Gram-negative US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Gram-negative. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Gram-negative US Medical PG Question 1: A 42-year-old man with a history of gout and hypertension presents to his family physician with a complaint of increased left knee pain over the past 2 days. He also reports swelling and redness of the left knee and is unable to bear weight on that side. He denies any prior surgery or inciting trauma to the knee. His temperature is 97.0°F (36.1°C), blood pressure is 137/98 mm Hg, pulse is 80/min, respirations are 13/min, and oxygen saturation is 98% on room air. Physical examination reveals a left knee that is erythematous, swollen, warm-to-touch, and extremely tender to palpation and with attempted flexion/extension movement. His left knee range of motion is markedly reduced compared to the contralateral side. Joint aspiration of the left knee is performed with synovial fluid analysis showing turbid fluid with a leukocyte count of 95,000/mm^3, 88% neutrophils, and a low glucose. Gram stain of the synovial fluid is negative. Results from synovial fluid culture are pending. Which of the following is the best treatment regimen for this patient?
- A. Ceftriaxone
- B. Piperacillin-tazobactam
- C. Vancomycin and ceftazidime (Correct Answer)
- D. Indomethacin and colchicine
- E. Acetaminophen and ibuprofen
Gram-negative Explanation: ***Vancomycin and ceftazidime***
- The patient presents with **acute monoarticular arthritis** with signs of **septic arthritis**, including a highly inflammatory synovial fluid (95,000 WBCs/mm^3, 88% neutrophils, low glucose) and inability to bear weight [1].
- **Empiric broad-spectrum antibiotic coverage** is crucial before culture results, typically involving an agent effective against **Gram-positive bacteria** (e.g., vancomycin for MRSA) and an agent against **Gram-negative bacteria** (e.g., ceftazidime for *Pseudomonas* or other resistant gram-negatives), especially in patients with comorbidities like gout [1].
*Ceftriaxone*
- Ceftriaxone is a **third-generation cephalosporin** that provides good coverage for many Gram-negative bacteria and some Gram-positive bacteria.
- However, it **does not cover *Pseudomonas aeruginosa* or methicillin-resistant *Staphylococcus aureus* (MRSA)**, which are important pathogens to consider in septic arthritis, especially without a definitive causative organism.
*Piperacillin-tazobactam*
- Piperacillin-tazobactam is a **broad-spectrum antibiotic** with good coverage for Gram-positive, Gram-negative (including *Pseudomonas*), and anaerobic bacteria.
- While it offers good broad coverage, it also **does not reliably cover MRSA**, which is a significant concern in empirical treatment of septic arthritis given its prevalence.
*Indomethacin and colchicine*
- This regimen is specific for the treatment of **acute gout flares** by reducing inflammation.
- While the patient has a history of gout, the **highly elevated leukocyte count** with a predominance of neutrophils and low glucose in the synovial fluid strongly indicate **septic arthritis**, which is a medical emergency requiring antibiotic treatment [1].
*Acetaminophen and ibuprofen*
- These medications provide **analgesia and anti-inflammatory effects** for pain relief.
- They are **insufficient as primary treatment** for septic arthritis, which necessitates urgent antimicrobial therapy to prevent joint destruction and systemic complications.
Gram-negative US Medical PG Question 2: A 42-year-old woman with a history of multiple sclerosis and recurrent urinary tract infections comes to the emergency department because of flank pain and fever. Her temperature is 38.8°C (101.8°F). Examination shows left-sided costovertebral angle tenderness. She is admitted to the hospital and started on intravenous vancomycin. Three days later, her symptoms have not improved. Urine culture shows growth of Enterococcus faecalis. Which of the following best describes the most likely mechanism of antibiotic resistance in this patient?
- A. Increased efflux across bacterial cell membranes
- B. Production of beta-lactamase
- C. Alteration of penicillin-binding proteins
- D. Alteration of peptidoglycan synthesis (Correct Answer)
- E. Alteration of ribosomal targets
Gram-negative Explanation: ***Alteration of peptidoglycan synthesis***
- **Vancomycin** targets the **D-Ala-D-Ala terminus** on the peptidoglycan precursor, preventing cross-linking during bacterial cell wall synthesis.
- **Vancomycin resistance in Enterococcus faecalis** occurs through acquisition of resistance genes (vanA, vanB) that encode enzymes modifying the peptidoglycan precursor from **D-Ala-D-Ala to D-Ala-D-Lac**.
- This structural change reduces vancomycin's binding affinity by approximately 1000-fold, rendering the antibiotic ineffective.
- The mechanism directly involves **alteration of the peptidoglycan synthesis pathway**, specifically the terminal amino acid residues of the pentapeptide precursor.
*Increased efflux across bacterial cell membranes*
- This mechanism involves **efflux pumps that actively transport antibiotics out of the bacterial cell**, reducing intracellular concentration.
- While efflux pumps contribute to resistance for antibiotics like **tetracyclines, fluoroquinolones, and macrolides**, this is not the primary mechanism of vancomycin resistance in Enterococcus.
*Production of beta-lactamase*
- **Beta-lactamase enzymes** hydrolyze the **beta-lactam ring** of antibiotics like **penicillins and cephalosporins**, rendering them inactive.
- **Vancomycin is a glycopeptide antibiotic, not a beta-lactam**, so its efficacy is not affected by beta-lactamase production.
*Alteration of ribosomal targets*
- This mechanism confers resistance to antibiotics that target **bacterial ribosomes** to inhibit protein synthesis, such as **macrolides, aminoglycosides, and tetracyclines**.
- **Vancomycin acts on cell wall synthesis**, not protein synthesis, so alteration of ribosomal targets is not relevant to vancomycin resistance.
*Alteration of penicillin-binding proteins*
- **Penicillin-binding proteins (PBPs)** are the targets of **beta-lactam antibiotics** (penicillins, cephalosporins, carbapenems).
- Alterations in PBPs cause resistance to beta-lactams, not to vancomycin.
- **Vancomycin does not interact with PBPs**; it binds directly to the D-Ala-D-Ala terminus of peptidoglycan precursors in the cell wall.
Gram-negative US Medical PG Question 3: An 18-year old college freshman presents to his university clinic because he has not been feeling well for the past two weeks. He has had a persistent headache, occasional cough, and chills without rigors. The patient’s vital signs are normal and physical exam is unremarkable. His radiograph shows patchy interstitial lung infiltrates and he is diagnosed with atypical pneumonia. The patient is prescribed azithromycin and takes his medication as instructed. Despite adherence to his drug regimen, he returns to the clinic one week later because his symptoms have not improved. The organism responsible for this infection is likely resistant to azithromycin through which mechanism?
- A. Mutation in topoisomerase II
- B. Methylation of ribosomal binding site
- C. Presence of a beta-lactamase
- D. Decreased binding to RNA polymerase
- E. Insertion of drug efflux pumps (Correct Answer)
Gram-negative Explanation: ***Insertion of drug efflux pumps***
- **Azithromycin** is a macrolide antibiotic that inhibits bacterial protein synthesis by binding to the **50S ribosomal subunit**.
- In **Mycoplasma pneumoniae** (the most common cause of atypical pneumonia in young adults), the **most common** mechanism of macrolide resistance is through **efflux pumps**, particularly the **mef genes**.
- These efflux pumps actively transport macrolides out of the bacterial cell, reducing intracellular drug concentration and conferring resistance.
- This mechanism is responsible for the majority of macrolide-resistant *M. pneumoniae* isolates worldwide.
*Methylation of ribosomal binding site*
- **Methylation** of the ribosomal binding site (specifically the **23S rRNA** via erm genes) does prevent azithromycin from binding effectively.
- While this is a valid macrolide resistance mechanism seen in organisms like *Streptococcus pneumoniae* and *Streptococcus pyogenes*, it is **less common** in *Mycoplasma pneumoniae*.
- Efflux pumps (mef) are the predominant mechanism in *M. pneumoniae* resistant strains.
*Mutation in topoisomerase II*
- **Topoisomerase II** (DNA gyrase) is the target of **fluoroquinolone antibiotics**, not macrolides.
- Mutations in this enzyme lead to resistance against fluoroquinolones, such as **ciprofloxacin**.
*Presence of a beta-lactamase*
- **Beta-lactamase enzymes** inactivate **beta-lactam antibiotics** (e.g., penicillin, cephalosporins) by hydrolyzing their beta-lactam ring.
- Additionally, *Mycoplasma pneumoniae* **lacks a cell wall**, making it inherently resistant to all beta-lactam antibiotics regardless of beta-lactamase production.
*Decreased binding to RNA polymerase*
- **RNA polymerase** is the target for antibiotics like **rifampin**, which inhibits bacterial transcription.
- Decreased binding to RNA polymerase would lead to rifampin resistance, not azithromycin resistance.
Gram-negative US Medical PG Question 4: A 77-year-old woman is brought to the emergency department from her nursing home because she was found down overnight. On presentation she was found to be delirious and was unable to answer questions. Chart review shows that she is allergic to cephalosporins. Her temperature is 102.2°F (39°C), blood pressure is 105/52 mmHg, pulse is 94/min, and respirations are 23/min. Physical exam reveals a productive cough. A metabolic panel is obtained with the following results:
Serum:
Na+: 135 mEq/L
Cl-: 95 mEq/L
K+: 4 mEq/L
HCO3-: 19 mEq/L
BUN: 40 mg/dL
Creatinine: 2.5 mg/dL
Glucose: 150 mg/dL
Based on these findings two different drugs are started empirically. Gram stain on a blood sample is performed showing the presence of gram-positive organisms on all samples. One of the drugs is subsequently stopped. The drug that was most likely stopped has which of the following characteristics?
- A. Resistance conveyed through acetylation
- B. Associated with red man syndrome
- C. Single-ringed ß-lactam structure (Correct Answer)
- D. Causes discolored teeth in children
- E. Accumulates inside bacteria via O2-dependent uptake
Gram-negative Explanation: ***Single-ringed ß-lactam structure***
- The patient presents with **sepsis** due to **pneumonia** likely caused by **gram-positive organisms**. Given a cephalosporin allergy, **aztreonam** (a monobactam) would be an initial empirical antibiotic choice to cover gram-negative bacteria, alongside a drug for gram-positive coverage (like vancomycin).
- Since the **blood cultures** confirmed **gram-positive organisms**, the drug covering gram-negative bacteria (aztreonam) would be stopped. Aztreonam is characterized by its **single-ringed β-lactam structure**.
*Resistance conveyed through acetylation*
- This mechanism of resistance is typical of **aminoglycosides** (e.g., gentamicin) and **chloramphenicol**.
- Aminoglycosides were unlikely to be one of the empirically started drugs, as they are often used in combination with β-lactams, and this patient has a cephalosporin allergy.
*Associated with red man syndrome*
- **Red man syndrome** is a common adverse effect associated with **vancomycin** administration, especially with rapid infusion.
- Vancomycin would likely be continued, as it effectively targets gram-positive organisms, including **MRSA**, and is a suitable alternative given the cephalosporin allergy.
*Causes discolored teeth in children*
- This is a characteristic side effect of **tetracyclines** (e.g., doxycycline), which are contraindicated in young children and pregnant women due to their effects on bone and teeth development.
- Tetracyclines are not typically first-line empiric therapy for severe pneumonia or sepsis, especially in an elderly patient.
*Accumulates inside bacteria via O2-dependent uptake*
- This describes the mechanism of uptake for **aminoglycosides**. Their entry into bacteria is an **energy-dependent process** requiring oxygen.
- As mentioned, aminoglycosides are less likely to be the initial drug stopped in this scenario, as they target gram-negative bacteria.
Gram-negative US Medical PG Question 5: A 26-year-old woman presents to the emergency department with fever, chills, lower quadrant abdominal pain, and urinary frequency for the past week. Her vital signs include temperature 38.9°C (102.0°F), pulse 110/min, respirations 16/min, and blood pressure 122/78 mm Hg. Physical examination is unremarkable. Urinalysis reveals polymorphonuclear leukocytes (PMNs) > 10 cells/HPF and the presence of bacteria (> 105 CFU/mL). Which of the following is correct concerning the most likely microorganism responsible for this patient’s condition?
- A. Gram-positive cocci that grow in clusters
- B. Pear-shaped motile protozoa
- C. Gram-positive cocci that grow in chains
- D. Gram-negative rod-shaped bacilli (Correct Answer)
- E. Nonmotile, pleomorphic rod-shaped, gram-negative bacilli
Gram-negative Explanation: ***Gram-negative rod-shaped bacilli***
- The symptoms of **fever, chills, abdominal pain, and urinary frequency**, along with urinalysis showing **pyuria (>10 PMNs/HPF) and bacteriuria (>10^5 CFU/mL)**, are highly suggestive of a **urinary tract infection (UTI)**.
- **_Escherichia coli_**, a **gram-negative rod-shaped bacillus**, is the most common cause of community-acquired UTIs, accounting for over 80% of cases.
*Gram-positive cocci that grow in clusters*
- This morphology describes **Staphylococcus species**, which can cause UTIs but are **less common** than *E. coli* in uncomplicated cases.
- **_Staphylococcus saprophyticus_** is a notable cause of UTIs in young, sexually active women, but it is not the most likely overall.
*Pear-shaped motile protozoa*
- This description typically refers to **_Giardia lamblia_**, which causes **gastrointestinal infections** (giardiasis) and is not associated with UTIs.
- UTIs are bacterial infections, not protozoal.
*Gram-positive cocci that grow in chains*
- This morphology describes **Streptococcus species**, such as **_Streptococcus agalactiae_** (Group B Streptococcus), which can cause UTIs, especially during pregnancy.
- However, they are **less frequent causes** of uncomplicated UTIs compared to *E. coli*.
*Nonmotile, pleomorphic rod-shaped, gram-negative bacilli*
- This description might fit certain bacteria like **_Haemophilus influenzae_**, which typically causes **respiratory tract infections or meningitis**, not UTIs.
- While *E. coli* is a gram-negative rod, it is **motile**, distinguishing it from this option.
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