A 44-year-old man presents to urgent care with severe vomiting. He states that he was at a camping ground for a party several hours ago and then suddenly began vomiting profusely. He denies experiencing any diarrhea and otherwise states he feels well. The patient only has a past medical history of lactose intolerance and hypertension managed with exercise and a low salt diet. His temperature is 99.3°F (37.4°C), blood pressure is 123/65 mmHg, pulse is 110/min, respirations are 14/min, and oxygen saturation is 98% on room air. Physical exam is notable only for tachycardia and diffuse abdominal discomfort. Which of the following foods is associated with the most likely cause of this patient's presentation?
Q12
A previously healthy 21-year-old college student is brought to the emergency department because of a 10-hour history of increasing headache, stiff neck, and sensitivity to light. He returned from a mission trip to Haiti 3 weeks ago where he worked in a rural health clinic. He appears lethargic. He is oriented to person, place, and time. His temperature is 39°C (102°F), pulse is 115/min, respirations are 20/min, and blood pressure is 100/70 mm Hg. Examination shows equal and reactive pupils. There are scattered petechiae over the trunk and lower extremities. Range of motion of the neck is decreased due to pain. Neurologic examination shows no focal findings. Blood cultures are obtained and a lumbar puncture is performed. Cerebrospinal fluid (CSF) analysis shows neutrophilic pleocytosis and decreased glucose concentration. Which of the following is most likely to have prevented this patient's condition?
Q13
A 31-year-old man with no medical history presents to his provider for infertility. He states that he and his partner have had unprotected intercourse for 1 year and have been unable to conceive. Upon further workup, he is determined to have antisperm antibodies (ASA), but he does not have any other signs or labs suggesting systemic autoimmune disease. A breakdown of which of the following may have played a role in the pathogenesis of his infertility?
Q14
Immunology researchers attempt to characterize the role of several cytokines in a 5-year-old male’s allergic reaction to peanuts. Months after initial exposure to peanuts, the child was brought to the ER due to repeat exposure with symptoms of anaphylaxis that resolved following epinephrine injection and supportive therapy. Which of the following best describes the role of IL-4 in the child’s response:
Q15
A biology graduate student is performing an experiment in the immunology laboratory. He is researching the recombination activation genes RAG1 and RAG2 in order to verify the function of these genes. He then decides to carry out the experiment on knock-out mice so that these genes will be turned off. Which of the following changes should he be expecting to see?
Q16
A scientist is studying the process of thymus-dependent B cell activation in humans. He observes that, after bacterial infections, the germinal centers of secondary lymphoid organs become highly metabolically active. After subsequent reinfection with the same pathogen, the organism is able to produce immunoglobulins at a much faster pace. Which of the following processes is likely taking place in the germinal centers at the beginning of an infection?
Q17
A 5-year-old boy is referred to an immunologist because of episodes of recurrent infections. He complains of ear pain, nasal discharge, congestion, and headache. His medical history is significant for neonatal sepsis, recurrent bronchitis, and otitis media. The boy also had pneumocystis pneumonia when he was 11 months old. His mother reports that she had a younger brother who had multiple serious infections and died when he was 4 years old because of otogenic sepsis. Her grandfather frequently developed pneumonia and had multiple episodes of diarrhea. The patient is in the 10th percentile for height and 40th percentile for weight. The vital signs include: blood pressure 90/60 mm Hg, heart rate 111/min, respiratory rate 26/min, and temperature 38.3°C (100.9°F). Physical examination reveals a red, swollen, and bulging eardrum and enlarged retroauricular lymph nodes. Meningeal signs are negative and the physician suspects the presence of a primary immunodeficiency. After a thorough laboratory investigation, the patient is found to be CD40L deficient. Despite this deficiency, which of the following chains would still be expressed normally in this patient’s B lymphocytes?
Q18
A 48-year-old woman presents to an urgent care clinic with the complaints of bloody diarrhea, mid-abdominal discomfort, and cramping for 3 days. She also has nausea, vomiting, and fever. She reports that she had eaten raw oysters at a local seafood restaurant almost 3 days ago, but she denies any other potentially infectious exposures. Her temperature is 37.5°C (99.6°F), respiratory rate is 15/min, pulse is 67/min, and blood pressure is 102/68 mm Hg. Physical examination is non-contributory. What is the most likely diagnosis?
Q19
A 16-year-old man presents to the clinic accompanied by his father, with the complaints of high fever, sore throat, and bloody diarrhea for 4 days. He adds that he is also nauseous and vomited several times in the past 2 days. He denies any recent travel or eating outside. He recently started a dog-walking business. The father relates that two of the dogs had been unwell. His temperature is 38.9°C (102°F), respiratory rate is 16/min, pulse is 77/min, and blood pressure is 100/88 mm Hg. A physical examination is performed and is within normal limits including the abdominal exam. Blood test results are given below:
Hb%: 14 gm/dL
Total count (WBC): 13,100/mm3
Differential count:
Neutrophils: 80%
Lymphocytes: 15%
Monocytes: 5%
What is the most likely diagnosis?
Q20
A 33-year-old woman presents to the urgent care center with 4 days of abdominal pain and increasingly frequent bloody diarrhea. She states that she is currently having 6 episodes of moderate volume diarrhea per day with streaks of blood mixed in. She says she recently returned from a long camping trip with her friends where they cooked all of their own food and drank water from nearby streams. Physical examination is negative for acute tenderness, rebound tenderness, or abnormal bowel sounds. Her vital signs include temperature 38.0°C (100.4°F), blood pressure 106/74 mm Hg, heart rate 94/min, and respiratory rate 14/min. Given the following options, which is the most likely pathogen responsible for her presentation?
Bacteria US Medical PG Practice Questions and MCQs
Question 11: A 44-year-old man presents to urgent care with severe vomiting. He states that he was at a camping ground for a party several hours ago and then suddenly began vomiting profusely. He denies experiencing any diarrhea and otherwise states he feels well. The patient only has a past medical history of lactose intolerance and hypertension managed with exercise and a low salt diet. His temperature is 99.3°F (37.4°C), blood pressure is 123/65 mmHg, pulse is 110/min, respirations are 14/min, and oxygen saturation is 98% on room air. Physical exam is notable only for tachycardia and diffuse abdominal discomfort. Which of the following foods is associated with the most likely cause of this patient's presentation?
A. Egg salad
B. Home-made ice cream
C. Fish
D. Unfiltered stream water
E. Refried rice (Correct Answer)
Explanation: ***Refried rice***
- The rapid onset of symptoms after eating foods like refried rice, especially following
social gatherings where food may have been left out, is highly suggestive of
**_Bacillus cereus_ food poisoning**.
- **_Bacillus cereus_** produces toxins (emetic toxin) that cause **vomiting** within 1-6 hours of
ingestion, often without diarrhea.
*Egg salad*
- Egg salad, particularly if improperly refrigerated, is a common source of **_Salmonella_**
infection.
- However, **_Salmonella_** typically causes **diarrhea, fever, and abdominal cramps**, often with a
longer incubation period of 6-72 hours, which is not consistent with this patient's
presentation of rapid onset **vomiting** and no diarrhea.
*Home-made ice cream*
- Home-made ice cream made with raw eggs can be a source of **_Salmonella_ infection**.
- Similar to commercially prepared egg salad, **_Salmonella_** presents with symptoms of
**diarrhea, fever**, and a longer incubation period, distinguishing it from this case.
*Fish*
- Poorly preserved fish can lead to **scombroid poisoning** (histamine toxicity), which can
cause rapid onset symptoms including flushing, headache, and gastrointestinal
disturbances.
- While it can cause rapid symptoms, **scombroid poisoning** typically causes a wider range
of symptoms including **rash, flushing, and headaches**, which are absent here.
*Unfiltered stream water*
- Ingestion of unfiltered stream water increases the risk of parasitic infections such as
**_Giardia lamblia_** or bacterial infections like **_E. coli_ (traveler's diarrhea)**.
- These infections typically cause **diarrhea, abdominal cramps**, and sometimes nausea
and vomiting, but their onset is usually delayed (days to weeks), and diarrhea is a
prominent symptom, unlike this patient's presentation.
Question 12: A previously healthy 21-year-old college student is brought to the emergency department because of a 10-hour history of increasing headache, stiff neck, and sensitivity to light. He returned from a mission trip to Haiti 3 weeks ago where he worked in a rural health clinic. He appears lethargic. He is oriented to person, place, and time. His temperature is 39°C (102°F), pulse is 115/min, respirations are 20/min, and blood pressure is 100/70 mm Hg. Examination shows equal and reactive pupils. There are scattered petechiae over the trunk and lower extremities. Range of motion of the neck is decreased due to pain. Neurologic examination shows no focal findings. Blood cultures are obtained and a lumbar puncture is performed. Cerebrospinal fluid (CSF) analysis shows neutrophilic pleocytosis and decreased glucose concentration. Which of the following is most likely to have prevented this patient's condition?
A. Fluconazole therapy
B. Polysaccharide conjugate vaccine (Correct Answer)
C. Inactivated whole-cell vaccine
D. Toxoid vaccine
E. Erythromycin therapy
Explanation: ***Polysaccharide conjugate vaccine***
- The patient presents with classic symptoms of **bacterial meningitis** (**fever, stiff neck, headache, lethargy, petechiae**), and CSF analysis showing **neutrophilic pleocytosis** and **decreased glucose** further supports this diagnosis.
- Given the patient's age and travel history, **Neisseria meningitidis** is a strong possibility, and being vaccinated with a **polysaccharide conjugate vaccine** against this bacterium would have likely prevented the condition.
*Fluconazole therapy*
- **Fluconazole** is an **antifungal medication** used to treat fungal infections, such as candidiasis or cryptococcosis.
- It would not be effective against **bacterial meningitis**, which is the most likely diagnosis in this case.
*Inactivated whole-cell vaccine*
- An **inactivated whole-cell vaccine** is a type of vaccine, but the specific type of pathogen targeted (e.g., *Bordetella pertussis* for whooping cough) is not specified and is unlikely to be relevant here.
- While various inactivated vaccines exist, a non-specific inactivated whole-cell vaccine would not necessarily target the organism causing **meningitis** in this patient.
*Toxoid vaccine*
- A **toxoid vaccine** uses an inactivated bacterial toxin to stimulate an immune response, protecting against diseases caused by bacterial toxins (e.g., tetanus, diphtheria).
- This patient's symptoms are indicative of a direct bacterial infection causing **meningitis**, not a toxin-mediated disease.
*Erythromycin therapy*
- **Erythromycin** is a **macrolide antibiotic** used to treat certain bacterial infections, such as respiratory tract infections (e.g., *Mycoplasma pneumoniae*, *Chlamydia pneumoniae*).
- While an antibiotic, it is not the most appropriate first-line treatment or preventative measure for **bacterial meningitis**, especially in the context of preventing **Neisseria meningitidis** infection compared to vaccination.
Question 13: A 31-year-old man with no medical history presents to his provider for infertility. He states that he and his partner have had unprotected intercourse for 1 year and have been unable to conceive. Upon further workup, he is determined to have antisperm antibodies (ASA), but he does not have any other signs or labs suggesting systemic autoimmune disease. A breakdown of which of the following may have played a role in the pathogenesis of his infertility?
A. Connexons
B. Integrins
C. Desmoplakins
D. E-cadherins
E. Occludins (Correct Answer)
Explanation: ***Occludins***
- **Occludins** are key components of **tight junctions** (zona occludens) in the Sertoli cells of the testes, which form the **blood-testis barrier**.
- A disruption or breakdown of these tight junctions can expose germ cells to the immune system, leading to the formation of **antisperm antibodies** and subsequent infertility.
*Connexons*
- **Connexons** form **gap junctions**, which are crucial for intercellular communication through the direct passage of small molecules between adjacent cells.
- While important for various cellular processes, their breakdown is not a direct cause of autoantibody formation against sperm.
*Integrins*
- **Integrins** are cell surface receptors that mediate **cell-matrix** and **cell-cell adhesion**, playing roles in cell signaling and migration.
- Their primary function is not in forming the tight junctions that prevent immune exposure of sperm, so their breakdown would not directly cause antisperm antibodies.
*Desmoplakins*
- **Desmoplakins** are major proteins of **desmosomes**, which provide strong mechanical adhesion between cells through intermediate filaments.
- Desmosomes are important for tissue integrity but do not form the selective barrier that protects sperm from immune surveillance.
*E-cadherins*
- **E-cadherins** are cell adhesion molecules primarily involved in forming **adherens junctions** and maintaining epithelial cell polarity.
- While critical for cell-cell adhesion, they are not the primary constituents of the tight junctions that comprise the blood-testis barrier.
Question 14: Immunology researchers attempt to characterize the role of several cytokines in a 5-year-old male’s allergic reaction to peanuts. Months after initial exposure to peanuts, the child was brought to the ER due to repeat exposure with symptoms of anaphylaxis that resolved following epinephrine injection and supportive therapy. Which of the following best describes the role of IL-4 in the child’s response:
A. Neutrophil chemotaxis
B. Macrophage and Th1 cell activation
C. Stimulates IgA production
D. B cell class switching (Correct Answer)
E. Growth of cytotoxic T cells
Explanation: ***B cell class switching***
- **IL-4** is a key cytokine in allergic reactions, primarily promoting **B cell class switching** to IgE, which mediates immediate hypersensitivity.
- This **IgE** then binds to mast cells and basophils, leading to histamine release upon re-exposure to the allergen and causing symptoms of anaphylaxis.
*Neutrophil chemotaxis*
- **Neutrophil chemotaxis** is primarily mediated by cytokines like **IL-8** (CXCL8) and C5a, not IL-4.
- Neutrophils are more involved in bacterial infections and acute inflammatory responses, not typically central to allergic anaphylaxis.
*Macrophage and Th1 cell activation*
- **Macrophage** and **Th1 cell activation** are mainly driven by **interferon-gamma (IFN-γ)** and **IL-12**, which promote cell-mediated immunity.
- **IL-4** typically inhibits **Th1 responses** and promotes **Th2 responses**, which are characteristic of allergic reactions.
*Stimulates IgA production*
- **IgA production** is primarily induced by cytokines such as **transforming growth factor-beta (TGF-β)**, **IL-5**, and **IL-6**, often in mucosal immunity.
- While IL-4 plays a role in humoral immunity, its main role in allergic reactions is not the stimulation of IgA.
*Growth of cytotoxic T cells*
- The **growth of cytotoxic T cells (CTLs)** is primarily stimulated by **IL-2** and **IL-15**, which are crucial for antiviral and antitumor immunity.
- **IL-4** is not directly involved in the proliferation or differentiation of cytotoxic T cells.
Question 15: A biology graduate student is performing an experiment in the immunology laboratory. He is researching the recombination activation genes RAG1 and RAG2 in order to verify the function of these genes. He then decides to carry out the experiment on knock-out mice so that these genes will be turned off. Which of the following changes should he be expecting to see?
A. Absence of CD18 in leukocytes
B. Deficiency in CD40L on activated T cells
C. Total lack of B and T cells (Correct Answer)
D. Defect of NADPH oxidase in phagocyte
E. The mice should be asymptomatic
Explanation: ***Total lack of B and T cells***
- **RAG1 and RAG2** genes are essential for **V(D)J recombination**, the process by which B and T cell receptors are diversified.
- Without functional RAG genes, **lymphocytes cannot properly mature**, leading to a severe combined immunodeficiency characterized by the absence of B and T cells.
*Absence of CD18 in leukocytes*
- The absence of **CD18** in leukocytes is characteristic of **Leukocyte Adhesion Deficiency (LAD)**, a disorder affecting neutrophil and lymphocyte adhesion and migration.
- This condition is caused by a defect in the **integrin B2 subunit (CD18)**, which is unrelated to RAG genes.
*Deficiency in CD40L on activated T cells*
- A deficiency in **CD40L (CD154)** on activated T cells is the hallmark of **X-linked Hyper-IgM Syndrome**.
- This defect primarily affects B cell class switching and antibody production, with normal numbers of B and T cells initially.
*Defect of NADPH oxidase in phagocyte*
- A defect in **NADPH oxidase** in phagocytes is responsible for **Chronic Granulomatous Disease (CGD)**.
- This leads to impaired phagocytic killing of microorganisms, but does not affect B or T cell development.
*The mice should be asymptomatic*
- Given the critical role of RAG genes in adaptive immunity, **knock-out mice would suffer from severe immunodeficiency**.
- They would be highly susceptible to infections and would likely develop a severe, life-threatening condition, not be asymptomatic.
Question 16: A scientist is studying the process of thymus-dependent B cell activation in humans. He observes that, after bacterial infections, the germinal centers of secondary lymphoid organs become highly metabolically active. After subsequent reinfection with the same pathogen, the organism is able to produce immunoglobulins at a much faster pace. Which of the following processes is likely taking place in the germinal centers at the beginning of an infection?
A. T cell positive selection
B. T cell negative selection
C. Development of early pro-B cells
D. Development of immature B cells
E. Affinity maturation (Correct Answer)
Explanation: ***Affinity maturation***
- This process involves **somatic hypermutation** in the germinal centers, leading to B cells with receptors having higher affinity for the antigen.
- Coupled with **clonal selection**, this ensures that subsequent immune responses are faster and more effective due to the improved binding of antibodies to the pathogen.
*T cell positive selection*
- This process occurs in the **thymic cortex** and selects T cells capable of recognizing self-MHC molecules.
- It is crucial for the development of the T cell repertoire and does not occur in germinal centers during B cell activation.
*T cell negative selection*
- This process takes place primarily in the **thymic medulla** and eliminates T cells that bind too strongly to self-peptide/MHC complexes, preventing autoimmunity.
- It is a central tolerance mechanism and is not related to B cell responses in germinal centers.
*Development of early pro-B cells*
- The development of pro-B cells, and indeed all early stages of B cell development (pro-B, pre-B, immature B), occurs primarily in the **bone marrow**.
- These are early developmental stages, distinct from the antigen-driven processes occurring in secondary lymphoid organs during an infection.
*Development of immature B cells*
- Immature B cells develop from pre-B cells in the **bone marrow** and then migrate to secondary lymphoid organs to complete maturation.
- This step occurs prior to encountering an antigen in the germinal centers and is part of initial B cell development rather than the refinement of the immune response to an infection.
Question 17: A 5-year-old boy is referred to an immunologist because of episodes of recurrent infections. He complains of ear pain, nasal discharge, congestion, and headache. His medical history is significant for neonatal sepsis, recurrent bronchitis, and otitis media. The boy also had pneumocystis pneumonia when he was 11 months old. His mother reports that she had a younger brother who had multiple serious infections and died when he was 4 years old because of otogenic sepsis. Her grandfather frequently developed pneumonia and had multiple episodes of diarrhea. The patient is in the 10th percentile for height and 40th percentile for weight. The vital signs include: blood pressure 90/60 mm Hg, heart rate 111/min, respiratory rate 26/min, and temperature 38.3°C (100.9°F). Physical examination reveals a red, swollen, and bulging eardrum and enlarged retroauricular lymph nodes. Meningeal signs are negative and the physician suspects the presence of a primary immunodeficiency. After a thorough laboratory investigation, the patient is found to be CD40L deficient. Despite this deficiency, which of the following chains would still be expressed normally in this patient’s B lymphocytes?
A. α and γ heavy chains
B. μ and δ heavy chains (Correct Answer)
C. α and ε heavy chains
D. μ and ε heavy chains
E. μ heavy chain only
Explanation: ***μ and δ heavy chains***
- **CD40L deficiency** prevents T-cell help for B cells, thus inhibiting **class-switch recombination (CSR)** and the production of IgG, IgA, and IgE.
- Despite the inability to class-switch, B cells can still produce **IgM** and **IgD** as these immunoglobulins are generated via alternative splicing of mRNA and do not depend on CD40L signaling or class switching.
*α and γ heavy chains*
- **Alpha (α) heavy chains** characterize IgA, and **gamma (γ) heavy chains** characterize IgG.
- The production of IgA and IgG requires **CD40L-CD40 interaction** for class-switch recombination, which is deficient in this patient.
*α and ε heavy chains*
- **Alpha (α) heavy chains** characterize IgA, and **epsilon (ε) heavy chains** characterize IgE.
- The production of IgA and IgE is dependent on **CD40L-CD40 signaling** for class-switch recombination, which is impaired in CD40L deficiency.
*μ and ε heavy chains*
- While **mu (μ) heavy chains** are normally expressed, **epsilon (ε) heavy chains** (IgE) require class-switch recombination.
- Class switching to IgE requires **CD40L-CD40 interaction** and IL-4, which is defective in this patient.
*μ heavy chain only*
- While **mu (μ) heavy chains** are indeed expressed, **delta (δ) heavy chains** (IgD) are also normally expressed by naive B cells.
- Both IgM and IgD are **co-expressed** on the surface of naive B cells through differential splicing of mRNA, independent of CD40L signaling.
Question 18: A 48-year-old woman presents to an urgent care clinic with the complaints of bloody diarrhea, mid-abdominal discomfort, and cramping for 3 days. She also has nausea, vomiting, and fever. She reports that she had eaten raw oysters at a local seafood restaurant almost 3 days ago, but she denies any other potentially infectious exposures. Her temperature is 37.5°C (99.6°F), respiratory rate is 15/min, pulse is 67/min, and blood pressure is 102/68 mm Hg. Physical examination is non-contributory. What is the most likely diagnosis?
A. Bacillus cereus infection
B. Rotavirus infection
C. Norovirus infection
D. C. difficile colitis
E. Vibrio parahaemolyticus infection (Correct Answer)
Explanation: ***Vibrio parahaemolyticus infection***
- The patient's symptoms of **bloody diarrhea**, **abdominal discomfort**, **nausea**, **vomiting**, and **fever** occurring 3 days after consuming **raw oysters** are highly suggestive of *Vibrio parahaemolyticus* infection.
- This bacterium is a common cause of **seafood-associated gastroenteritis**, particularly linked to exposure to **raw shellfish**.
*Bacillus cereus infection*
- This typically causes **food poisoning** from **reheated rice** or other starchy foods, presenting with a much shorter incubation period (1-6 hours for emetic toxin, 6-15 hours for diarrheal toxin).
- While it causes vomiting and diarrhea, the incubation period and association with raw oysters are not consistent with *B. cereus*.
*Rotavirus infection*
- Rotavirus primarily affects **infants and young children**, causing **watery diarrhea**, vomiting, and fever.
- It is highly contagious and spread **fecal-orally**, but it is not typically associated with seafood consumption in adults.
*Norovirus infection*
- Norovirus is a common cause of **viral gastroenteritis** and can cause outbreaks, often linked to contaminated food or water, but typically not specifically raw oysters.
- While it causes nausea, vomiting, and diarrhea, **bloody diarrhea** is uncommon, and the specific exposure to raw oysters points away from norovirus.
*C. difficile colitis*
- *Clostridium difficile* infection is characterized by **diarrhea** (often foul-smelling and watery) and **abdominal pain**, but it typically occurs after **antibiotic use** or in healthcare settings.
- The patient has no history of recent antibiotic use, and the link to raw oysters is inconsistent with *C. difficile*.
Question 19: A 16-year-old man presents to the clinic accompanied by his father, with the complaints of high fever, sore throat, and bloody diarrhea for 4 days. He adds that he is also nauseous and vomited several times in the past 2 days. He denies any recent travel or eating outside. He recently started a dog-walking business. The father relates that two of the dogs had been unwell. His temperature is 38.9°C (102°F), respiratory rate is 16/min, pulse is 77/min, and blood pressure is 100/88 mm Hg. A physical examination is performed and is within normal limits including the abdominal exam. Blood test results are given below:
Hb%: 14 gm/dL
Total count (WBC): 13,100/mm3
Differential count:
Neutrophils: 80%
Lymphocytes: 15%
Monocytes: 5%
What is the most likely diagnosis?
A. Rotavirus infection
B. Norovirus infection
C. C. difficile colitis
D. Bacillus cereus infection
E. Yersiniosis (Correct Answer)
Explanation: ***Yersiniosis***
- The patient's symptoms of **fever**, **sore throat**, **bloody diarrhea**, nausea, and vomiting, combined with a history of exposure to sick dogs (potential carriers of *Yersinia*), strongly suggest **Yersiniosis**.
- **Elevated WBC count** with neutrophilia is consistent with a bacterial infection, and *Yersinia enterocolitica* is a known cause of bloody diarrhea, particularly in children and adolescents, often mimicking appendicitis.
*Rotavirus infection*
- While Rotavirus causes **diarrhea** and vomiting, it is typically **watery diarrhea** and does not usually present with bloody stool or significant neutrophilia.
- It is more common in **infants** and young children and symptoms usually resolve within a week.
*Norovirus infection*
- Norovirus causes acute **gastroenteritis** with vomiting and watery diarrhea, but usually **without blood** in the stool.
- The symptoms appear more abruptly and resolve within 1-3 days, and it typically does not present with a significant fever or sore throat.
*C. difficile colitis*
- This typically occurs after **antibiotic use** and presents with severe **watery diarrhea** (which can occasionally be bloody), abdominal pain, and fever.
- The patient has **no history of antibiotic exposure**, making *C. difficile* less likely.
*Bacillus cereus infection*
- **Bacillus cereus** gastroenteritis is usually associated with **food poisoning**, particularly from reheated rice or other starchy foods.
- It presents with either a **vomiting type** (short incubation, severe nausea/vomiting) or a **diarrheal type** (longer incubation, watery diarrhea), neither of which perfectly fit the bloody diarrhea, fever, and sore throat described.
Question 20: A 33-year-old woman presents to the urgent care center with 4 days of abdominal pain and increasingly frequent bloody diarrhea. She states that she is currently having 6 episodes of moderate volume diarrhea per day with streaks of blood mixed in. She says she recently returned from a long camping trip with her friends where they cooked all of their own food and drank water from nearby streams. Physical examination is negative for acute tenderness, rebound tenderness, or abnormal bowel sounds. Her vital signs include temperature 38.0°C (100.4°F), blood pressure 106/74 mm Hg, heart rate 94/min, and respiratory rate 14/min. Given the following options, which is the most likely pathogen responsible for her presentation?
A. Campylobacter
B. Shigella
C. Clostridium difficile
D. Salmonella
E. E. coli O157:H7 (Correct Answer)
Explanation: ***E. coli 0157:H7***
- This patient's presentation with **bloody diarrhea** and a history of consuming potentially contaminated **stream water** during a camping trip is highly suggestive of enterohemorrhagic *E. coli* O157:H7 infection.
- While other pathogens can cause bloody diarrhea, the combination of environmental exposure, moderate fever, and the absence of severe systemic illness points towards this specific *E. coli* strain, known for producing toxins that damage the intestinal lining.
*Campylobacter*
- While *Campylobacter* can cause **bloody diarrhea** and is associated with consuming contaminated water or undercooked poultry, its incubation period is typically longer (2-5 days) than the 4 days described, and it more frequently causes **abdominal cramping with fever**.
- Although it's a possibility, *E. coli* O157:H7 is a stronger fit given the rapid onset of bloody diarrhea after likely water exposure.
*Shigella*
- *Shigella* causes **dysentery** (bloody, mucoid stools) and is highly contagious, often transmitted person-to-person or through contaminated food/water.
- However, *Shigella* infections commonly present with **high fever**, severe abdominal cramps, and tenesmus, which are not prominently described in this patient.
*Clostridium difficile*
- *Clostridium difficile* infection (CDI) is typically associated with **recent antibiotic use** or hospitalization, which is not mentioned in this patient's history.
- While CDI can cause bloody diarrhea, it is more commonly characterized by watery diarrhea, severe abdominal pain, and often a distinctive smell.
*Salmonella*
- *Salmonella* can cause **gastroenteritis** with watery or sometimes bloody diarrhea, often linked to contaminated food (e.g., poultry, eggs).
- However, bloody diarrhea is less common than with *E. coli* O157:H7, and this patient's specific exposure to stream water during camping makes *E. coli* O157:H7 a more direct link.
