A 47-year-old woman presents to a local medical shelter while on a mission trip with her church to help rebuild homes after a hurricane. She has been experiencing severe nausea, vomiting, and diarrhea for the last 2 days and was feeling too fatigued to walk this morning. On presentation, her temperature is 99.2°F (37.3°C), blood pressure is 95/62 mmHg, pulse is 121/min, and respirations are 17/min. Physical exam reveals decreased skin turgor, and a stool sample reveals off-white watery stools. Gram stain reveals a gram-negative, comma-shaped organism that produces a toxin. Which of the following is consistent with the action of the toxin most likely involved in the development of this patient's symptoms?
Q2
A 62-year-old woman presents to the emergency department for evaluation of a spreading skin infection that began from an ulcer on her foot. The patient has type 2 diabetes mellitus that is poorly controlled. On examination, there is redness and erythema to the lower limb with skin breakdown along an extensive portion of the leg. The patient’s tissues separate readily from the fascial plane, prompting a diagnosis of necrotizing fasciitis. What is the exotoxin most likely associated with this patient’s presentation?
Q3
A 27-year-old man presents to the emergency department with unrelenting muscle spasms for the past several hours. The patient’s girlfriend states that he started having jaw spasms and soreness last night but now his neck, back, and arms are spasming. She also states that he stepped on a nail about 1 week ago. Past medical history is noncontributory. The patient's vaccination status is unknown at this time. Today, the vital signs include temperature 39.1°C (102.4°F), heart rate 115/min, blood pressure 145/110 mm Hg, and respiratory rate 10/min. On exam, the patient is in obvious discomfort, with a clenched jaw and extended neck. Labs are drawn and a basic metabolic panel comes back normal and the white blood cell (WBC) count is moderately elevated. Which of the following is the most likely etiology of this patient’s symptoms?
Q4
A 24-year-old woman presents to the ED with symptoms of pelvic inflammatory disease despite being previously treated with azithromycin for chlamydial infection. Based on your clinical understanding about the epidemiology of PID, you decide to obtain a gram stain which shows a gram-negative diplococci. What is the next step in order to confirm the identity of the organism described?
Q5
A microbiology student was given a swab containing an unknown bacteria taken from the wound of a soldier and asked to identify the causative agent. She determined that the bacteria was a gram-positive, spore-forming bacilli, but had difficulty narrowing it down to the specific bacteria. The next test she performed was the Nagler's test, in which she grew the bacteria on a plate made from egg yolk, which would demonstrate the ability of the bacteria to hydrolyze phospholipids and produce an area of opacity. Half the plate contained a specific antitoxin which prevented hydrolysis of phospholipids while the other half did not contain any antitoxin. The bacteria produced an area of opacity only on half of the plate containing no antitoxin. Which of the following toxins was the antitoxin targeting?
Q6
A 48-year-old woman comes to the physician for a follow-up examination. Six months ago, she was diagnosed with overactive bladder syndrome and began treatment with oxybutynin. She continues to have involuntary loss of urine with sudden episodes of significant bladder discomfort that is only relieved by voiding. A substance is injected into the detrusor muscle to treat her symptoms. The physician informs the patient that she will have transitory relief for several months before symptoms return and will require repeated treatment. The injected substance is most likely produced by an organism with which of the following microbiological properties?
Q7
A 6-year-old boy presents with fever, sore throat, hoarseness, and neck enlargement. The symptoms started 3 days ago and progressed gradually with an elevated temperature and swollen lymph nodes. His family immigrated recently from Honduras. He was born via spontaneous vaginal delivery at 39 weeks after an uneventful gestational period and he is now on a catch-up vaccination schedule. He lives with several family members, including his parents, in a small apartment. No one in the apartment smokes tobacco. On presentation, the patient’s blood pressure is 110/75 mm Hg, heart rate is 103/min, respiratory rate is 20/min, and temperature is 39.4°C (102.9°F). On physical examination, the child is acrocyanotic and somnolent. There is widespread cervical edema and enlargement of the cervical lymph nodes. The tonsils are covered with a gray, thick membrane which spreads beyond the tonsillar bed and reveals bleeding, erythematous mucosa with gentle scraping. The lungs are clear to auscultation. Which of the following is the target of the virulence factor produced by the pathologic organism infecting this child?
Q8
An 81-year-old man comes to the emergency department with severe left ear pain and drainage for 3 days. He has a history of poorly-controlled type 2 diabetes mellitus. He appears uncomfortable. Physical examination of the ear shows marked periauricular erythema, exquisite tenderness on palpation, and granulation tissue in the external auditory canal. The most likely causal pathogen produces an exotoxin that acts by a mechanism most similar to a toxin produced by which of the following organisms?
Q9
A 3-year-old male is brought to the ER with a sore throat and fever. Examination of the pharynx reveals a dark, inflammatory exudate. Cysteine-tellurite agar culture produces black, iridescent colonies. Microscopic features of the causal organism most likely include which of the following?
Q10
A 16-year-old male is brought to the clinic by his mother for the complaints of fever, nonproductive cough, fatigue, lack of appetite, and sore throat for the past 2 months. Several other students at his high school have had similar symptoms. Physical exam shows a whitish membrane in his oropharynx, bilateral enlarged cervical lymphadenopathy, and mild splenomegaly. Which of the following tests is most likely to diagnose his condition?
Corynebacterium species US Medical PG Practice Questions and MCQs
Question 1: A 47-year-old woman presents to a local medical shelter while on a mission trip with her church to help rebuild homes after a hurricane. She has been experiencing severe nausea, vomiting, and diarrhea for the last 2 days and was feeling too fatigued to walk this morning. On presentation, her temperature is 99.2°F (37.3°C), blood pressure is 95/62 mmHg, pulse is 121/min, and respirations are 17/min. Physical exam reveals decreased skin turgor, and a stool sample reveals off-white watery stools. Gram stain reveals a gram-negative, comma-shaped organism that produces a toxin. Which of the following is consistent with the action of the toxin most likely involved in the development of this patient's symptoms?
A. Decreased ribosomal activity
B. Increased membrane permeability
C. Cleavage of junctional proteins
D. Increased adenylyl cyclase activity (Correct Answer)
E. Activation of receptor tyrosine kinase
Explanation: ***Increased adenylyl cyclase activity***
- The patient's symptoms (severe nausea, vomiting, diarrhea, dehydration, **hypotension**, and **tachycardia**) along with the presence of a **gram-negative, comma-shaped organism** producing an off-white watery stool are highly suggestive of **Cholera** caused by *Vibrio cholerae*.
- Cholera toxin is an **AB5 toxin** that irreversibly activates **adenylyl cyclase** in intestinal epithelial cells, leading to increased intracellular cyclic AMP (cAMP) levels. This increased cAMP then causes massive secretion of chloride and bicarbonate into the intestinal lumen, followed by water, resulting in the characteristic **"rice-water stool"**.
*Decreased ribosomal activity*
- This mechanism is characteristic of toxins like **Shiga toxin** (produced by *Shigella dysenteriae* and enterohemorrhagic *E. coli*) and **diphtheria toxin** (produced by *Corynebacterium diphtheriae*).
- These toxins inhibit protein synthesis by inactivating the 60S ribosomal subunit, which typically leads to **cytotoxicity** rather than the profuse watery diarrhea seen in cholera.
*Increased membrane permeability*
- Some toxins, such as **alpha-toxin** of *Clostridium perfringens* (gas gangrene) or **pore-forming toxins**, increase membrane permeability by creating pores in cell membranes.
- While this can lead to cell damage and lysis, it is not the primary mechanism by which the cholera toxin causes massive fluid secretion.
*Cleavage of junctional proteins*
- Toxins that cleave **tight junction proteins** (e.g., *Clostridium difficile* toxins A and B) can disrupt the intestinal barrier and lead to fluid leakage.
- However, the main mechanism of cholera toxin is fluid secretion due to ion channel activation rather than direct disruption of intercellular junctions.
*Activation of receptor tyrosine kinase*
- Activation of **receptor tyrosine kinases** is typically involved in cell growth, differentiation, and metabolism, not directly in acute, severe secretory diarrhea.
- While some bacterial toxins can modulate host signaling pathways, direct activation of receptor tyrosine kinases is not the primary mechanism of action for toxins causing cholera-like symptoms.
Question 2: A 62-year-old woman presents to the emergency department for evaluation of a spreading skin infection that began from an ulcer on her foot. The patient has type 2 diabetes mellitus that is poorly controlled. On examination, there is redness and erythema to the lower limb with skin breakdown along an extensive portion of the leg. The patient’s tissues separate readily from the fascial plane, prompting a diagnosis of necrotizing fasciitis. What is the exotoxin most likely associated with this patient’s presentation?
A. Streptococcal pyogenic exotoxin A
B. TSST-1
C. Diphtheria toxin
D. Exfoliative toxin
E. Streptococcal pyogenic exotoxin B (Correct Answer)
Explanation: ***Streptococcal pyogenic exotoxin B***
- **Streptococcal pyogenic exotoxin B** is a **cysteine protease** that directly degrades tissue, including collagen and fibronectin, leading to the rapid tissue destruction characteristic of **necrotizing fasciitis**.
- This exotoxin is frequently associated with **Group A Streptococcus (GAS)** infections, a common cause of severe soft tissue infections, especially in immunocompromised individuals like diabetics.
*Streptococcal pyogenic exotoxin A*
- This exotoxin acts as a **superantigen**, primarily causing symptoms of **streptococcal toxic shock syndrome** (STSS), characterized by fever, rash, and organ failure.
- While GAS can cause necrotizing fasciitis, Exotoxin A is more closely linked to toxic shock phenomena rather than direct tissue destruction.
*TSST-1*
- **Toxic Shock Syndrome Toxin-1 (TSST-1)** is produced by **Staphylococcus aureus** and is a classic cause of **staphylococcal toxic shock syndrome**.
- It acts as a **superantigen** but is not directly responsible for the extensive tissue necrosis seen in necrotizing fasciitis caused by streptococci.
*Diphtheria toxin*
- **Diphtheria toxin**, produced by *Corynebacterium diphtheriae*, inhibits **protein synthesis** by inactivating elongation factor-2 (EF-2), leading to cell death.
- It causes diphtheria, characterized by a **pseudomembrane** in the throat and myocarditis, not necrotizing fasciitis.
*Exfoliative toxin*
- **Exfoliative toxins A and B** are produced by **Staphylococcus aureus** and are responsible for **Staphylococcal Scalded Skin Syndrome (SSSS)**.
- These toxins cause cleavage of desmoglein-1 in the epidermis, leading to widespread blistering and desquamation, not deep tissue necrosis.
Question 3: A 27-year-old man presents to the emergency department with unrelenting muscle spasms for the past several hours. The patient’s girlfriend states that he started having jaw spasms and soreness last night but now his neck, back, and arms are spasming. She also states that he stepped on a nail about 1 week ago. Past medical history is noncontributory. The patient's vaccination status is unknown at this time. Today, the vital signs include temperature 39.1°C (102.4°F), heart rate 115/min, blood pressure 145/110 mm Hg, and respiratory rate 10/min. On exam, the patient is in obvious discomfort, with a clenched jaw and extended neck. Labs are drawn and a basic metabolic panel comes back normal and the white blood cell (WBC) count is moderately elevated. Which of the following is the most likely etiology of this patient’s symptoms?
A. An exotoxin that causes ADP-ribosylation of EF-2
B. An edema factor that functions as adenylate cyclase
C. A heat-labile toxin that inhibits ACh release at the NMJ
D. An exotoxin that cleaves SNARE proteins (Correct Answer)
E. A toxin that disables the G-protein coupled receptor
Explanation: ***An exotoxin that cleaves SNARE proteins***
- The patient's presentation with **unrelenting muscle spasms**, jaw spasms (**trismus**), extended neck (**opisthotonus**), and a recent **puncture wound** are classic signs of **tetanus**.
- **Tetanospasmin**, the neurotoxin produced by *Clostridium tetani*, acts by cleaving **SNARE proteins**, which are essential for the release of **inhibitory neurotransmitters** (glycine and GABA) from spinal interneurons, leading to uncontrolled muscle contraction.
*An exotoxin that causes ADP-ribosylation of EF-2*
- This mechanism describes **diphtheria toxin**, which is produced by *Corynebacterium diphtheriae* and inhibits protein synthesis in eukaryotic cells.
- While *C. diphtheriae* can cause systemic effects, it primarily manifests as **upper respiratory tract infection** with pseudomembrane formation, lymphadenopathy, and myocarditis, not generalized muscle spasms.
*An edema factor that functions as adenylate cyclase*
- This describes the **edema factor** component of **anthrax toxin**, produced by *Bacillus anthracis*.
- Anthrax typically causes cutaneous, inhalational, or gastrointestinal infections, and its symptoms do not include the generalized muscle spasms seen in this patient.
*A heat-labile toxin that inhibits ACh release at the NMJ*
- This mechanism describes **botulinum toxin**, produced by *Clostridium botulinum*, which causes **flaccid paralysis** by preventing the release of acetylcholine at the neuromuscular junction.
- The patient exhibits muscle spasms and rigidity (**spastic paralysis**), which is directly opposite to the effects of botulinum toxin.
*A toxin that disables the G-protein coupled receptor*
- While various toxins can affect G-protein coupled receptors (e.g., cholera toxin or pertussis toxin), this general description does not specifically match the clinical presentation of tetanus.
- Toxins affecting G-protein coupled receptors typically lead to symptoms like **severe diarrhea** (cholera) or **whooping cough** (pertussis) rather than generalized muscle spasms.
Question 4: A 24-year-old woman presents to the ED with symptoms of pelvic inflammatory disease despite being previously treated with azithromycin for chlamydial infection. Based on your clinical understanding about the epidemiology of PID, you decide to obtain a gram stain which shows a gram-negative diplococci. What is the next step in order to confirm the identity of the organism described?
A. Perform an RT-PCR
B. Culture in TCBS agar
C. Culture in Thayer-Martin media (Correct Answer)
D. Obtain an acid fast stain
E. Culture in Bordet-Gengou agar
Explanation: ***Culture in Thayer-Martin media***
- The presence of **gram-negative diplococci** in a patient with PID symptoms strongly suggests *Neisseria gonorrhoeae*.
- **Thayer-Martin media** is a selective **agar** specifically designed for the isolation and identification of *Neisseria* species, including *N. gonorrhoeae*, by inhibiting the growth of most commensal bacteria and fungi.
*Perform an RT-PCR*
- While **RT-PCR** can detect *Neisseria gonorrhoeae* nucleic acids, it is primarily used for **molecular diagnosis** and not directly for confirming the identity of a cultured organism visualized on gram stain.
- **RT-PCR** is generally used for direct detection from clinical samples and is particularly useful in situations where culture is difficult or unavailable.
*Culture in TCBS agar*
- **TCBS (Thiosulfate Citrate Bile Salts Sucrose) agar** is a selective medium primarily used for the isolation of *Vibrio* species, which are not typically associated with pelvic inflammatory disease or characterized as gram-negative diplococci.
- This medium is designed to differentiate between different *Vibrio* species based on sucrose fermentation.
*Obtain an acid fast stain*
- An **acid-fast stain** (e.g., Ziehl-Neelsen stain) is used to identify bacteria with a **waxy cell wall**, such as *Mycobacterium* species (e.g., *Mycobacterium tuberculosis*).
- *Neisseria gonorrhoeae* is not acid-fast, and this stain would not be appropriate for its identification.
*Culture in Bordet-Gengou agar*
- **Bordet-Gengou agar** is a specialized culture medium used for the isolation of *Bordetella pertussis*, the causative agent of whooping cough.
- This medium is not suitable for the isolation of *Neisseria gonorrhoeae*.
Question 5: A microbiology student was given a swab containing an unknown bacteria taken from the wound of a soldier and asked to identify the causative agent. She determined that the bacteria was a gram-positive, spore-forming bacilli, but had difficulty narrowing it down to the specific bacteria. The next test she performed was the Nagler's test, in which she grew the bacteria on a plate made from egg yolk, which would demonstrate the ability of the bacteria to hydrolyze phospholipids and produce an area of opacity. Half the plate contained a specific antitoxin which prevented hydrolysis of phospholipids while the other half did not contain any antitoxin. The bacteria produced an area of opacity only on half of the plate containing no antitoxin. Which of the following toxins was the antitoxin targeting?
A. Alpha toxin (Correct Answer)
B. Exotoxin A
C. Tetanus toxin
D. Diphtheria toxin
E. Botulinum toxin
Explanation: ***Alpha toxin***
- The scenario describes a **Nagler's test**, which is specifically used to detect the presence of **alpha toxin (lecithinase)** produced by *Clostridium perfringens*.
- The antitoxin prevents the hydrolysis of phospholipids and the formation of opacity, confirming that the opacity is due to the alpha toxin.
*Exotoxin A*
- **Exotoxin A** is a toxin produced by *Pseudomonas aeruginosa* and inhibits protein synthesis.
- It is not associated with the **Nagler's test** or phospholipid hydrolysis on egg yolk agar.
*Tetanus toxin*
- **Tetanus toxin** is produced by *Clostridium tetani* and causes spastic paralysis by inhibiting inhibitory neurotransmitter release.
- It is not involved in phospholipid hydrolysis or detected by the **Nagler's test**.
*Diphtheria toxin*
- **Diphtheria toxin** is produced by *Corynebacterium diphtheriae* and inhibits protein synthesis, leading to cellular death.
- This toxin is not detected by the **Nagler's test** and does not cause phospholipid hydrolysis.
*Botulinum toxin*
- **Botulinum toxin** is produced by *Clostridium botulinum* and causes flaccid paralysis by inhibiting acetylcholine release at the neuromuscular junction.
- It is not associated with the **Nagler's test** or the hydrolysis of phospholipids.
Question 6: A 48-year-old woman comes to the physician for a follow-up examination. Six months ago, she was diagnosed with overactive bladder syndrome and began treatment with oxybutynin. She continues to have involuntary loss of urine with sudden episodes of significant bladder discomfort that is only relieved by voiding. A substance is injected into the detrusor muscle to treat her symptoms. The physician informs the patient that she will have transitory relief for several months before symptoms return and will require repeated treatment. The injected substance is most likely produced by an organism with which of the following microbiological properties?
A. Gram-negative, encapsulated diplococcus
B. Gram-negative, comma-shaped rod
C. Gram-positive, club-shaped rod
D. Gram-negative, aerobic coccobacillus
E. Gram-positive, spore-forming rod (Correct Answer)
Explanation: ***Gram-positive, spore-forming rod***
- The patient's symptoms of **overactive bladder** are being treated with a substance injected into the detrusor muscle, providing temporary relief, which is characteristic of **botulinum toxin**.
- **Botulinum toxin** is produced by *Clostridium botulinum*, a **Gram-positive, spore-forming anaerobic rod** known for producing potent neurotoxins.
*Gram-negative, encapsulated diplococcus*
- This describes organisms like *Neisseria meningitidis*, which causes **meningitis** and **sepsis**, not conditions treated with muscle relaxants.
- The clinical presentation and treatment are inconsistent with infections caused by such bacteria.
*Gram-negative, comma-shaped rod*
- This morphology is characteristic of *Vibrio cholerae*, which causes **cholera**, an acute diarrheal illness.
- There is no clinical relevance of *Vibrio cholerae* toxins in the treatment of overactive bladder.
*Gram-positive, club-shaped rod*
- This describes *Corynebacterium diphtheriae*, the causative agent of **diphtheria**, a respiratory illness.
- The diphtheria toxin causes tissue damage and systemic effects, but it is not used therapeutically for muscle relaxation.
*Gram-negative, aerobic coccobacillus*
- This describes bacteria such as *Bordetella pertussis*, which causes **pertussis (whooping cough),** and *Haemophilus influenzae*.
- Toxins from these organisms are not used for therapeutic detrusor muscle relaxation.
Question 7: A 6-year-old boy presents with fever, sore throat, hoarseness, and neck enlargement. The symptoms started 3 days ago and progressed gradually with an elevated temperature and swollen lymph nodes. His family immigrated recently from Honduras. He was born via spontaneous vaginal delivery at 39 weeks after an uneventful gestational period and he is now on a catch-up vaccination schedule. He lives with several family members, including his parents, in a small apartment. No one in the apartment smokes tobacco. On presentation, the patient’s blood pressure is 110/75 mm Hg, heart rate is 103/min, respiratory rate is 20/min, and temperature is 39.4°C (102.9°F). On physical examination, the child is acrocyanotic and somnolent. There is widespread cervical edema and enlargement of the cervical lymph nodes. The tonsils are covered with a gray, thick membrane which spreads beyond the tonsillar bed and reveals bleeding, erythematous mucosa with gentle scraping. The lungs are clear to auscultation. Which of the following is the target of the virulence factor produced by the pathologic organism infecting this child?
A. Desmoglein
B. SNAP-25
C. ADP-ribosylation factor 6
D. Eukaryotic elongation factor-2 (eEF-2) (Correct Answer)
E. RNA polymerase II
Explanation: ***Eukaryotic elongation factor-2 (eEF-2)***
- The clinical presentation (fever, sore throat, hoarseness, neck enlargement, and a **gray, thick membrane** on tonsils that bleeds on scraping, accompanied by **acrocyanosis and somnolence**) is highly suggestive of **diphtheria**, caused by *Corynebacterium diphtheriae*.
- The **diphtheria toxin** produced by *C. diphtheriae* is an **exotoxin** that acts by **ADP-ribosylating and inactivating eukaryotic elongation factor-2 (eEF-2)**, thereby inhibiting protein synthesis and leading to cell death.
*Desmoglein*
- **Desmoglein** is a component of **desmosomes** targeted by **autoantibodies** in **pemphigus vulgaris**, a blistering skin disease.
- This is not related to the mechanism of action of the diphtheria toxin.
*SNAP-25*
- **SNAP-25** is a protein involved in the release of **neurotransmitters** at the **neuromuscular junction**.
- It is cleaved by **botulinum toxin** (produced by *Clostridium botulinum*), leading to flaccid paralysis; it is not the target of diphtheria toxin.
*ADP-ribosylation factor 6*
- **ADP-ribosylation factor 6 (ARF6)** is a small GTPase involved in regulating vesicular trafficking and actin dynamics.
- While other bacterial toxins, such as **cholera toxin** and **pertussis toxin**, also cause ADP-ribosylation, their targets and clinical effects differ significantly from diphtheria toxin.
*RNA polymerase II*
- **RNA polymerase II** is responsible for transcribing **mRNA** in eukaryotes.
- Some toxins, like **alpha-amanitin** from *Amanita phalloides* mushrooms, inhibit RNA polymerase II, but this is not the target of the diphtheria toxin.
Question 8: An 81-year-old man comes to the emergency department with severe left ear pain and drainage for 3 days. He has a history of poorly-controlled type 2 diabetes mellitus. He appears uncomfortable. Physical examination of the ear shows marked periauricular erythema, exquisite tenderness on palpation, and granulation tissue in the external auditory canal. The most likely causal pathogen produces an exotoxin that acts by a mechanism most similar to a toxin produced by which of the following organisms?
A. Corynebacterium diphtheriae (Correct Answer)
B. Bacillus anthracis
C. Staphylococcus aureus
D. Bordetella pertussis
E. Shigella dysenteriae
Explanation: ***Corynebacterium diphtheriae***
- The clinical picture describes **malignant otitis externa** (MOE), likely caused by *Pseudomonas aeruginosa*, particularly in an elderly diabetic patient. Both *Pseudomonas aeruginosa* exotoxin A and *Corynebacterium diphtheriae* diphtheria toxin **inhibit protein synthesis by ADP-ribosylation of elongation factor-2 (EF-2)**.
- This shared mechanism of action makes *Corynebacterium diphtheriae* the most appropriate comparative organism based on the question's premise of exotoxin mechanism.
*Bacillus anthracis*
- Produces **anthrax toxin**, which consists of Protective Antigen (PA), Edema Factor (EF), and Lethal Factor (LF). LF is a **zinc-dependent metalloprotease** that cleaves mitogen-activated protein kinase kinase (MAPKK) family proteins.
- This mechanism is distinct from the ADP-ribosylation of EF-2.
*Staphylococcus aureus*
- Produces several toxins, including **toxic shock syndrome toxin-1 (TSST-1)** and **exfoliatin**, which act as **superantigens** or **proteases** respectively.
- These mechanisms differ from the ADP-ribosylation of EF-2.
*Bordetella pertussis*
- Produces **pertussis toxin**, which **ADP-ribosylates Gi proteins**, leading to increased cAMP levels by disinhibiting adenylate cyclase.
- This is a different target and mechanism compared to the ADP-ribosylation of EF-2.
*Shigella dysenteriae*
- Produces **Shiga toxin**, which is an **N-glycosidase** that cleaves the adenine residue from the 28S rRNA of the 60S ribosomal subunit, thereby **inhibiting protein synthesis**.
- While it inhibits protein synthesis, the specific mechanism is different from ADP-ribosylation of EF-2.
Question 9: A 3-year-old male is brought to the ER with a sore throat and fever. Examination of the pharynx reveals a dark, inflammatory exudate. Cysteine-tellurite agar culture produces black, iridescent colonies. Microscopic features of the causal organism most likely include which of the following?
A. Serpentine growth patterns
B. Kidney bean-shaped diplococci
C. Long, branching filaments
D. Lancet-shape
E. Metachromic granules (Correct Answer)
Explanation: ***Metachromic granules***
- The constellation of **sore throat**, **fever**, **dark inflammatory exudate** in the pharynx, and growth on **cysteine-tellurite agar** with **black, iridescent colonies** is highly characteristic of *Corynebacterium diphtheriae*.
- *Corynebacterium diphtheriae* is known for exhibiting **metachromatic granules** (Babes-Ernst bodies) when stained, which are reserves of inorganic polyphosphate.
*Serpentine growth patterns*
- **Serpentine growth patterns** are characteristic of *Mycobacterium tuberculosis* in liquid culture, not *Corynebacterium diphtheriae*.
- This growth pattern is due to the arrangement of bacterial cells in long, cord-like structures.
*Kidney bean-shaped diplococci*
- **Kidney bean-shaped diplococci** are characteristic of *Neisseria* species, such as *Neisseria gonorrhoeae* or *Neisseria meningitidis*.
- These Gram-negative cocci are typically found in pairs with adjacent flattened sides, giving them a kidney bean appearance.
- These organisms cause different clinical syndromes and have distinct culture characteristics.
*Long, branching filaments*
- **Long, branching filaments** are a microscopic feature of certain bacteria like *Actinomyces* and *Nocardia*.
- These organisms are responsible for actinomycosis and nocardiosis, which are typically chronic infections distinct from diphtheria.
*Lancet-shape*
- The term **lancet-shape** is used to describe the morphology of *Streptococcus pneumoniae*, which are Gram-positive cocci typically found in pairs (diplococci).
- *Streptococcus pneumoniae* causes pneumonia, meningitis, and otitis media, which differ from the presentation of diphtheria.
Question 10: A 16-year-old male is brought to the clinic by his mother for the complaints of fever, nonproductive cough, fatigue, lack of appetite, and sore throat for the past 2 months. Several other students at his high school have had similar symptoms. Physical exam shows a whitish membrane in his oropharynx, bilateral enlarged cervical lymphadenopathy, and mild splenomegaly. Which of the following tests is most likely to diagnose his condition?
A. Monospot test (Correct Answer)
B. Enzyme-linked immunosorbent assay
C. Throat culture
D. Urine culture
E. Chest X-ray
Explanation: ***Monospot test***
- The Monospot test detects **heterophile antibodies**, which are commonly produced during an acute Epstein-Barr virus (EBV) infection, the cause of **infectious mononucleosis**.
- The patient's symptoms (fever, fatigue, nonproductive cough, sore throat, cervical lymphadenopathy, splenomegaly) and the epidemiological context (several other students with similar symptoms) are highly suggestive of **infectious mononucleosis**.
*Enzyme-linked immunosorbent assay (ELISA)*
- While ELISA can detect antibodies to various pathogens, including EBV-specific antigens, the **Monospot test** is the more common and rapid initial diagnostic tool for infectious mononucleosis.
- ELISA for EBV-specific antibodies (e.g., VCA-IgM, VCA-IgG) might be used if the Monospot test is negative but clinical suspicion remains high, especially in younger children or atypical presentations.
*Throat culture*
- A throat culture is used to identify bacterial infections, such as **Streptococcus pyogenes** (strep throat).
- Although the patient has a sore throat and a whitish membrane, his other systemic symptoms (fatigue, splenomegaly, lack of appetite for 2 months) are not typical for a bacterial pharyngitis which usually responds to antibiotics. A **nonproductive cough** also makes bacterial pharyngitis less likely.
*Urine culture*
- A urine culture is used to diagnose **urinary tract infections**.
- The patient's symptoms are not indicative of a urinary tract infection.
*Chest X-ray*
- A chest X-ray is used to evaluate the lungs for conditions such as **pneumonia**, **bronchitis**, or other respiratory pathologies.
- While the patient has a nonproductive cough, the predominant systemic symptoms (fever, fatigue, lymphadenopathy, splenomegaly) point towards a systemic viral infection rather than primarily a lung issue that would be definitively diagnosed by a chest X-ray.
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