This practice set contains high-yield board review questions covering key concepts in Infection, Pharmacology & VTE. Each clinical scenario is designed to test your diagnostic and management skills relevant to this subspecialty.
Question 401
Topic: Infection, Pharmacology & VTE
In the pathogenesis of fracture-related infections, Staphylococcus aureus utilizes a mechanism to regulate virulence factors and biofilm dispersion based on population density. Which specific system is the primary regulator of this quorum sensing?
Correct Answer & Explanation
. agr (accessory gene regulator) system
Explanation
The accessory gene regulator (agr) system in S. aureus is a quorum-sensing system that controls the expression of numerous virulence factors and is critical for biofilm maturation and dispersion.
Question 402
Topic: Infection, Pharmacology & VTE
In the United States, groups at risk for community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) within the pediatric and adolescent populations include
Correct Answer & Explanation
. toddlers in daycare.
Explanation
CA-MRSA is a growing problem in the United States. Groups at risk for CA-MRSA include athletes in contact sports and children in daycare. Tennis players, golfers, and runners are at lowest risk because their sports do not require close contact with teammates or competitors. Outbreaks have been noted in Alaskan native, Native American, and Pacific Islander minority populations.
Question 403
Topic: Infection, Pharmacology & VTE
What is the most common causative bacteria in septic arthritis in children?
Correct Answer & Explanation
. Staphylococcus aureus
Explanation
The spectrum of causative bacteria and frequency of occurrence of specific pathogens in septic arthritis are similar to those seen in osteomyelitis, with Staphylococcus aureus being the most common. Other common causative organisms include Kingella kingae, Streptococcus pneumoniae, Klebsiella species, Salmonella, Brucella melitensis, and Haemophilus influenzae.
Question 404
Topic: Infection, Pharmacology & VTE
A 10-year-old boy who has had progressive low back and right buttock pain for the past 3 days is now unable to bear weight on the right side secondary to pain. He has a temperature of 101.3 degrees F (38.5 degrees C). Examination reveals full hip range of motion; but he reports pain on the right side with external rotation. Pain is elicited with compression of the iliac wings and with direct palpation of the right sacroiliac (SI) joint. An MRI scan of the pelvis shows no abscess, but there is inflammation of the SI joint. Management should consist of
Correct Answer & Explanation
. bed rest, blood cultures, and IV antibiotics.
Explanation
The clinical presentation and MRI findings are consistent with an acute infection of the SI joint. Bed rest and nonsteroidal anti-inflammatory drugs alone are insufficient to treat the problem. Staphylococcus aureus is the causative organism in most of these infections; therefore, unless there is an unusual factor in the history such as IV drug use, immune system compromise, or unusual travel, SI joint aspiration is unnecessary. It is often difficult to enter the SI joint, even under radiographic guidance. Management should consist of hospital admission and IV antibiotics. Blood cultures may be positive and should be obtained prior to starting antibiotics. Surgical fusion of the SI joint is not indicated.
Question 405
Topic: Infection, Pharmacology & VTE
What do the bone scan findings represent?
Correct Answer & Explanation
. Osteonecrosis
Explanation
The bone scan shows no uptake of the tracer, which indicates osteonecrosis. The MRI scan of the shoulder reveals sepsis with a focus on osteomyelitis. A CT scan will not add more useful information and will delay treatment. A sequestrum is a piece of dead bone that has become separated from normal/solid bone during the osteonecrosis process. It appears as a radiopacity on plain radiograph. The involucrum is new bone formed by an elevated periosteum and can be seen on radiograph. Heterotopic ossification appears as radiopacity within the soft tissues. Late recognition of pediatric shoulder sepsis has been reported, and damage to the joint can be extensive. Septic joints are one of the few true orthopaedic emergencies. The methodology of drainage in shoulder sepsis is controversial (aspiration vs open drainage vs arthroscopic lavage), but the joint must be drained and sterilized to prevent cartilage damage.
Question 406
Topic: Infection, Pharmacology & VTE
An adult patient has had low-grade fevers, malaise, and joint pain for the past several days. Examination reveals an erythematous "bulls eye" rash over the right thigh (erythema migrans). What is the organism associated with this infection? Review Topic
Correct Answer & Explanation
. Borrelia burgdorferi
Explanation
Erythema migrans is typically found in patients with Lyme's disease, which is caused by Borrelia burgdorferi and is usually transmitted by ticks. Bartonella henselae is associated with cat-scratch fever. Kingella kingae can be a cause of pediatric osteomyelitis; whereas, Nocardia and Brucella cause mono-articular forms of septic arthritis in adults, and are not associated with erythema migrans.
Question 407
Topic: Infection, Pharmacology & VTE
A 25-year-old male sustains an anterior knee dislocation after a motorcycle accident. The knee is reduced in the emergency department. The patient has palpable, symmetric 2+ dorsalis pedis and posterior tibial pulses. However, the ankle-brachial index (ABI) is measured at 0.85. What is the most appropriate next step in management?
Correct Answer & Explanation
. Perform a CT angiogram of the lower extremity
Explanation
In the setting of a knee dislocation, palpable pulses do not definitively rule out an intimal flap tear of the popliteal artery. An ABI less than 0.9 is highly sensitive for vascular injury and mandates advanced imaging, such as a CT angiogram.
Question 408
Topic: Infection, Pharmacology & VTE
Methicillin-resistant staphylococcus aureus (MRSA) develops its resistance to penicillinase-stable antibiotics via which of the following actions?
Correct Answer & Explanation
. Genetic mutation
Explanation
After the introduction of penicillins, bacteria developed the ability to hydrolyze these antibiotics using B-lactamase. In response, penicillinase-stable antibiotics were developed, the first of which was methicillin, since replaced with oxacillin and nafcillin. Drug resistance to this class of antibiotics is achieved via a genetic mutation of mecA encoding an altered penicillin binding protein. The gene product of this mutation, PBPa has a low affinity for these antibiotics and cannot be inhibited by them. Altering cell wall permeability is found in resistance to tetracyclines, quinolones, and trimethoprim, as well as B-lactam antibiotics. Biofilm barriers are produced by bacteria such as salmonella. Active efflux pumps provide resistance to erythromycin and tetracycline, and altering the peptidoglycan subunit is found in resistance to vancomycin.
Question 409
Topic: Infection, Pharmacology & VTE
A 70-year-old female with a chronic staphylococcal periprosthetic joint infection of the hip is treated with a two-stage exchange arthroplasty. Her systemic antibiotic regimen includes rifampin due to its excellent biofilm penetration. What is the molecular mechanism of action of rifampin?
Correct Answer & Explanation
. Inhibits DNA-dependent RNA polymerase
Explanation
Rifampin is highly active against Staphylococcal biofilms because it penetrates the biofilm matrix well and acts by binding to and inhibiting bacterial DNA-dependent RNA polymerase, thereby preventing RNA transcription. It must always be used in combination with other agents to prevent the rapid emergence of resistance.
Question 410
Topic: Infection, Pharmacology & VTE
A 2-year-old girl is diagnosed with septic arthritis of the knee. Synovial fluid PCR identifies Kingella kingae. Which of the following best describes the bacteriology of this organism?
Correct Answer & Explanation
. Gram-negative coccobacillus
Explanation
Kingella kingae is a Gram-negative coccobacillus that is increasingly recognized as a leading cause of pediatric septic arthritis and osteomyelitis in children aged 6 months to 4 years. It is a fastidious organism that is often missed on standard solid agar cultures; thus, inoculation into BACTEC liquid blood culture vials or detection via PCR is required for reliable identification.
Question 411
Topic: Infection, Pharmacology & VTE
A 4-year-old boy is evaluated in the emergency department for an acute limp and refusal to bear weight on his right leg. His temperature is 38.6°C (101.5°F). Laboratory studies reveal a WBC count of 10,500/mm3 and an ESR of 45 mm/hr. According to the original Kocher criteria for differentiating pediatric septic arthritis of the hip from transient synovitis, what is the approximate probability that this child has septic arthritis?
Correct Answer & Explanation
. 93%
Explanation
The original Kocher criteria consist of four predictors for septic arthritis: 1) Non-weight-bearing, 2) Temperature > 38.5°C, 3) ESR > 40 mm/hr, and 4) WBC > 12,000/mm3. This patient meets exactly three criteria: Non-weight-bearing, Temperature > 38.5°C, and ESR > 40 mm/hr (his WBC is < 12,000). According to the Kocher study, the predicted probability of septic arthritis is 3% for 1 criterion, 40% for 2 criteria, 93% for 3 criteria, and 99% for 4 criteria.
Question 412
Topic: Infection, Pharmacology & VTE
A 6-year-old boy with homozygous sickle cell disease develops acute diaphyseal osteomyelitis. While Staphylococcus aureus remains the most common overall cause of osteomyelitis in this patient population, which of the following organisms is uniquely characteristic and disproportionately prevalent due to the underlying pathophysiology of his disease?
Correct Answer & Explanation
. Salmonella typhimurium
Explanation
Patients with sickle cell disease are uniquely susceptible to Salmonella osteomyelitis due to intermittent bowel ischemia leading to bacteremia, coupled with autosplenectomy impairing the clearance of encapsulated organisms. Though S. aureus is still extremely common, Salmonella is the classic, uniquely associated pathogen in this population.
Question 413
Topic: Infection, Pharmacology & VTE
A 3-year-old girl is evaluated in the emergency department for a limp and refusal to bear weight on her right leg. Her temperature is 38.0°C (100.4°F). Laboratory testing reveals an Erythrocyte Sedimentation Rate (ESR) of 50 mm/hr and a White Blood Cell (WBC) count of 13,000/mm3. According to the Kocher criteria, what is her approximate probability of having septic arthritis of the hip?
Correct Answer & Explanation
. 93%
Explanation
The Kocher criteria for pediatric septic hip are: non-weight-bearing, temperature >38.5°C, ESR >40 mm/hr, and WBC >12,000/mm3. This patient meets three criteria (non-weight-bearing, ESR >40, WBC >12,000; she fails the temperature criterion at 38.0°C). The probabilities of septic arthritis are approximately 3% for 1 criterion, 40% for 2, 93% for 3, and 99% for 4 criteria.
Question 414
Topic: Infection, Pharmacology & VTE
A 45-year-old agricultural worker from the San Joaquin Valley develops chronic osteomyelitis of the lumbar spine following a severe respiratory illness. Histopathologic examination of a vertebral biopsy reveals large, thick-walled spherules containing numerous endospores. What is the most appropriate systemic pharmacological therapy for this bone infection?
Correct Answer & Explanation
. Intravenous Amphotericin B or oral Itraconazole
Explanation
The clinical history (San Joaquin Valley) and histology ('spherules containing endospores') are pathognomonic for Coccidioidomycosis. Disseminated extrapulmonary disease, such as Coccidioidal osteomyelitis, is classically treated with prolonged antifungal therapy utilizing azoles (Itraconazole or Fluconazole) or, in severe/refractory cases, intravenous Amphotericin B.
Question 415
Topic: Infection, Pharmacology & VTE
In the management of a staphylococcal periprosthetic joint infection treated with debridement, antibiotics, and implant retention (DAIR), rifampin is often added to the postoperative antibiotic regimen. What is the primary mechanism of action of rifampin that makes it uniquely effective against biofilm-associated staphylococci?
Correct Answer & Explanation
. Inhibition of DNA-dependent RNA polymerase.
Explanation
Rifampin exerts its bactericidal effect by binding to the beta subunit of DNA-dependent RNA polymerase, thereby inhibiting bacterial RNA synthesis. It is highly lipophilic, penetrates biofilms well, and is active against stationary-phase staphylococci, making it a critical adjunct in DAIR procedures for staphylococcal PJIs. To prevent resistance, it must never be used as monotherapy.
Question 416
Topic: Infection, Pharmacology & VTE
A 4-year-old boy presents with right hip pain, a limp, and refusal to bear weight. Which inflammatory marker has been shown in subsequent validation studies (e.g., Caird et al.) to be the single best independent predictor of pediatric septic arthritis when combined with the classic Kocher criteria?
Correct Answer & Explanation
. C-reactive protein (CRP) > 2.0 mg/dL
Explanation
While the classic Kocher criteria utilized ESR, WBC, inability to bear weight, and fever, subsequent studies (Caird et al.) demonstrated that a CRP > 2.0 mg/dL is an excellent independent predictor. When added as a fifth variable to the original criteria, it significantly increases the diagnostic accuracy for pediatric septic arthritis.
Question 417
Topic: Infection, Pharmacology & VTE
A 4-year-old boy presents with right hip pain and refusal to bear weight. Which of the following combinations of parameters comprises the original Kocher criteria, utilized to differentiate pediatric septic arthritis of the hip from transient synovitis?
Correct Answer & Explanation
. Temperature > 38.5 C, ESR > 40 mm/hr, WBC > 12,000 cells/mm3, and refusal to bear weight
Explanation
The original Kocher criteria for differentiating septic arthritis from transient synovitis in children include four parameters: Non-weight-bearing status on the affected side, Erythrocyte sedimentation rate (ESR) > 40 mm/hr, Fever > 38.5 C (101.3 F), and a Serum white blood cell (WBC) count > 12,000 cells/mm3. (Note: CRP > 2.0 mg/dL was later added as a fifth independent predictor in Caird's modification).
Question 418
Topic: Infection, Pharmacology & VTE
In the pathogenesis of periprosthetic joint infection, biofilm maturation allows bacteria to become highly resistant to host defenses and antimicrobials. Which biological process allows bacteria to coordinate gene expression according to the local density of their population?
Correct Answer & Explanation
. Quorum sensing
Explanation
Quorum sensing is an intercellular communication mechanism that allows bacteria to share information about cell density and adjust gene expression accordingly. This process is critical for biofilm maturation and the development of antibiotic resistance.
Question 419
Topic: Infection, Pharmacology & VTE
A 4-year-old boy presents with a limp, right hip pain, and a temperature of 38.8°C. His WBC count is 14,000/mm3, ESR is 50 mm/hr, and CRP is 3.5 mg/dL. He refuses to bear weight. Based on the Kocher and Caird criteria, what is the approximate probability that this child has septic arthritis rather than transient synovitis?
Correct Answer & Explanation
. 99%
Explanation
This patient has 5 out of 5 criteria (fever > 38.5°C, non-weight bearing, ESR > 40, WBC > 12k, and CRP > 2.0 mg/dL). The presence of 4 or more criteria yields a 93-99% predictive probability for pediatric septic arthritis of the hip.
Question 420
Topic: Infection, Pharmacology & VTE
During a medial opening wedge high tibial osteotomy (HTO) for medial compartment osteoarthritis, which of the following technical errors will most likely result in an unintended increase in the posterior tibial slope?
Correct Answer & Explanation
. Opening the wedge more posteriorly than anteriorly
Explanation
Because the proximal tibia is triangular, opening the wedge uniformly or more anteriorly will inadvertently increase the posterior tibial slope. To maintain normal slope, the wedge must be opened approximately twice as much posteromedially as anteromedially.
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