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Infection, Pharmacology & VTE MCQs

Practice Set 63 of 68

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 1241

Topic: Infection, Pharmacology & VTE
A 2-year-old child presents with a suspected native joint septic arthritis of the knee. Given the high prevalence of Kingella kingae in this age group, which of the following microbiological techniques will maximize the likelihood of isolating this fastidious organism?
. Prolonged anaerobic culture for 14 days on Brucella agar.
. Inoculation onto Sabouraud dextrose agar at room temperature.
. Direct inoculation of the synovial fluid into aerobic blood culture vials (e.g., BACTEC).
. Plating exclusively on Thayer-Martin chocolate agar under standard aerobic conditions.

Correct Answer & Explanation

. Direct inoculation of the synovial fluid into aerobic blood culture vials (e.g., BACTEC).

Explanation

Kingella kingae is a fastidious Gram-negative organism that is a leading cause of pediatric septic arthritis in children under 4 years old. Standard solid agar plating often yields false-negative results. Directly inoculating the synovial fluid into aerobic blood culture vials (such as BACTEC) significantly increases the diagnostic yield for this organism.

Question 1242

Topic: Infection, Pharmacology & VTE

In the evaluation of pediatric septic arthritis of the hip, the original Kocher criteria utilized four predictors. Subsequent studies (e.g., Caird et al.) added a fifth predictor which was found to be the strongest independent predictor of septic arthritis. Which biomarker is this?

. Procalcitonin > 0.5 ng/mL
. Erythrocyte sedimentation rate (ESR) > 40 mm/hr
. White blood cell (WBC) count > 12,000 cells/mm3
. Synovial fluid lactic acid > 5.0 mmol/L
. C-reactive protein (CRP) > 2.0 mg/dL (20 mg/L)

Correct Answer & Explanation

. C-reactive protein (CRP) > 2.0 mg/dL (20 mg/L)

Explanation

Caird et al. modified the original Kocher criteria by adding C-reactive protein (CRP) > 2.0 mg/dL (20 mg/L) as a fifth variable. In their study, a CRP > 2.0 mg/dL was identified as the single strongest independent clinical predictor of septic arthritis in pediatric patients.

Question 1243

Topic: Infection, Pharmacology & VTE

A 22-year-old healthy female presents with an acutely swollen, painful left wrist. She reports a history of migratory polyarthralgia over the past week and multiple scant vesiculopustular skin lesions. Synovial fluid aspiration shows a WBC count of 40,000 cells/uL. If this condition is suspected, what is the optimal culture medium required for diagnosis?

. Sabouraud dextrose agar
. Lowenstein-Jensen medium
. Thayer-Martin agar
. Eosin Methylene Blue (EMB) agar

Correct Answer & Explanation

. Thayer-Martin agar

Explanation

The clinical presentation (migratory polyarthralgia, tenosynovitis, skin lesions, and an infected joint with slightly lower WBC than typical S. aureus septic arthritis) strongly suggests gonococcal arthritis (Neisseria gonorrhoeae). N. gonorrhoeae is a fastidious organism that requires specialized chocolate agar enriched with antibiotics, specifically Thayer-Martin agar, to selectively grow while inhibiting normal flora.

Question 1244

Topic: Infection, Pharmacology & VTE

A 65-year-old male with a history of recurrent gout presents with acute severe pain, swelling, and redness in his native left knee. Aspiration yields cloudy fluid with 65,000 WBCs/uL. Polarized light microscopy confirms the presence of abundant needle-shaped, strongly negative birefringent crystals. Gram stain is negative. What is the most appropriate next step in management regarding the diagnosis of infection?

. Rule out septic arthritis entirely, as crystal deposition and bacterial infection are mutually exclusive.
. Administer intra-articular corticosteroids immediately to treat the acute gout flare, bypassing further infectious workup.
. Perform a rapid synovial fluid uric acid level to confirm gout, negating the need for culture.
. Send the fluid for definitive aerobic and anaerobic cultures, as concomitant septic arthritis can occur.

Correct Answer & Explanation

. Send the fluid for definitive aerobic and anaerobic cultures, as concomitant septic arthritis can occur.

Explanation

The presence of monosodium urate crystals strongly supports an acute gout flare; however, gout and septic arthritis can coexist (found concurrently in ~1.5% to 5% of cases). Given the extremely high WBC count (which can be seen in severe gout but also indicates infection) and clinical picture, one cannot definitively rule out infection based on crystals alone. The fluid must always be sent for definitive bacterial cultures.

Question 1245

Topic: Infection, Pharmacology & VTE

A 60-year-old male presents with an acutely swollen, erythematous native knee. Aspiration yields cloudy fluid with a WBC count of 65,000 cells/uL. Polarized microscopy reveals negatively birefringent needle-shaped crystals. What is the most critical next step?

. Administer intra-articular corticosteroids and discharge the patient.
. Send the fluid for Gram stain and culture despite the presence of crystals.
. Perform an immediate open arthrotomy without waiting for culture results.
. Order a serum uric acid level to confirm the isolated diagnosis of gout.
. Begin empiric oral colchicine and hold antibiotics.

Correct Answer & Explanation

. Send the fluid for Gram stain and culture despite the presence of crystals.

Explanation

The presence of crystals does not rule out septic arthritis, as concurrent crystal arthropathy and joint infection can occur in up to 2% of cases. Synovial fluid must still be cultured.

Question 1246

Topic: Infection, Pharmacology & VTE

A 5-year-old boy presents with a temperature of 38.8 C, refusal to bear weight on the right leg, an ESR of 55 mm/hr, and a peripheral WBC of 14,000 cells/mcL. According to the Kocher criteria, what is the most appropriate next step in management?

. Immediate open surgical drainage without prior imaging or aspiration
. Prescribe oral cephalexin and observe as an outpatient for 24 hours
. Ultrasound-guided hip aspiration and synovial fluid analysis
. MRI of the pelvis with and without intravenous contrast
. Technetium-99m bone scan to evaluate for contiguous osteomyelitis

Correct Answer & Explanation

. Ultrasound-guided hip aspiration and synovial fluid analysis

Explanation

This patient meets 4 out of 4 Kocher criteria, predicting a 99% probability of septic arthritis. The next most appropriate step is an ultrasound-guided hip aspiration to confirm the diagnosis and secure cultures prior to surgical intervention.

Question 1247

Topic: Infection, Pharmacology & VTE

Which of the following synovial fluid profiles is most characteristic of disseminated gonococcal infection of a native joint?

. WBC of 120,000 cells/mcL with copious Gram-positive cocci in clusters
. WBC of 40,000 cells/mcL with a negative Gram stain and frequently negative routine cultures
. WBC of 2,000 cells/mcL with a positive India ink stain
. WBC of 15,000 cells/mcL with a 90% lymphocytic predominance
. WBC of 5,000 cells/mcL with intracellular monosodium urate crystals

Correct Answer & Explanation

. WBC of 40,000 cells/mcL with a negative Gram stain and frequently negative routine cultures

Explanation

Gonococcal septic arthritis typically presents with a moderately elevated synovial WBC count (30,000 to 50,000 cells/mcL), which is lower than non-gonococcal bacterial arthritis. Gram stains and standard synovial cultures are frequently negative, often requiring nucleic acid amplification testing (NAAT) from mucosal sites.

Question 1248

Topic: Infection, Pharmacology & VTE

Aspiration of a swollen, erythematous first metatarsophalangeal joint yields cloudy fluid. Microscopy reveals negatively birefringent, needle-shaped crystals, and the WBC count is 65,000 cells/mcL. What is the most appropriate next step regarding the synovial fluid analysis?

. Discard the remaining fluid as the diagnosis of gout is definitively confirmed.
. Ensure the fluid is sent for Gram stain and bacterial culture despite the presence of crystals.
. Send the fluid for specialized fungal cultures only.
. Add uricase to the fluid to dissolve the crystals and repeat the WBC count.
. Perform a rapid point-of-care alpha-defensin test to rule out pseudogout.

Correct Answer & Explanation

. Ensure the fluid is sent for Gram stain and bacterial culture despite the presence of crystals.

Explanation

The identification of monosodium urate crystals establishes a diagnosis of gout, but it does not rule out a concomitant septic arthritis. Because crystal arthropathy and joint infection can coexist, especially with WBC counts exceeding 50,000, Gram stain and culture must always be obtained.

Question 1249

Topic: Infection, Pharmacology & VTE

Which of the following best describes the primary mechanism by which sonication of explanted orthopedic hardware improves the diagnostic yield for periprosthetic joint infection?

. It lyses host immune cells to release intracellular bacteria.
. It physically disrupts the bacterial biofilm to release sessile organisms into the fluid.
. It increases the metabolic rate of dormant persister cells to promote rapid growth.
. It concentrates planktonic bacteria via acoustic cavitation.
. It denatures the bacterial glycocalyx without dislodging the actual organisms.

Correct Answer & Explanation

. It physically disrupts the bacterial biofilm to release sessile organisms into the fluid.

Explanation

Sonication uses low-frequency ultrasound to physically disrupt the extracellular polymeric substance of the biofilm on explanted hardware. This releases the sessile bacteria into the surrounding fluid, significantly improving culture yields.

Question 1250

Topic: Infection, Pharmacology & VTE

A 55-year-old male presents with an acute, swollen knee. Aspiration yields cloudy fluid with 65,000 WBCs/mcL and intracellular monosodium urate crystals. Gram stain is negative. What is the most appropriate management plan?

. Administer intra-articular corticosteroids and discharge the patient.
. Begin treatment for gout and closely monitor while awaiting final culture results.
. Proceed immediately to surgical arthroscopic irrigation and debridement.
. Prescribe oral allopurinol as monotherapy and discharge.
. Schedule an outpatient MRI to rule out osteomyelitis.

Correct Answer & Explanation

. Begin treatment for gout and closely monitor while awaiting final culture results.

Explanation

The presence of crystals confirms gout but does not definitive rule out concomitant septic arthritis. The safest management is to treat the acute flare while observing the patient and awaiting final synovial fluid culture results.

Question 1251

Topic: Infection, Pharmacology & VTE

The Kocher criteria are utilized to risk-stratify children presenting with an irritable hip. Which of the following is NOT one of the classic four Kocher criteria used to differentiate septic arthritis from transient synovitis?

. Non-weight-bearing on the affected side
. Erythrocyte sedimentation rate > 40 mm/hr
. Serum white blood cell count > 12,000 cells/mm3
. Temperature > 38.5 C (101.3 F)
. C-reactive protein > 2.0 mg/dL

Correct Answer & Explanation

. C-reactive protein > 2.0 mg/dL

Explanation

The classic four Kocher criteria are non-weight-bearing, ESR > 40, WBC > 12,000, and Temp > 38.5 C. Although CRP > 2.0 mg/dL was later identified as a strong independent predictor by Caird et al., it is not one of the original four Kocher criteria.

Question 1252

Topic: Infection, Pharmacology & VTE

A 45-year-old male presents with an acutely swollen native knee. Synovial fluid analysis reveals an elevated WBC count. Above what threshold is native joint septic arthritis highly suspected?

. 3,000 cells/uL
. 10,000 cells/uL
. 25,000 cells/uL
. 50,000 cells/uL
. 100,000 cells/uL

Correct Answer & Explanation

. 50,000 cells/uL

Explanation

In native joints, a synovial WBC count greater than 50,000 cells/uL with >90% polymorphonuclear cells is highly suggestive of septic arthritis. Lower thresholds (e.g., 3,000 cells/uL) are typically used for chronic periprosthetic joint infections.

Question 1253

Topic: Infection, Pharmacology & VTE
A 68-year-old female is prescribed fondaparinux for deep vein thrombosis prophylaxis following a total hip arthroplasty. What is the mechanism of action of this agent?
. Direct thrombin inhibitor
. Vitamin K epoxide reductase antagonist
. Indirect Factor Xa inhibitor via antithrombin III
. Direct Factor Xa inhibitor
. Irreversible cyclooxygenase inhibitor

Correct Answer & Explanation

. Indirect Factor Xa inhibitor via antithrombin III

Explanation

Fondaparinux is a synthetic pentasaccharide that binds to antithrombin III, accelerating its inhibition of Factor Xa. It is an indirect Factor Xa inhibitor. Rivaroxaban and apixaban are direct Factor Xa inhibitors, while dabigatran is a direct thrombin (Factor IIa) inhibitor.

Question 1254

Topic: Infection, Pharmacology & VTE

A patient develops a surgical site infection after spinal fusion. Cultures grow Methicillin-resistant Staphylococcus aureus (MRSA). The mecA gene is responsible for this resistance by encoding for which of the following?

. A potent beta-lactamase enzyme
. A transmembrane efflux pump
. An altered penicillin-binding protein (PBP2a)
. An aminoglycoside-modifying enzyme
. A dense exopolysaccharide biofilm matrix

Correct Answer & Explanation

. An altered penicillin-binding protein (PBP2a)

Explanation

MRSA resistance is primarily mediated by the mecA gene, which encodes for an altered penicillin-binding protein (PBP2a). PBP2a has a markedly reduced affinity for beta-lactam antibiotics, allowing the bacteria to synthesize cell walls even in the presence of methicillin or other beta-lactams.

Question 1255

Topic: Infection, Pharmacology & VTE

A 35-year-old presents with a large, recurrent knee effusion. Synovial fluid analysis reveals 25,000 WBCs/mm3 with a negative Gram stain. Due to suspected late Lyme arthritis, serology is ordered. According to CDC criteria, what constitutes a positive Western blot for late Lyme disease?

. 2 of 3 specific IgM bands
. 5 of 10 specific IgG bands
. 2 of 3 specific IgG bands
. 5 of 10 specific IgM bands
. Detection of Borrelia burgdorferi DNA by PCR in the serum

Correct Answer & Explanation

. 5 of 10 specific IgG bands

Explanation

The CDC recommends two-tiered testing for Lyme disease. If the initial EIA or IFA is positive, a Western blot is performed. For late Lyme disease (symptoms present for > 1 month, such as arthritis), the criteria require the presence of 5 of 10 specific IgG bands. IgM bands are not used for late disease due to the high rate of false positives over time.

Question 1256

Topic: Infection, Pharmacology & VTE

A 9-year-old African American boy with homozygous hemoglobin SS disease presents with a 4-day history of fever, localized distal femoral pain, and an inability to bear weight. Blood cultures and a bone aspirate are obtained. While Staphylococcus aureus remains a common pathogen, this patient is at uniquely high risk for osteomyelitis caused by which of the following encapsulated, gram-negative bacilli?

. Pseudomonas aeruginosa
. Haemophilus influenzae
. Salmonella typhimurium
. Escherichia coli
. Klebsiella pneumoniae

Correct Answer & Explanation

. Salmonella typhimurium

Explanation

Correct Answer: C. Salmonella typhimuriumPatients with sickle cell disease are functionally asplenic and have a uniquely high risk of osteomyelitis caused bySalmonellaspecies, an encapsulated, gram-negative bacillus. WhileStaphylococcus aureusis still the most common overall cause of osteomyelitis in this population, the incidence ofSalmonellais disproportionately high compared to the general population. Microinfarctions in the bowel wall are thought to allowSalmonellato translocate into the bloodstream and seed infarcted bone.

Question 1257

Topic: Infection, Pharmacology & VTE
A 68-year-old male presents with insidious onset of right hip pain 14 months after a primary total hip arthroplasty. Inflammatory markers are mildly elevated. Joint aspiration yields a synovial fluid leukocyte count of 4,500 cells/μL with 75% neutrophils. Cultures grow a coagulase-negative, Gram-positive coccus that forms biofilms. Which of the following organisms is most likely responsible?
. Staphylococcus aureus
. Staphylococcus epidermidis
. Cutibacterium acnes
. Streptococcus pyogenes
. Enterococcus faecalis

Correct Answer & Explanation

. Staphylococcus epidermidis

Explanation

Prosthetic joint infections (PJIs) are most commonly caused by coagulase-negative staphylococci, particularly Staphylococcus epidermidis, especially in delayed or late-onset presentations (3-24 months post-op). S. epidermidis is a normal skin commensal that possesses a unique ability to adhere to orthopedic implants and form a protective glycocalyx biofilm. This biofilm makes the bacteria highly resistant to host immune responses and systemic antibiotics. S. aureus is coagulase-positive and typically presents more acutely.

Question 1258

Topic: Infection, Pharmacology & VTE
A 68-year-old male presents with a chronically painful total knee arthroplasty, 2 years post-operatively. Aspiration yields a synovial fluid white blood cell count of 45,000 cells/μL with 90% neutrophils. Cultures grow a coagulase-negative Staphylococcus. Which of the following characteristics of this organism makes eradication difficult without implant removal?
. Production of beta-lactamase
. Intracellular survival within macrophages
. Formation of a polysaccharide glycocalyx biofilm
. Rapid doubling time in synovial fluid
. Secretion of exotoxins that cause tissue necrosis

Correct Answer & Explanation

. Formation of a polysaccharide glycocalyx biofilm

Explanation

Coagulase-negative staphylococci (e.g., S. epidermidis) are a leading cause of chronic prosthetic joint infections. Their primary virulence factor in this setting is the ability to adhere to the implant surface and produce a polysaccharide glycocalyx, forming a dense biofilm. This biofilm protects the bacteria from the host immune system and systemic antibiotics, typically necessitating surgical removal of the implant (e.g., two-stage revision) for definitive cure.

Question 1259

Topic: Infection, Pharmacology & VTE

A 68-year-old female with severe varus gonarthrosis is undergoing a total knee arthroplasty. After making the initial bone cuts, the surgeon notes that the knee is tight medially in both flexion and extension. Which of the following structures should be released first to balance the knee?

. Superficial medial collateral ligament (sMCL).
. Deep medial collateral ligament (dMCL).
. Pes anserinus tendons.
. Semimembranosus insertion.
. Posterior cruciate ligament (PCL).

Correct Answer & Explanation

. Deep medial collateral ligament (dMCL).

Explanation

Correct Answer: BIn a varus knee, medial tightness in both flexion and extension requires a sequential medial release to achieve a balanced gap. The standard sequence begins with the removal of medial osteophytes, which often provides significant correction. If still tight, the next step is the release of the deep medial collateral ligament (dMCL). If further release is needed, the posteromedial capsule and semimembranosus are released, followed by the superficial MCL (sMCL), and finally the pes anserinus if absolutely necessary.

Question 1260

Topic: Infection, Pharmacology & VTE
A 68-year-old male presents with increasing pain and stiffness in his right knee, 14 months after a primary total knee arthroplasty. Inflammatory markers are elevated, and a joint aspiration yields a synovial fluid white blood cell count of 45,000 cells/μL with 90% neutrophils. Cultures grow a coagulase-negative Staphylococcus. Which of the following characteristics of this organism is the primary reason for its virulence in prosthetic joint infections?
. Production of Panton-Valentine leukocidin
. Ability to form a polysaccharide glycocalyx biofilm
. Secretion of toxic shock syndrome toxin-1
. Intracellular survival within osteoblasts
. Production of beta-lactamase

Correct Answer & Explanation

. Ability to form a polysaccharide glycocalyx biofilm

Explanation

Coagulase-negative staphylococci, such as Staphylococcus epidermidis, are the most common pathogens in delayed prosthetic joint infections (PJIs). Their primary virulence factor is the ability to adhere to polymer and metal surfaces and produce a thick polysaccharide glycocalyx, forming a biofilm. This biofilm protects the bacteria from the host's immune system (e.g., phagocytosis) and significantly decreases the penetration and efficacy of systemic antibiotics. This is why definitive treatment of chronic PJIs typically requires surgical removal of the hardware (e.g., two-stage exchange arthroplasty) rather than antibiotics alone.

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