated with contaminated **cooked rice** (like fried rice) that has been improperly stored, allowing the bacteria to proliferate and produce the heat-stable toxin, **cereulide**. *Clostridium botulinum* - This bacterium causes **botulism**, a severe neuroparalytic syndrome (e.g., descending flaccid paralysis, diplopia), not typically an acute emetic gastroenteritis. - The incubation period for botulism is usually longer (12 to 36 hours), and it is most often linked to improperly canned or vacuum-packed foods. *Escherichia coli* - Diarrheagenic *E. coli* strains typically require a longer incubation period (usually 1 or more days) and cause symptoms dominated by **diarrhea** (either watery or bloody), with vomiting being a less prominent feature. - Common infectious sources are often fecally contaminated water or uncooked produce, rather than cooked and cooled rice. *Shigella dysentriae* - *S. dysentriae* causes **bacillary dysentery**, characterized by a longer incubation period (1 to 7 days) and symptoms including fever, severe abdominal cramping, and **bloody, mucoid stools** (dysentery). - While systemic symptoms might occur, the rapid onset of vomiting alone is not characteristic of Shigellosis.

Question 6: A 70-year-old man with a prosthetic valve was admitted with a murmur and blood culture positive for Gram-positive cocci. Choose the most common agent causing prosthetic valve infective endocarditis.

A. Staphylococcus aureus
B. Streptococcus viridans
C. Enterococci
D. Staphylococcus epidermidis (Correct Answer)

Explanation: ***Staphylococcus epidermidis***- This is the most common cause of **early** prosthetic valve endocarditis (PVIE), which typically occurs within 60 days following surgery, often due to contamination during the replacement procedure.- This organism is a **coagulase-negative Staphylococci** known for forming protective **biofilms** on the prosthetic surface, leading to severe device-related infections.*Streptococcus viridans*- This is the most common cause of **native valve** infective endocarditis (NVIE), frequently associated with dental procedures or poor oral hygiene.- While it can cause PVIE, it is typically responsible for late-onset cases, and overall less common than **Staphylococcal species** in prosthetic valve disease.*Staphylococcus aureus*- This highly virulent organism causes a rapidly destructive form of endocarditis (both NVIE and PVIE), often associated with **IV drug use** or healthcare-associated infections.- Although a major cause of PVIE, *S. epidermidis* is **statistically more frequent** overall in this setting, especially in early postoperative infections.*Enterococci*- Enterococcal endocarditis is often seen in older men or patients who have recently undergone **genitourinary or gastrointestinal** instrumentation or surgery.- While it causes both NVIE and PVIE, its incidence in prosthetic valve disease is lower than the **Staphylococcal species**.

Question 7: A patient’s laboratory results were positive for malarial antigen, and the Widal test findings were as follows: O ag >1:300, H ag >1:20, A >1:20, B >1:20. What is the most likely diagnosis?

A. Past infection
B. Patient has taken TAB vaccine
C. Recent infection by Salmonella spp (Correct Answer)
D. Convalescence

Explanation: ***Recent infection by Salmonella spp*** - The **O antigen** titer (1:300) is significantly elevated (the diagnostic threshold is generally $\geq$1:160 or $\geq$1:320), strongly indicating a **recent or active infection** with *Salmonella typhi* (enteric fever). - The 'O' antigen is related to the **somatic LPS** and represents the IgM response, which is prominent during the acute phase of the infection. *Patient has taken TAB vaccine* - The **TAB vaccine** (Typhoid-Paratyphoid A and B) primarily generates high titers of **H (flagellar) antigen**, while the O antigen titer remains minimally elevated or low. - This patient exhibits a very high **O antigen** titer and low H antigen titer (1:20), ruling out recent vaccination as the sole cause of the serological picture. *Past infection* - **Past infection** is typically characterized by high and persistent **H (flagellar) antigen** titers, as the H antibody (IgG) persists longer than the O antibody (IgM). - The low H titer (1:20) and high O titer (1:300) suggest an **acute infection** rather than a remote or past infection. *Reconvalescence* - The **convalescent phase** (recovery) is marked by a decline in the acute-phase O antibody titers and sometimes a rise or persistence of H antibody titers. - The observed high O titer indicates an **ongoing acute process**, which is inconsistent with the late stage of recovery or reconvalescence.

Question 8: A patient's sample is positive for infectious Hepatitis B. Choose the correct serological marker combination for this condition.

A. HBcAg
B. HBeAg
C. HBs + Anti Hbe
D. HBsAg + HBeAg (Correct Answer)

Explanation: ***Correct: HBsAg + HBeAg*** - **HBsAg (Hepatitis B surface antigen)** indicates active HBV infection (acute or chronic) - **HBeAg (Hepatitis B e antigen)** indicates active viral replication and **high infectivity** - This combination is the hallmark of **infectious/highly contagious Hepatitis B** with active virus replication - High viral load (HBV DNA levels typically >10^5 copies/mL) *Incorrect: HBcAg* - Core antigen is **not detectable in serum** - only detectable in liver tissue - Anti-HBc antibodies are detected in serum, not HBcAg itself *Incorrect: HBeAg alone* - While HBeAg indicates infectivity, **HBsAg must be present** for active infection - HBeAg without HBsAg context is incomplete for diagnosis *Incorrect: HBsAg + Anti-HBe* - This pattern indicates **low replicative phase** (inactive carrier or chronic infection with seroconversion) - Anti-HBe suggests decreased viral replication and **lower infectivity** - This is the opposite of "infectious" Hepatitis B

Question 9: Which of the following is the culture medium used for tuberculosis?

A. Chocolate agar
B. MacConkey medium
C. Blood agar
D. LJ medium (Correct Answer)

Explanation: ***LJ medium*** - LJ medium, or **Löwenstein-Jensen medium**, is the standard, egg-based solid medium used worldwide for the culture of ***Mycobacterium tuberculosis*** (M.tb). - It contains **malachite green**, which acts as a selective agent, inhibiting the growth of most common contaminating bacteria but allowing slow growth of mycobacteria. *MacConkey medium* - This is a **selective and differential medium** primarily used for isolating and differentiating **Gram-negative enteric bacilli**, such as *Escherichia coli* and *Salmonella*. - It inhibits Gram-positive bacteria using bile salts and crystal violet, and differentiates organisms based on **lactose fermentation**. *Blood agar* - This is a **general-purpose enrichment medium** used for the cultivation of a wide range of fastidious and non-fastidious bacteria. - It is crucial for assessing the **hemolytic reaction** of bacteria (alpha, beta, or gamma hemolysis), especially for classifying streptococci. *Chocolate agar* - This medium is prepared by heating blood agar, lysing the red blood cells, which releases essential nutrients like **X and V factors** (hematin and NAD). - It is specifically used for the culture of highly **fastidious organisms** such as ***Haemophilus influenzae*** and **pathogenic Neisseria species**.

Question 10: What is the most common causative organism of prosthetic heart valve endocarditis?

A. S. aureus
B. S. epidermidis (Correct Answer)
C. S. viridans
D. Enterococcus

Explanation: ***S. epidermidis*** - **S. epidermidis** is the most common single causative organism of prosthetic heart valve (PHV) endocarditis, especially in the **early post-operative period** (within 60 days to 1 year of surgery). - This organism is a **coagulase-negative Staphylococcus** known for its ability to produce a **glycocalyx (biofilm)**, which adheres strongly to foreign materials like prosthetic valves. *S. viridans* - Typically the leading cause of **subacute infectious endocarditis** on native, damaged valves, often following minor **dental procedures**. - While it can cause late PHV endocarditis, it is much less frequent than staphylococcal species, which dominate PHV infections. *S. aureus* - **S. aureus** is the most common cause of **acute native valve endocarditis** and frequently implicated in endocarditis in **IV drug users**. - It is a major cause of PHV endocarditis, especially **late-onset** (more than 1 year post-surgery), but overall, **S. epidermidis** is considered the most common pathogen when considering all PHV infections. *Enterococcus* - **Enterococcus species** (especially *E. faecalis*) account for approximately **5-10% of prosthetic valve endocarditis** cases, particularly in late-onset infections. - While significant, enterococci are far less common than staphylococcal species in PHV endocarditis.