Bacteroides and anaerobic gram-negatives — MCQs

10 questions

A 37-year-old woman with a history of anorectal abscesses complains of pain in the perianal region. Physical examination reveals mild swelling, tenderness, and erythema of the perianal skin. She is prescribed oral ampicillin and asked to return for follow-up. Two days later, the patient presents with a high-grade fever, syncope, and increased swelling. Which of the following would be the most common mechanism of resistance leading to the failure of antibiotic therapy in this patient?

A 25-year-old woman comes to the physician because of a 2-day history of a burning sensation when urinating and increased urinary frequency. She is concerned about having contracted a sexually transmitted disease. Physical examination shows suprapubic tenderness. Urinalysis shows a negative nitrite test and positive leukocyte esterases. Urine culture grows organisms that show resistance to novobiocin on susceptibility testing. Which of the following is the most likely causal organism of this patient's symptoms?

A 51-year-old man with alcohol use disorder comes to the physician because of a fever and productive cough. An x-ray of the chest shows a right lower lobe consolidation and a diagnosis of aspiration pneumonia is made. The physician prescribes a drug that blocks peptide transfer by binding to the 50S ribosomal subunit. Which of the following drugs was most likely prescribed?

You are a resident in the surgical ICU. One of the patients you are covering is a 35-year-old pregnant G1P0 in her first trimester admitted for complicated appendicitis and awaiting appendectomy. Your attending surgeon would like you to start the patient on moxifloxacin IV preoperatively. You remember from your obstetrics clerkship, however, that moxifloxacin is Pregnancy Category C, and animal studies have shown that immature animals exposed to fluoroquinolones like moxifloxacin may experience cartilage damage. You know that there are potentially safer antibiotics, such as piperacillin/tazobactam, which is in Pregnancy Category B. What should you do?

A 28-year-old male presents to his primary care physician with complaints of intermittent abdominal pain and alternating bouts of constipation and diarrhea. His medical chart is not significant for any past medical problems or prior surgeries. He is not prescribed any current medications. Which of the following questions would be the most useful next question in eliciting further history from this patient?

A 62-year-old woman with type 2 diabetes mellitus is brought to the emergency department by her husband because of fever, chills, and purulent drainage from a foot ulcer for 2 days. Her hemoglobin A1c was 15.4% 16 weeks ago. Physical examination shows a 2-cm ulcer on the plantar surface of the left foot with foul-smelling, purulent drainage and surrounding erythema. Culture of the abscess fluid grows several bacteria species, including gram-negative, anaerobic, non-spore-forming bacilli that are resistant to bile and aminoglycoside antibiotics. Which of the following is the most likely source of this genus of bacteria?

A 20-year-old man comes to the physician because of a 3-day history of fever, myalgia, and swelling in his left groin after a recent camping trip in northern California. He appears acutely ill. Physical examination shows tender, left-sided inguinal lymphadenopathy and an enlarged, tender lymph node in the right axilla that is draining bloody necrotic material. Microscopic examination of a lymph node aspirate shows gram-negative coccobacilli with bipolar staining and a safety-pin appearance. This patient's condition is most likely caused by an organism with which of the following reservoirs?

A 15-year-old boy is brought to the Emergency department by ambulance from school. He started the day with some body aches and joint pain but then had several episodes of vomiting and started complaining of a terrible headache. The school nurse called for emergency services. The boy was born at 39 weeks gestation via spontaneous vaginal delivery. He is up to date on all vaccines and is meeting all developmental milestones. Past medical history is noncontributory. He is a good student and enjoys sports. At the hospital, his blood pressure is 120/80 mm Hg, heart rate is 105/min, respiratory rate is 21/min, and his temperature is 38.9°C (102.0°F). On physical exam, he appears drowsy with neck stiffness and sensitivity to light. Kernig’s sign is positive. An ophthalmic exam is performed followed by a lumbar puncture. An aliquot of cerebrospinal fluid is sent to microbiology. A gram stain shows gram-negative diplococci. A smear is prepared on blood agar and grows round, smooth, convex colonies with clearly defined edges. Which of the following would identify the described pathogen?

A 59-year-old man comes to the physician because of urinary frequency and perineal pain for the past 3 days. During this time, he has also had pain with defecation. He is sexually active with his wife only. His temperature is 39.1°C (102.3°F). His penis and scrotum appear normal. Digital rectal examination shows a swollen, exquisitely tender prostate. His leukocyte count is 13,400/mm3. A urine culture obtained prior to initiating treatment is most likely to show which of the following?

Q10

A 13-year-old boy is brought by his mother to the emergency department because he has had fever, chills, and severe coughing for the last two days. While they originally tried to manage his condition at home, he has become increasingly fatigued and hard to arouse. He has a history of recurrent lung infections and occasionally has multiple foul smelling stools. On presentation, his temperature is 102.2 °F (39 °C), blood pressure is 106/71 mmHg, pulse is 112/min, and respirations are 20/min. Physical exam reveals scattered rhonchi over both lung fields, rales at the base of the right lung base and corresponding dullness to percussion. The most likely organism responsible for this patient's symptoms has which of the following characteristics?

Bacteroides and anaerobic gram-negatives US Medical PG Practice Questions and MCQs

Question 1: A 37-year-old woman with a history of anorectal abscesses complains of pain in the perianal region. Physical examination reveals mild swelling, tenderness, and erythema of the perianal skin. She is prescribed oral ampicillin and asked to return for follow-up. Two days later, the patient presents with a high-grade fever, syncope, and increased swelling. Which of the following would be the most common mechanism of resistance leading to the failure of antibiotic therapy in this patient?

A. Intrinsic absence of a target site for the drug
B. Use of an altered metabolic pathway
C. Production of beta-lactamase enzyme (Correct Answer)
D. Altered structural target for the drug
E. Drug efflux pump

Explanation: ***Production of beta-lactamase enzyme*** - The patient's symptoms of a rapidly worsening infection despite ampicillin treatment suggest the presence of a **beta-lactamase producing organism**. Ampicillin is a **beta-lactam antibiotic** that is inactivated by these enzymes. - Anorectal abscesses and rapidly progressing soft tissue infections are often caused by **polymicrobial flora**, including staphylococci and enterococci, many of which can produce **beta-lactamase**. *Intrinsic absence of a target site for the drug* - While some bacteria inherently lack the target site for certain drugs (e.g., mycoplasma lacking a cell wall, thus being resistant to beta-lactams), this is less likely to be the **most common mechanism of acquired resistance** leading to treatment failure in a typical perianal infection. - The rapid progression and failed initial treatment point towards an **acquired mechanism of resistance** rather than an intrinsic one. *Use of an altered metabolic pathway* - This mechanism, such as altered **folate synthesis pathways** in resistance to trimethoprim-sulfamethoxazole, is less common as the primary mechanism for ampicillin resistance. - Ampicillin's mechanism of action primarily targets the **bacterial cell wall**, not a metabolic pathway in the same way. *Altered structural target for the drug* - This involves modifications to the **penicillin-binding proteins (PBPs)**, which are the targets of beta-lactam antibiotics like ampicillin. While a valid mechanism (e.g., in MRSA), the **production of beta-lactamase** is generally a more widespread and common cause of ampicillin failure, especially in infections involving mixed flora from the perianal region. - Given the abrupt failure of ampicillin, **beta-lactamase inactivation** is a more immediate and common cause than a rapid mutational change in PBPs. *Drug efflux pump* - **Efflux pumps** actively remove antibiotics from the bacterial cell, contributing to resistance against various drug classes. - While efflux pumps can play a role, the dominant mechanism for resistance to **ampicillin** in many common perianal pathogens is the **enzymatic degradation by beta-lactamases**.

Question 2: A 25-year-old woman comes to the physician because of a 2-day history of a burning sensation when urinating and increased urinary frequency. She is concerned about having contracted a sexually transmitted disease. Physical examination shows suprapubic tenderness. Urinalysis shows a negative nitrite test and positive leukocyte esterases. Urine culture grows organisms that show resistance to novobiocin on susceptibility testing. Which of the following is the most likely causal organism of this patient's symptoms?

A. Klebsiella pneumoniae
B. Proteus mirabilis
C. Pseudomonas aeruginosa
D. Staphylococcus epidermidis
E. Staphylococcus saprophyticus (Correct Answer)

Explanation: ***Staphylococcus saprophyticus*** - This organism is the **second most common cause of UTIs in young, sexually active women** (after *E. coli*), making it highly consistent with the patient's demographics and presentation. - *S. saprophyticus* is characterized by **resistance to novobiocin**, which is the key laboratory test differentiating it from *S. epidermidis* (novobiocin-sensitive). - It is **nitrite-negative** as it does not reduce nitrates to nitrites, consistent with the negative nitrite test. *Klebsiella pneumoniae* - While *K. pneumoniae* can cause UTIs, it is typically **nitrite-positive** because it reduces nitrates to nitrites, which contradicts the negative nitrite test result. - This gram-negative organism would not be tested for novobiocin susceptibility, as this antibiotic is used specifically to differentiate staphylococcal species. *Proteus mirabilis* - *P. mirabilis* is known for causing UTIs and is **nitrite-positive** due to its ability to reduce nitrates, which is inconsistent with the patient's negative nitrite test. - It also produces **urease**, leading to alkaline urine and struvite stones, which are not features of this acute presentation. *Pseudomonas aeruginosa* - *P. aeruginosa* is **nitrite-negative** (it does not reduce nitrates), which matches the test result. - However, it is typically associated with **hospital-acquired UTIs**, catheter-related infections, or infections in immunocompromised patients, not uncomplicated community-acquired UTIs in healthy young women. - Novobiocin testing is not routinely used for gram-negative organisms. *Staphylococcus epidermidis* - *S. epidermidis* is a common **skin commensal** and frequent contaminant in urine cultures. - Critically, it is **novobiocin-sensitive**, which distinguishes it from *S. saprophyticus* and makes it incompatible with the culture findings. - It rarely causes true UTIs unless associated with indwelling catheters or prosthetic devices.

Question 3: A 51-year-old man with alcohol use disorder comes to the physician because of a fever and productive cough. An x-ray of the chest shows a right lower lobe consolidation and a diagnosis of aspiration pneumonia is made. The physician prescribes a drug that blocks peptide transfer by binding to the 50S ribosomal subunit. Which of the following drugs was most likely prescribed?

A. Ceftriaxone
B. Doxycycline
C. Metronidazole
D. Clindamycin (Correct Answer)
E. Azithromycin

Explanation: ***Clindamycin*** - **Clindamycin** is a lincosamide antibiotic that **blocks peptide transfer** by binding to the **50S ribosomal subunit**, inhibiting bacterial protein synthesis. - It is a highly effective treatment for **aspiration pneumonia** due to its excellent activity against the **anaerobic bacteria** commonly found in oral flora, which are the primary pathogens in this condition. *Ceftriaxone* - **Ceftriaxone** is a third-generation cephalosporin that inhibits bacterial cell wall synthesis by binding to **penicillin-binding proteins**, not the 50S ribosomal subunit. - While it has broad-spectrum activity, it is typically used for community-acquired pneumonia and is less effective against the **anaerobic organisms** predominant in aspiration pneumonia. *Doxycycline* - **Doxycycline** is a tetracycline antibiotic that binds to the **30S ribosomal subunit**, preventing the attachment of aminoacyl-tRNA. - While effective against some respiratory pathogens, it is not the first-line choice for **aspiration pneumonia** as its anaerobic coverage is insufficient. *Metronidazole* - **Metronidazole** acts by forming **cytotoxic compounds** that damage bacterial DNA after reduction by anaerobic enzymes, rather than binding to ribosomal subunits. - While effective against many **anaerobes**, it is often used in combination with other antibiotics for aspiration pneumonia, and its mechanism of action is distinct from that described. *Azithromycin* - **Azithromycin** is a macrolide antibiotic that also binds to the **50S ribosomal subunit**, but it **inhibits translocation** of the growing peptide chain, not primarily peptide transfer. - While used for community-acquired pneumonia, its coverage for **oropharyngeal anaerobes** can be inconsistent, making clindamycin a more reliable choice for aspiration pneumonia.

Question 4: You are a resident in the surgical ICU. One of the patients you are covering is a 35-year-old pregnant G1P0 in her first trimester admitted for complicated appendicitis and awaiting appendectomy. Your attending surgeon would like you to start the patient on moxifloxacin IV preoperatively. You remember from your obstetrics clerkship, however, that moxifloxacin is Pregnancy Category C, and animal studies have shown that immature animals exposed to fluoroquinolones like moxifloxacin may experience cartilage damage. You know that there are potentially safer antibiotics, such as piperacillin/tazobactam, which is in Pregnancy Category B. What should you do?

A. Administer moxifloxacin since it is only Pregnancy Category C and, although studies may have revealed adverse effects in animals, there is no definite evidence that it causes risk in humans.
B. Administer piperacillin/tazobactam instead of moxifloxacin without discussing with the attending since your obligation is to "first, do no harm" and both are acceptable antibiotics for complicated appendicitis.
C. Discuss the adverse effects of each antibiotic with the patient, and then let the patient decide which antibiotic she would prefer.
D. Wait to administer any antibiotics until you discuss your safety concerns with your attending. (Correct Answer)
E. Administer moxifloxacin since the attending is the executive decision maker and had to know the patient was pregnant when deciding on an antibiotic.

Explanation: **Wait to administer any antibiotics until you discuss your safety concerns with your attending.** - As a resident, you have a **professional and ethical obligation** to voice concerns about patient safety, especially regarding medication choices in vulnerable populations like pregnant women. - Discussing your concerns with the attending physician allows for a **re-evaluation of the treatment plan** based on current evidence and patient-specific factors, ensuring the safest care. *Administer moxifloxacin since it is only Pregnancy Category C and, although studies may have revealed adverse effects in animals, there is no definite evidence that it causes risk in humans.* - While Category C means risk cannot be ruled out and benefits *may* outweigh risks, the presence of **known adverse effects in animal studies** and the availability of a safer alternative warrant reconsideration. - Administering a drug with known potential harm without discussing alternatives or concerns goes against the principle of **prudence and patient safety**. *Administer piperacillin/tazobactam instead of moxifloxacin without discussing with the attending since your obligation is to "first, do no harm" and both are acceptable antibiotics for complicated appendicitis.* - While "first, do no harm" is paramount, **unilaterally changing a treatment plan** ordered by an attending physician is inappropriate and breaches professional hierarchy and communication protocols. - The correct approach is to **communicate concerns** to the attending, allowing for a collaborative decision, rather than making independent substitutions. *Discuss the adverse effects of each antibiotic with the patient, and then let the patient decide which antibiotic she would prefer.* - While patient autonomy and informed consent are crucial, decisions about specific antibiotic choices, especially for a complicated condition like appendicitis, require **medical expertise**. - As a resident, it is your role to present information but not to delegate such complex medical decisions to a patient, particularly when you yourself have **unresolved concerns** with the attending's order. *Administer moxifloxacin since the attending is the executive decision maker and had to know the patient was pregnant when deciding on an antibiotic.* - While the attending is the senior decision-maker, it is possible for **oversights or errors to occur**, even with experienced physicians. - Assuming the attending "had to know" and therefore dismissing your own clinical judgment and knowledge of potential harm is **irresponsible** and compromises patient safety.

Question 5: A 28-year-old male presents to his primary care physician with complaints of intermittent abdominal pain and alternating bouts of constipation and diarrhea. His medical chart is not significant for any past medical problems or prior surgeries. He is not prescribed any current medications. Which of the following questions would be the most useful next question in eliciting further history from this patient?

A. "Does the diarrhea typically precede the constipation, or vice-versa?"
B. "Is the diarrhea foul-smelling?"
C. "Please rate your abdominal pain on a scale of 1-10, with 10 being the worst pain of your life"
D. "Are the symptoms worse in the morning or at night?"
E. "Can you tell me more about the symptoms you have been experiencing?" (Correct Answer)

Explanation: ***Can you tell me more about the symptoms you have been experiencing?*** - This **open-ended question** encourages the patient to provide a **comprehensive narrative** of their symptoms, including details about onset, frequency, duration, alleviating/aggravating factors, and associated symptoms, which is crucial for diagnosis. - In a patient presenting with vague, intermittent symptoms like alternating constipation and diarrhea, allowing them to elaborate freely can reveal important clues that might not be captured by more targeted questions. *Does the diarrhea typically precede the constipation, or vice-versa?* - While knowing the sequence of symptoms can be helpful in understanding the **pattern of bowel dysfunction**, it is a very specific question that might overlook other important aspects of the patient's experience. - It prematurely narrows the focus without first obtaining a broad understanding of the patient's overall symptomatic picture. *Is the diarrhea foul-smelling?* - Foul-smelling diarrhea can indicate **malabsorption** or **bacterial overgrowth**, which are important to consider in some gastrointestinal conditions. - However, this is a **specific symptom inquiry** that should follow a more general exploration of the patient's symptoms, as it may not be relevant if other crucial details are missed. *Please rate your abdominal pain on a scale of 1-10, with 10 being the worst pain of your life* - Quantifying pain intensity is useful for assessing the **severity of discomfort** and monitoring changes over time. - However, for a patient with intermittent rather than acute, severe pain, understanding the **character, location, and triggers** of the pain is often more diagnostically valuable than just a numerical rating initially. *Are the symptoms worse in the morning or at night?* - Diurnal variation can be relevant in certain conditions, such as inflammatory bowel diseases where nocturnal symptoms might be more concerning, or functional disorders whose symptoms might be stress-related. - This is another **specific question** that should come after gathering a more complete initial picture of the patient's symptoms to ensure no key information is overlooked.

Question 6: A 62-year-old woman with type 2 diabetes mellitus is brought to the emergency department by her husband because of fever, chills, and purulent drainage from a foot ulcer for 2 days. Her hemoglobin A1c was 15.4% 16 weeks ago. Physical examination shows a 2-cm ulcer on the plantar surface of the left foot with foul-smelling, purulent drainage and surrounding erythema. Culture of the abscess fluid grows several bacteria species, including gram-negative, anaerobic, non-spore-forming bacilli that are resistant to bile and aminoglycoside antibiotics. Which of the following is the most likely source of this genus of bacteria?

A. Stomach
B. Oropharynx
C. Vagina
D. Colon (Correct Answer)
E. Skin

Explanation: ***Colon*** - The description of the bacteria—**gram-negative, anaerobic, non-spore-forming bacilli** that are **resistant to bile** and **aminoglycoside antibiotics**—is highly characteristic of the genus *Bacteroides*, especially *Bacteroides fragilis*. - *Bacteroides fragilis* is a prominent component of the normal **colonic microflora** and is frequently implicated in infections originating from breaches in the gastrointestinal tract, such as a diabetic foot ulcer with a mixed infection. *Stomach* - The stomach's highly acidic environment generally limits significant bacterial colonization, and it is not a primary source of mixed anaerobic infections as described. - While *Helicobacter pylori* can colonize the stomach, it does not fit the described microbiological characteristics. *Oropharynx* - The oropharynx contains a diverse microbiota, including anaerobes like **Peptostreptococcus** and **Fusobacterium**, but it is not the typical source for *Bacteroides fragilis* or the specific resistance profile mentioned. - Oropharyngeal anaerobes are more commonly associated with head and neck infections, aspiration pneumonia, or dental abscesses. *Vagina* - The vaginal flora includes various anaerobes such as **Gardnerella vaginalis** and some *Bacteroides* species, but it is not the most common or primary source of widespread mixed anaerobic infections matching this description. - Infections originating from the vagina would typically be linked to pelvic or genitourinary conditions. *Skin* - The skin surface predominantly harbors **aerobic** and **facultative anaerobic bacteria** like **Staphylococcus** and **Streptococcus** species. - While skin breaches can lead to infections, the described **anaerobic, gram-negative, bile-resistant** profile points away from the typical skin flora as the primary source for the specific bacterial characteristics given.

Question 7: A 20-year-old man comes to the physician because of a 3-day history of fever, myalgia, and swelling in his left groin after a recent camping trip in northern California. He appears acutely ill. Physical examination shows tender, left-sided inguinal lymphadenopathy and an enlarged, tender lymph node in the right axilla that is draining bloody necrotic material. Microscopic examination of a lymph node aspirate shows gram-negative coccobacilli with bipolar staining and a safety-pin appearance. This patient's condition is most likely caused by an organism with which of the following reservoirs?

A. Squirrels (Correct Answer)
B. Deer
C. Bats
D. Dogs
E. Birds

Explanation: ***Squirrels*** - The clinical presentation of **fever**, **myalgia**, **tender lymphadenopathy (buboes)**, especially with **bloody necrotic material drainage**, in a patient with recent outdoor exposure in **northern California**, is highly suggestive of **bubonic plague**. - Microscopic examination revealing **gram-negative coccobacilli with bipolar staining** and a **safety-pin appearance** is **pathognomonic for *Yersinia pestis***, the causative agent of plague. - The primary reservoir for *Y. pestis* is **wild rodents**, particularly **ground squirrels, prairie dogs, and rock squirrels** in the western United States, including California. - Transmission occurs via flea bites from infected rodents, or through direct contact with infected animals. *Deer* - **Deer** are not reservoirs for *Yersinia pestis*. - Deer serve as reservoirs for **Lyme disease** (*Borrelia burgdorferi*) transmitted by *Ixodes* ticks, which presents with erythema migrans, not buboes with bipolar-staining bacteria. - Deer may also harbor ticks that transmit other diseases (ehrlichiosis, anaplasmosis), but none match this clinical picture. *Bats* - **Bats** are not associated with *Yersinia pestis* infection. - Bats are reservoirs for **rabies virus** and **Histoplasma capsulatum** (histoplasmosis from bat guano in caves). - Neither presents with the characteristic bubonic lymphadenopathy and gram-negative coccobacilli with bipolar staining seen here. *Dogs* - **Dogs** are not primary reservoirs for plague, though they can become infected and rarely transmit to humans. - Dogs are reservoirs for **rabies**, **leptospirosis**, and **Capnocytophaga** infections. - These do not match the clinical presentation of buboes and the pathognomonic microscopic findings of *Y. pestis*. *Birds* - **Birds** are not reservoirs for *Yersinia pestis*. - Birds can harbor **Chlamydophila psittaci** (causing psittacosis/atypical pneumonia) and **Cryptococcus neoformans** (in pigeon droppings). - These present with respiratory symptoms, not bubonic lymphadenopathy with bipolar-staining bacteria.

Question 8: A 15-year-old boy is brought to the Emergency department by ambulance from school. He started the day with some body aches and joint pain but then had several episodes of vomiting and started complaining of a terrible headache. The school nurse called for emergency services. The boy was born at 39 weeks gestation via spontaneous vaginal delivery. He is up to date on all vaccines and is meeting all developmental milestones. Past medical history is noncontributory. He is a good student and enjoys sports. At the hospital, his blood pressure is 120/80 mm Hg, heart rate is 105/min, respiratory rate is 21/min, and his temperature is 38.9°C (102.0°F). On physical exam, he appears drowsy with neck stiffness and sensitivity to light. Kernig’s sign is positive. An ophthalmic exam is performed followed by a lumbar puncture. An aliquot of cerebrospinal fluid is sent to microbiology. A gram stain shows gram-negative diplococci. A smear is prepared on blood agar and grows round, smooth, convex colonies with clearly defined edges. Which of the following would identify the described pathogen?

A. Oxidase-positive and ferments glucose and maltose (Correct Answer)
B. Oxidase-positive test and ferments glucose only
C. Catalase-negative and oxidase-positive
D. No growth on Thayer-Martin medium
E. Growth in anaerobic conditions

Explanation: ***Oxidase-positive and ferments glucose and maltose*** - The patient's symptoms (fever, headache, neck stiffness, sensitivity to light, positive Kernig's sign) are classic for **meningitis**, and the CSF showing **gram-negative diplococci** points to *Neisseria meningitidis*. - *Neisseria meningitidis* is identified by its positive **oxidase test** and its ability to ferment both **glucose and maltose**. *Oxidase-positive test and ferments glucose only* - This description corresponds to *Neisseria gonorrhoeae*, which primarily causes **gonorrhea** and occasionally meningitis due to disseminated infection but is less common in this age group and presentation. - While *Neisseria gonorrhoeae* is also an **oxidase-positive gram-negative diplococcus**, it specifically ferments only *glucose*, not maltose. *Catalase-negative and oxidase-positive* - While *Neisseria meningitidis* is **oxidase-positive**, stating it is "catalase-negative" is incorrect; *Neisseria* species are actually **catalase-positive**. - This option incorrectly describes a general metabolic property that would rule out *Neisseria meningitidis*. *No growth on Thayer-Martin medium* - Thayer-Martin medium is a **selective medium** specifically designed to isolate pathogenic *Neisseria species* by inhibiting the growth of commensal bacteria and fungi. - Therefore, *Neisseria meningitidis* would **grow well** on Thayer-Martin medium, making "no growth" an incorrect identifier. *Growth in anaerobic conditions* - *Neisseria meningitidis* is an **obligate aerobe**, meaning it requires oxygen for growth. - It would **not grow** in anaerobic conditions, making this statement false for identifying the described pathogen.

Question 9: A 59-year-old man comes to the physician because of urinary frequency and perineal pain for the past 3 days. During this time, he has also had pain with defecation. He is sexually active with his wife only. His temperature is 39.1°C (102.3°F). His penis and scrotum appear normal. Digital rectal examination shows a swollen, exquisitely tender prostate. His leukocyte count is 13,400/mm3. A urine culture obtained prior to initiating treatment is most likely to show which of the following?

A. Gram-negative, lactose-fermenting rods in pink colonies (Correct Answer)
B. Gram-negative, oxidase-positive rods in green colonies
C. Gram-negative, encapsulated rods in mucoid colonies
D. Gram-negative, aerobic, intracellular diplococci
E. Weakly staining, obligate intracellular bacilli

Explanation: ***Gram-negative, lactose-fermenting rods in pink colonies*** - The patient's symptoms (urinary frequency, perineal pain, fever, tender prostate) are highly suggestive of **acute bacterial prostatitis**. - **Uropathogenic Escherichia coli** (a Gram-negative, lactose-fermenting rod) is the most common cause of acute bacterial prostatitis and UTIs, typically producing pink colonies on **MacConkey agar**. *Gram-negative, oxidase-positive rods in green colonies* - This description typically fits *Pseudomonas aeruginosa*, which can cause UTIs but is less common in uncomplicated acute prostatitis and often produces a characteristic **green pigment** and fruity odor. - *Pseudomonas* infections are more frequently seen in **hospital-acquired infections** or in patients with indwelling catheters or immunocompromise. *Gram-negative, encapsulated rods in mucoid colonies* - This describes organisms like **Klebsiella pneumoniae**, which can cause UTIs and prostatitis, often presenting with **mucoid colonies** due to their prominent capsule. - While possible, *Klebsiella* is less common than *E. coli* in uncomplicated community-acquired prostatitis. *Gram-negative, aerobic, intracellular diplococci* - This description refers to **Neisseria gonorrhoeae**, which causes **gonorrhea**, a sexually transmitted infection. - While it can cause urethritis, it is not a typical cause of acute bacterial prostatitis in a patient described as exclusively sexually active with his wife and without urethral discharge. - *N. gonorrhoeae* requires **specialized culture media** (Thayer-Martin agar) and is not routinely detected on standard urine culture media; modern diagnosis typically uses NAAT/PCR testing. *Weakly staining, obligate intracellular bacilli* - This description refers to organisms like **Chlamydia trachomatis** or **Rickettsia** species. - *Chlamydia* can cause urethritis and, less commonly, epididymitis, but it typically causes a more subacute prostatitis if involved and requires specialized non-culture-based testing (e.g., PCR) for detection due to its obligate intracellular nature.

Question 10: A 13-year-old boy is brought by his mother to the emergency department because he has had fever, chills, and severe coughing for the last two days. While they originally tried to manage his condition at home, he has become increasingly fatigued and hard to arouse. He has a history of recurrent lung infections and occasionally has multiple foul smelling stools. On presentation, his temperature is 102.2 °F (39 °C), blood pressure is 106/71 mmHg, pulse is 112/min, and respirations are 20/min. Physical exam reveals scattered rhonchi over both lung fields, rales at the base of the right lung base and corresponding dullness to percussion. The most likely organism responsible for this patient's symptoms has which of the following characteristics?

A. Mixed anaerobic rods
B. Lancet-shaped diplococci
C. Mucoid lactose-fermenting rod
D. Green gram-negative rod (Correct Answer)
E. Coagulase-positive, gram-positive cocci

Explanation: ***Green gram-negative rod*** - The patient's history of **recurrent lung infections** and **foul-smelling stools (malabsorption)** is highly suggestive of **cystic fibrosis (CF)**. - **Pseudomonas aeruginosa**, a **green gram-negative rod** (due to pyocyanin pigment), is a common cause of severe pulmonary infections in CF patients and is a significant contributor to morbidity and mortality. *Mixed anaerobic rods* - This typically causes **aspiration pneumonia**, often involving the posterior segments of the upper lobes or superior segments of the lower lobes. - While patients with CF can have aspiration, the **recurrent nature** and specific **malabsorption symptoms** point more strongly to *Pseudomonas*. *Lancet-shaped diplococci* - This describes **Streptococcus pneumoniae**, a common cause of **community-acquired pneumonia**. - While possible, it does not explain the recurrent infections or the patient's underlying condition of malabsorption and is less specific for CF-related pneumonia than *Pseudomonas*. *Mucoid lactose-fermenting rod* - This describes **Klebsiella pneumoniae**, which can cause severe pneumonia, often with **currant jelly sputum**. - While *Klebsiella* can cause lung infections, it is not as characteristic of recurrent infections in CF patients as *Pseudomonas*, and the malabsorption connection is weaker. *Coagulase-positive, gram-positive cocci* - This describes **Staphylococcus aureus**, which is another common pathogen in CF, especially in younger patients. - However, the description of a "green" gram-negative rod in the correct option points more specifically to *Pseudomonas aeruginosa*, which becomes increasingly prevalent and problematic in older CF patients.

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