A scientist is studying the mechanisms by which bacteria become resistant to antibiotics. She begins by obtaining a culture of vancomycin-resistant Enterococcus faecalis and conducts replicate plating experiments. In these experiments, colonies are inoculated onto a membrane and smeared on 2 separate plates, 1 containing vancomycin and the other with no antibiotics. She finds that all of the bacterial colonies are vancomycin resistant because they grow on both plates. She then maintains the bacteria in liquid culture without vancomycin while she performs her other studies. Fifteen generations of bacteria later, she conducts replicate plating experiments again and finds that 20% of the colonies are now sensitive to vancomycin. Which of the following mechanisms is the most likely explanation for why these colonies have become vancomycin sensitive?
Q152
A 68-year-old man comes to the physician because of a 1-month history of fatigue, low-grade fevers, and cough productive of blood-tinged sputum. He has type 2 diabetes mellitus and chronic kidney disease and underwent kidney transplantation 8 months ago. His temperature is 38.9°C (102.1°F) and pulse is 98/min. Examination shows rhonchi in the right lower lung field. An x-ray of the chest shows a right-sided lobar consolidation. A photomicrograph of specialized acid-fast stained tissue from a blood culture is shown. Which of the following is the strongest predisposing factor for this patient's condition?
Q153
A 48-year-old man comes to the physician because of a hypopigmented skin lesion on his finger. He first noticed it 4 weeks ago after cutting his finger with a knife while preparing food. He did not feel the cut. For the past week, he has also had fever, fatigue, and malaise. He has not traveled outside the country since he immigrated from India to join his family in the United States 2 years ago. His temperature is 38.7°C (101.7°F). Physical examination shows a small, healing laceration on the dorsal aspect of the left index finger and an overlying well-defined, hypopigmented macule with raised borders. Sensation to pinprick and light touch is decreased over this area. Which of the following is the most likely causal pathogen of this patient's condition?
Q154
An investigator inoculates three different broths with one colony-forming unit of Escherichia coli. Broth A contains 100 μmol of lactose, broth B contains 100 μmol of glucose, and broth C contains both 100 μmol of lactose and 100 μmol of glucose. After 24 hours, the amounts of lactose, galactose, and glucose in the three broths are measured. The results of the experiment are shown:
Lactose Galactose Glucose
Broth A 43 μmol 11 μmol 9 μmol
Broth B 0 μmol 0 μmol 39 μmol
Broth C 94 μmol 1 μmol 66 μmol
The observed results are most likely due to which of the following properties of broth A compared to broth C?
Q155
A 34-year-old, previously healthy woman is admitted to the hospital with abdominal pain and bloody diarrhea. She reports consuming undercooked beef a day before the onset of her symptoms. Her medical history is unremarkable. Vital signs include: blood pressure 100/70 mm Hg, pulse rate 70/min, respiratory rate 16/min, and temperature 36.6℃ (97.9℉). Physical examination shows paleness, face and leg edema, and abdominal tenderness in the lower right quadrant. Laboratory investigation shows the following findings:
Erythrocytes 3 x 106/mm3
Hemoglobin 9.4 g/dL
Hematocrit 0.45 (45%)
Corrected reticulocyte count 5.5%
Platelet count 18,000/mm3
Leukocytes 11,750/mm3
Total bilirubin 2.33 mg/dL (39.8 µmol/L)
Direct bilirubin 0.2 mg/dL (3.4 µmol/L)
Serum creatinine 4.5 mg/dL (397.8 µmol/L)
Blood urea nitrogen 35.4 mg/dL (12.6 mmol/L)
E. coli O157: H7 was identified in the patient’s stool. Which toxin is likely responsible for her symptoms?
Q156
A 20-week-old infant is brought to an urgent care clinic by her mother because she has not been eating well for the past 2 days. The mother said her daughter has also been "floppy" since yesterday morning and has been unable to move or open her eyes since the afternoon of the same day. The child has recently started solid foods, like cereals sweetened with honey. There is no history of loose, watery stools. On examination, the child is lethargic with lax muscle tone. She does not have a fever or apparent respiratory distress. What is the most likely mode of transmission of the pathogen responsible for this patient’s condition?
Q157
A 34-year-old man presents with a 2-day history of loose stools, anorexia, malaise, and abdominal pain. He describes the pain as moderate, cramping in character, and diffusely localized to the periumbilical region. His past medical history is unremarkable. He works as a wildlife photographer and, 1 week ago, he was in the Yucatan peninsula capturing the flora and fauna for a magazine. The vital signs include blood pressure 120/60 mm Hg, heart rate 90/min, respiratory rate 18/min, and body temperature 38.0°C (100.4°F). Physical examination is unremarkable. Which of the following is a characteristic of the microorganism most likely responsible for this patient’s symptoms?
Q158
A 40-year-old woman comes to the emergency department due to severe right flank pain, fever, chills, and decreased urine output. The vital signs include a temperature of 39.0°C (102.2°F), heart rate of 120/min, a regular breathing pattern, and blood pressure of 128/70 mm Hg. Cardiopulmonary auscultation is normal. In addition, tenderness is elicited by right lumbar percussion. After initiating intravenous antibiotics empirically, the condition of the patient improves significantly. However, a low urine output persists. The results of the ordered laboratory tests are as follows:
Urine culture Proteus mirabilis, > 150,000 CFU/mL (normal range: < 100,000 CFU/mL to no bacterial growth in asymptomatic patients)
Density 1.030; Leukocyte esterase (+); Nitrites (+)
pH 7.8 (normal range: 4.5–8.0)
C-reactive protein 60 mg/dL (normal range: 0–10 mg/dL)
Serum creatinine 1.8 mg/dL (normal range: 0.6–1.2 mg/dL)
BUN 40 mg/dL (normal range: 7–20 mg/dL)
Plain abdominal film Complex renal calculus in the right kidney
Which of the following is the most likely type of stone the patient has?
Q159
A 35-year-old man comes to the emergency department with fever, chills, dyspnea, and a productive cough. His symptoms began suddenly 2 days ago. He was diagnosed with HIV 4 years ago and has been on triple antiretroviral therapy since then. He smokes one pack of cigarettes daily. He is 181 cm (5 ft 11 in) tall and weighs 70 kg (154 lb); BMI is 21.4 kg/m2. He lives in Illinois and works as a carpenter. His temperature is 38.8°C (101.8°F), pulse is 110/min, respirations are 24/min, and blood pressure is 105/74 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 92%. Examinations reveals crackles over the right lower lung base. The remainder of the examination shows no abnormalities. Laboratory studies show:
Hemoglobin 11.5 g/dL
Leukocyte count 12,800/mm3
Segmented neutrophils 80%
Eosinophils 1%
Lymphocytes 17%
Monocytes 2%
CD4+ T-lymphocytes 520/mm3(N ≥ 500)
Platelet count 258,000/mm3
Serum
Na+ 137 mEq/L
Cl- 102 mEq/L
K+ 5.0 mEq/L
HCO3- 22 mEq/L
Glucose 92 mg/dL
An x-ray of the chest shows a right lower-lobe infiltrate of the lung. Which of the following is the most likely causal organism?
Q160
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?
Bacteria US Medical PG Practice Questions and MCQs
Question 151: A scientist is studying the mechanisms by which bacteria become resistant to antibiotics. She begins by obtaining a culture of vancomycin-resistant Enterococcus faecalis and conducts replicate plating experiments. In these experiments, colonies are inoculated onto a membrane and smeared on 2 separate plates, 1 containing vancomycin and the other with no antibiotics. She finds that all of the bacterial colonies are vancomycin resistant because they grow on both plates. She then maintains the bacteria in liquid culture without vancomycin while she performs her other studies. Fifteen generations of bacteria later, she conducts replicate plating experiments again and finds that 20% of the colonies are now sensitive to vancomycin. Which of the following mechanisms is the most likely explanation for why these colonies have become vancomycin sensitive?
A. Point mutation
B. Gain of function mutation
C. Viral infection
D. Plasmid loss (Correct Answer)
E. Loss of function mutation
Explanation: ***Plasmid loss***
- The initial **vancomycin resistance** in *Enterococcus faecalis* is often mediated by genes located on **plasmids**, which are extrachromosomal DNA.
- In the absence of selective pressure (vancomycin), bacteria that lose the plasmid (and thus the resistance genes) have a **growth advantage** over those that retain the energetically costly plasmid, leading to an increase in sensitive colonies over generations.
*Point mutation*
- A **point mutation** typically involves a change in a single nucleotide and could lead to loss of resistance if it occurred in a gene conferring resistance.
- However, since there was no selective pressure for loss of resistance, it is less likely that 20% of the population would acquire such a specific point mutation to revert resistance.
*Gain of function mutation*
- A **gain of function mutation** would imply that the bacteria acquired a *new* advantageous trait, not the *loss* of resistance.
- This type of mutation would not explain why some colonies became sensitive to vancomycin after the drug was removed.
*Viral infection*
- **Viral infection** (bacteriophages) can transfer genes through transduction or cause bacterial lysis, but it's not the primary mechanism for a widespread reversion of resistance in the absence of antibiotic pressure.
- It would not explain the observed increase in vancomycin-sensitive colonies due to evolutionary pressure.
*Loss of function mutation*
- While a **loss of function mutation** in a gene conferring resistance could lead to sensitivity, it's generally less likely to explain a 20% shift without selective pressure than **plasmid loss**.
- Plasmids are often unstable and are easily lost in the absence of selection, whereas a specific gene mutation causing loss of function would need to arise and become prevalent in the population.
Question 152: A 68-year-old man comes to the physician because of a 1-month history of fatigue, low-grade fevers, and cough productive of blood-tinged sputum. He has type 2 diabetes mellitus and chronic kidney disease and underwent kidney transplantation 8 months ago. His temperature is 38.9°C (102.1°F) and pulse is 98/min. Examination shows rhonchi in the right lower lung field. An x-ray of the chest shows a right-sided lobar consolidation. A photomicrograph of specialized acid-fast stained tissue from a blood culture is shown. Which of the following is the strongest predisposing factor for this patient's condition?
A. Sharing of unsterile IV needles
B. Poor oral hygiene
C. Exposure to contaminated air-conditioning unit
D. Exposure to contaminated soil (Correct Answer)
E. Crowded living situation
Explanation: ***Exposure to contaminated soil***
- The photomicrograph shows **acid-fast stain** demonstrating **filamentous, branching gram-positive rods**, consistent with **Nocardia species**.
- **Nocardiosis** is acquired through **inhalation of Nocardia spores from contaminated soil or dust**, which is the primary environmental source and route of transmission.
- While this patient's **immunocompromised status** (post-kidney transplant on immunosuppressive therapy) is the critical host factor that predisposes him to infection, **soil exposure** is the specific environmental predisposing factor among the options listed.
- Nocardia is an opportunistic pathogen that primarily affects immunocompromised individuals, causing pulmonary infection that can disseminate.
*Sharing of unsterile IV needles*
- Sharing unsterile IV needles is a common route for transmitting **bloodborne pathogens** (e.g., HIV, hepatitis B/C) or causing bacterial endocarditis and soft tissue infections.
- This is not the typical route of acquisition for **pulmonary nocardiosis**, which is acquired via inhalation.
*Poor oral hygiene*
- Poor oral hygiene predisposes to dental caries, periodontal disease, and aspiration of oral flora causing pneumonia or lung abscess.
- **Actinomyces** (not acid-fast) is associated with poor oral hygiene and can be confused with Nocardia morphologically, but Actinomyces is not acid-fast positive.
- This is not a risk factor for acquiring **Nocardia** infection.
*Exposure to contaminated air-conditioning unit*
- Contaminated air-conditioning units and water systems are associated with **Legionella pneumophila**, causing Legionnaires' disease.
- Legionella is not acid-fast and does not show the branching filamentous morphology seen with Nocardia.
*Crowded living situation*
- Crowded living situations increase risk of person-to-person transmission of respiratory pathogens such as **Mycobacterium tuberculosis**, influenza, and other droplet-spread infections.
- **Nocardia** is acquired from environmental sources (soil, dust), not through person-to-person transmission.
Question 153: A 48-year-old man comes to the physician because of a hypopigmented skin lesion on his finger. He first noticed it 4 weeks ago after cutting his finger with a knife while preparing food. He did not feel the cut. For the past week, he has also had fever, fatigue, and malaise. He has not traveled outside the country since he immigrated from India to join his family in the United States 2 years ago. His temperature is 38.7°C (101.7°F). Physical examination shows a small, healing laceration on the dorsal aspect of the left index finger and an overlying well-defined, hypopigmented macule with raised borders. Sensation to pinprick and light touch is decreased over this area. Which of the following is the most likely causal pathogen of this patient's condition?
A. Epidermophyton floccosum
B. Mycobacterium leprae (Correct Answer)
C. Malassezia furfur
D. Pseudomonas aeruginosa
E. Leishmania donovani
Explanation: ***Mycobacterium leprae***
- The patient's history of immigration from **India**, a country endemic for leprosy, coupled with the **hypopigmented, anesthetic skin lesion** with raised borders, is classic for **leprosy** (Hansen's disease).
- The diminished sensation to pinprick and light touch in the affected area points to **nerve involvement**, a hallmark of *Mycobacterium leprae* infection.
*Epidermophyton floccosum*
- This fungus primarily causes **tinea infections** (ringworm), such as athlete's foot and jock itch.
- While it can cause skin lesions, they are typically **erythematous** and scaly, not hypopigmented with sensory loss.
*Malassezia furfur*
- This yeast is responsible for **tinea versicolor**, characterized by **hypopigmented or hyperpigmented patches** that typically scale.
- However, it does not cause **nerve damage** or accompanying sensory loss, and systemic symptoms like fever and malaise are not typical.
*Pseudomonas aeruginosa*
- This bacterium can cause various opportunistic infections, including skin infections like **ecthyma gangrenosum** or **folliculitis** in immunocompromised patients.
- *Pseudomonas* infections are usually painful, often associated with a characteristic **grape-like odor**, and do not typically present with chronic, anesthetic, hypopigmented lesions.
*Leishmania donovani*
- This parasite causes **visceral leishmaniasis** (kala-azar), which presents with fever, weight loss, hepatosplenomegaly, and **hyperpigmentation of the skin** (darkening), not hypopigmentation.
- It does not cause localized anesthetic skin lesions like those described in the patient.
Question 154: An investigator inoculates three different broths with one colony-forming unit of Escherichia coli. Broth A contains 100 μmol of lactose, broth B contains 100 μmol of glucose, and broth C contains both 100 μmol of lactose and 100 μmol of glucose. After 24 hours, the amounts of lactose, galactose, and glucose in the three broths are measured. The results of the experiment are shown:
Lactose Galactose Glucose
Broth A 43 μmol 11 μmol 9 μmol
Broth B 0 μmol 0 μmol 39 μmol
Broth C 94 μmol 1 μmol 66 μmol
The observed results are most likely due to which of the following properties of broth A compared to broth C?
A. Absence of glucose preventing catabolite repression (Correct Answer)
B. Higher lactose concentration allowing increased metabolism
C. Lower pH favoring lactose utilization
D. Presence of additional nutrients enhancing growth
E. Reduced osmotic pressure facilitating enzyme activity
Explanation: ***Absence of glucose preventing catabolite repression***
- In Broth A, the absence of **glucose** means that **catabolite repression** does not occur, allowing the *E. coli* to immediately and more extensively utilize **lactose**.
- This leads to a greater breakdown of lactose into galactose and glucose, and subsequent further metabolism of these sugars, evidenced by the lower final lactose concentration and higher galactose/glucose conversion than in Broth C.
*Higher lactose concentration allowing increased metabolism*
- All broths initially contain **100 μmol of lactose**, so there is no higher initial concentration in Broth A to explain the difference in metabolism.
- The observed difference is in the *extent* of lactose utilization, not in the initial amount available.
*Lower pH favoring lactose utilization*
- The experiment does not provide any information about the **pH levels** in the broths.
- While pH can affect enzyme activity, there is no basis in the given data to suggest a pH difference as the cause for varied lactose utilization.
*Presence of additional nutrients enhancing growth*
- All broths are inoculated with the same organism and the only specified difference is the sugar content (lactose, glucose, or both).
- There is no mention of **additional nutrients** being present in Broth A compared to Broth C.
*Reduced osmotic pressure facilitating enzyme activity*
- The initial total solute concentration in Broth A (100 μmol lactose) is similar to Broth B (100 μmol glucose) and lower than Broth C (100 μmol lactose + 100 μmol glucose).
- While **osmotic pressure** can affect bacterial growth, there is nothing in the data to suggest it is the primary factor explaining the difference in lactose metabolism between Broth A and Broth C.
Question 155: A 34-year-old, previously healthy woman is admitted to the hospital with abdominal pain and bloody diarrhea. She reports consuming undercooked beef a day before the onset of her symptoms. Her medical history is unremarkable. Vital signs include: blood pressure 100/70 mm Hg, pulse rate 70/min, respiratory rate 16/min, and temperature 36.6℃ (97.9℉). Physical examination shows paleness, face and leg edema, and abdominal tenderness in the lower right quadrant. Laboratory investigation shows the following findings:
Erythrocytes 3 x 106/mm3
Hemoglobin 9.4 g/dL
Hematocrit 0.45 (45%)
Corrected reticulocyte count 5.5%
Platelet count 18,000/mm3
Leukocytes 11,750/mm3
Total bilirubin 2.33 mg/dL (39.8 µmol/L)
Direct bilirubin 0.2 mg/dL (3.4 µmol/L)
Serum creatinine 4.5 mg/dL (397.8 µmol/L)
Blood urea nitrogen 35.4 mg/dL (12.6 mmol/L)
E. coli O157: H7 was identified in the patient’s stool. Which toxin is likely responsible for her symptoms?
A. Erythrogenic toxin
B. Shiga toxin (Correct Answer)
C. Enterotoxin type B
D. α-hemolysin
E. Exotoxin A
Explanation: ***Shiga toxin***
- The clinical presentation of **abdominal pain, bloody diarrhea, thrombocytopenia, microangiopathic hemolytic anemia** (indicated by low hemoglobin, elevated reticulocyte count, and elevated indirect bilirubin), and **acute kidney injury** (elevated creatinine and BUN with edema) strongly suggests **hemolytic uremic syndrome (HUS)**.
- ***E. coli* O157:H7** is a common cause of HUS, and it produces **Shiga toxins** (Stx1 and Stx2, also known as verotoxins), which cause endothelial damage leading to the characteristic features of HUS.
- Shiga toxins inhibit protein synthesis by cleaving ribosomal RNA, resulting in **endothelial cell damage, thrombotic microangiopathy**, and the classic triad of HUS.
*Erythrogenic toxin*
- This toxin is produced by **_Streptococcus pyogenes_** and is responsible for the rash seen in **scarlet fever**.
- It is not associated with **_E. coli_ O157:H7 infection** or the development of HUS.
*Enterotoxin type B*
- **Enterotoxin type B** is typically associated with **_Staphylococcus aureus_** and acts as a **superantigen** causing food poisoning with symptoms like vomiting and diarrhea.
- It does not cause HUS or the specific hematological and renal manifestations described in the patient.
*α-hemolysin*
- **Alpha-hemolysin** is a pore-forming cytolytic toxin produced by various bacteria, including **_Staphylococcus aureus_** and **_E. coli_**.
- While some *E. coli* strains produce alpha-hemolysin, it is not the primary toxin responsible for HUS caused by **_E. coli_ O157:H7**.
*Exotoxin A*
- **Exotoxin A** is produced by **_Pseudomonas aeruginosa_** and inhibits protein synthesis by ADP-ribosylation of elongation factor-2 (EF-2).
- It is not produced by *E. coli* O157:H7 and is not associated with HUS or the clinical presentation in this patient.
Question 156: A 20-week-old infant is brought to an urgent care clinic by her mother because she has not been eating well for the past 2 days. The mother said her daughter has also been "floppy" since yesterday morning and has been unable to move or open her eyes since the afternoon of the same day. The child has recently started solid foods, like cereals sweetened with honey. There is no history of loose, watery stools. On examination, the child is lethargic with lax muscle tone. She does not have a fever or apparent respiratory distress. What is the most likely mode of transmission of the pathogen responsible for this patient’s condition?
A. Vertical transmission
B. Vector-borne disease
C. Direct contact
D. Contaminated food (Correct Answer)
E. Airborne transmission
Explanation: ***Contaminated food***
- The infant's symptoms of **lethargy**, widespread **flaccid paralysis** (floppy, unable to move or open eyes), and recent ingestion of **honey** (a known source of **Clostridium botulinum** spores) strongly suggest **infant botulism**.
- **Infant botulism** is acquired through the ingestion of **Clostridium botulinum spores**, typically from environmental sources or contaminated food like honey, which then germinate in the infant's immature gut.
*Vertical transmission*
- **Vertical transmission** refers to the passage of a pathogen from mother to offspring during pregnancy, birth, or breastfeeding.
- The clinical picture of **flaccid paralysis** and association with **honey ingestion** in this case does not align with typical vertically transmitted infections.
*Vector-borne disease*
- **Vector-borne diseases** are transmitted by an arthropod vector, such as mosquitoes or ticks.
- There is no clinical or epidemiological evidence in the scenario to suggest an **arthropod vector** as the source of this infant's illness.
*Direct contact*
- Diseases transmitted by **direct contact** typically require close physical interaction with an infected individual or their body fluids.
- The onset of **neurological symptoms** and the specific history of **honey ingestion** do not point to direct contact as the mode of transmission for botulism.
*Airborne transmission*
- **Airborne transmission** occurs when pathogens are spread through respiratory droplets or aerosols.
- The symptoms of **flaccid paralysis** and the history of recent **honey ingestion** are not consistent with an airborne pathogen.
Question 157: A 34-year-old man presents with a 2-day history of loose stools, anorexia, malaise, and abdominal pain. He describes the pain as moderate, cramping in character, and diffusely localized to the periumbilical region. His past medical history is unremarkable. He works as a wildlife photographer and, 1 week ago, he was in the Yucatan peninsula capturing the flora and fauna for a magazine. The vital signs include blood pressure 120/60 mm Hg, heart rate 90/min, respiratory rate 18/min, and body temperature 38.0°C (100.4°F). Physical examination is unremarkable. Which of the following is a characteristic of the microorganism most likely responsible for this patient’s symptoms?
A. Disabling Gi alpha subunit
B. Production of lecithinase
C. Overactivation of guanylate cyclase (Correct Answer)
D. Inactivation of the 60S ribosomal subunit
E. Presynaptic vesicle dysregulation
Explanation: ***Overactivation of guanylate cyclase***
- The patient's symptoms (loose stools, abdominal cramping, recent travel to the Yucatan Peninsula) are highly suggestive of **traveler's diarrhea**, most commonly caused by **enterotoxigenic E. coli (ETEC)**.
- ETEC produces a **heat-stable toxin (ST)** that binds to the **guanylate cyclase C receptor** on intestinal epithelial cells, leading to **increased intracellular cGMP** and subsequent **chloride and water secretion**.
*Disabling Gi alpha subunit*
- This mechanism is characteristic of **pertussis toxin** (from *Bordetella pertussis*), which **ADP-ribosylates and inactivates the Gi protein**, preventing inhibition of adenylate cyclase.
- Pertussis toxin is associated with **whooping cough**, not gastrointestinal disease or traveler's diarrhea.
- Note: **Cholera toxin** works via a different mechanism—it **activates Gs alpha subunit** to increase cAMP, causing severe watery diarrhea, but the clinical presentation here (mild symptoms, travel to endemic area) favors ETEC over cholera.
*Production of lecithinase*
- **Lecithinase (alpha-toxin)** is a characteristic virulence factor of **Clostridium perfringens**, causing gas gangrene and some food poisoning, not the watery diarrhea described here.
- It acts as a phospholipase, disrupting cell membranes.
*Inactivation of the 60S ribosomal subunit*
- This mechanism is associated with **Shiga toxin** produced by **enterohemorrhagic E. coli (EHEC)** and **Shigella dysenteriae**.
- These typically cause **bloody diarrhea** and **hemolytic uremic syndrome (HUS)**, which are not described in this patient.
*Presynaptic vesicle dysregulation*
- This mechanism is characteristic of **botulinum toxin** (from *Clostridium botulinum*), which **cleaves SNARE proteins** and prevents acetylcholine release, causing flaccid paralysis.
- It is not involved in bacterial gastroenteritis causing diarrhea.
Question 158: A 40-year-old woman comes to the emergency department due to severe right flank pain, fever, chills, and decreased urine output. The vital signs include a temperature of 39.0°C (102.2°F), heart rate of 120/min, a regular breathing pattern, and blood pressure of 128/70 mm Hg. Cardiopulmonary auscultation is normal. In addition, tenderness is elicited by right lumbar percussion. After initiating intravenous antibiotics empirically, the condition of the patient improves significantly. However, a low urine output persists. The results of the ordered laboratory tests are as follows:
Urine culture Proteus mirabilis, > 150,000 CFU/mL (normal range: < 100,000 CFU/mL to no bacterial growth in asymptomatic patients)
Density 1.030; Leukocyte esterase (+); Nitrites (+)
pH 7.8 (normal range: 4.5–8.0)
C-reactive protein 60 mg/dL (normal range: 0–10 mg/dL)
Serum creatinine 1.8 mg/dL (normal range: 0.6–1.2 mg/dL)
BUN 40 mg/dL (normal range: 7–20 mg/dL)
Plain abdominal film Complex renal calculus in the right kidney
Which of the following is the most likely type of stone the patient has?
A. Cystine
B. Uric acid
C. Xanthine
D. Calcium oxalate
E. Struvite (Correct Answer)
Explanation: ***Struvite***
- The presence of **_Proteus mirabilis_** in the urine culture, combined with a **high urine pH (7.8)** and a **"complex renal calculus"** (often a **staghorn calculus**), strongly points to a **struvite stone**.
- **_Proteus_** and other urease-producing bacteria hydrolyze urea to ammonia, creating an alkaline environment that favors the precipitation of **magnesium ammonium phosphate (struvite)**.
*Cystine*
- **Cystine stones** are associated with the genetic disorder **cystinuria** and often present at a younger age; they are uncommon and typically appear as hexagonal crystals.
- The urine pH in cystinuria is usually acidic or neutral, contrasting with the **alkaline urine** seen in this patient.
*Uric acid*
- **Uric acid stones** are typically found in patients with **gout** or those with high purine intake, and they are usually seen in **acidic urine** environments.
- They are **radiolucent** (not visible on plain X-ray), but the association with **_Proteus_ infection** and **alkaline urine** is absent.
*Xanthine*
- **Xanthine stones** are extremely rare and result from a deficiency of **xanthine oxidase** or treatment with **allopurinol** for conditions like gout.
- The patient's presentation does not align with the typical causes or characteristics of **xanthine urolithiasis**.
*Calcium oxalate*
- **Calcium oxalate stones** are the most common type of kidney stone but are not typically associated with **urinary tract infections** (UTIs) caused by **urease-producing bacteria**.
- While they can be complex, the presence of **_Proteus_** and **alkaline urine** strongly suggests an infectious **struvite stone**.
Question 159: A 35-year-old man comes to the emergency department with fever, chills, dyspnea, and a productive cough. His symptoms began suddenly 2 days ago. He was diagnosed with HIV 4 years ago and has been on triple antiretroviral therapy since then. He smokes one pack of cigarettes daily. He is 181 cm (5 ft 11 in) tall and weighs 70 kg (154 lb); BMI is 21.4 kg/m2. He lives in Illinois and works as a carpenter. His temperature is 38.8°C (101.8°F), pulse is 110/min, respirations are 24/min, and blood pressure is 105/74 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 92%. Examinations reveals crackles over the right lower lung base. The remainder of the examination shows no abnormalities. Laboratory studies show:
Hemoglobin 11.5 g/dL
Leukocyte count 12,800/mm3
Segmented neutrophils 80%
Eosinophils 1%
Lymphocytes 17%
Monocytes 2%
CD4+ T-lymphocytes 520/mm3(N ≥ 500)
Platelet count 258,000/mm3
Serum
Na+ 137 mEq/L
Cl- 102 mEq/L
K+ 5.0 mEq/L
HCO3- 22 mEq/L
Glucose 92 mg/dL
An x-ray of the chest shows a right lower-lobe infiltrate of the lung. Which of the following is the most likely causal organism?
A. Streptococcus pneumoniae (Correct Answer)
B. Legionella pneumophila
C. Pneumocystis jirovecii
D. Staphylococcus aureus
E. Cryptococcus neoformans
Explanation: ***Streptococcus pneumoniae***
- This patient presents with **fever, chills, productive cough, dyspnea, leukocytosis with neutrophilia, and a lobar infiltrate on chest X-ray**, which are classic signs of **community-acquired bacterial pneumonia**.
- Although the patient is **HIV-positive**, his CD4+ count is >500/mm3 and he is on antiretroviral therapy, indicating relatively preserved immune function, making *S. pneumoniae* the most common cause of pneumonia even in HIV-infected individuals with controlled disease.
*Legionella pneumophila*
- While *Legionella* can cause pneumonia with fever and dyspnea, it is often associated with **gastrointestinal symptoms** (e.g., diarrhea) and **hyponatremia**, which are not present here.
- Exposure to contaminated water sources is a common risk factor, and the lobar infiltrate is less typical than diffuse or patchy infiltrates.
*Pneumocystis jirovecii*
- *Pneumocystis pneumonia (PJP)* is typically seen in **HIV patients with severely suppressed immune systems (CD4+ count <200/mm3)**.
- The patient's CD4+ count (520/mm3) is above this threshold, and PJP usually presents with diffuse interstitial infiltrates rather than a lobar infiltrate.
*Staphylococcus aureus*
- *S. aureus* pneumonia often occurs in the context of recent **influenza infection, intravenous drug use, or hospitalization**, or can present rapidly with **necrotizing pneumonia** or **empyema**.
- While possible, the absence of these specific risk factors or severe features makes it less likely than *S. pneumoniae* in this specific presentation.
*Cryptococcus neoformans*
- *Cryptococcus neoformans* is an opportunistic fungus that typically causes **pulmonary or central nervous system infections**, especially in severely immunocompromised patients (CD4+ count usually <100/mm3).
- Pulmonary cryptococcosis often manifests as **nodules or cavitary lesions**, or can be asymptomatic, which differs from the acute lobar pneumonia presented.
Question 160: 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.
