Question 1

Which of the following groups is not at increased risk for community- acquired methicillin-resistant Staphylococcus aureus (C AMRSA):

Explanation

People traveling abroad are not considered at increased risk for C A-MRSA. Athletes who participate in physical contact sports, patients who have had recent antibiotics, people living in crowded conditions, and prison inmates are considered at increased risk for C A-MRSA.

Question 2

Which of the following toxins is responsible for the virulence of community- acquired methicillin-resistant Staphylococcus aureus (C A-MRSA):

Explanation

Panton-Valentine leukocidin is a toxin that produces necrosis of tissue and white blood cells. This toxin is much more common in CA-MRSA than in hospital-acquired MRSA.

Question 3

A 5-year-old girl presents to the emergency department with pain in her hip. Her temperature is 102° F, pulse is 96, and blood pressure is 104/60. Magnetic resonance imaging reveals edema in the obturator muscles. Blood cultures are negative. Ultrasound-guided aspiration reveals no abscess, but fluid obtained grows methicillin-resistant Staphylococcus aureus (MRSA). In addition to antibiotic treatment, what other treatment option is recommended:

Explanation

This patient has community-acquired MRSA but does not have an evident fluid collection. There is no evidence of systemic infection or necrotizing fasciitis. There is no indication for drainage or debridement of any type. Therefore, antibiotics alone with clinical observation are indicated.

Question 4

A 5-year-old girl who presents to the emergency department has obturator muscle infection without abscess formation, which is seen on magnetic resonance imaging. She has no clinical evidence of sepsis. Aspiration yields methicillin-resistant Staphylococcus aureus (MRSA). Which of the following antibiotics is recommended:

Explanation

To treat this patient, clindamycin is the drug of choice. Vancomycin is not recommended for uncomplicated MRSA in the nonseptic patient. Rifampin is not recommended to be used alone because of the rapid development of resistance. Tetracycline is not recommended for children under 8 years old because it causes permanent staining of the teeth. Linezolid is to be used only after infectious disease consultation because of its toxicity (thrombocytopenia in 3.5% of patients) and expense. Another option for this patient would be trimetoprim/sulfamethoxazole.

Question 5

A nonverbal 12-year-old patient with totally involved cerebral palsy and developmental delay has been in pain for the past week. The source of the pain cannot be localized. Pelvis radiographs and urinalysis are unremarkable. The next recommended study is:

Explanation

In a nonverbal child with longstanding pain, the whole-body bone scan is the most sensitive and specific test. Whole-body bone scans detected a cause of pain in more than half of such children in one study; most causes of pain were fractures. MRI cannot be focused enough, and C T has the same limitation as well as increased radiation exposure. Thermography is not widely available and has not been studied in children with cerebral palsy.

Question 6

A 2-month-old infant has a midshaft humeral fracture. The fracture has 45Â° angulation with an anterolateral apex and is 6 mm short. Recommended treatment includes:

Explanation

The humerus has unlimited remodeling potential at this age. The goal of treatment is pain relief. Immobilization against the torso is the most practical method of treatment.

Question 7

A 23-month-old patient demonstrates intoeing. He walked at 21 months. His height is in the 3rd quantile, and he is cognitively normal. Possible diagnoses include all of the following except:

Explanation

The upper limit of normal age for onset of walking is 18 months. All of the diagnoses, except for internal tibial torsion, may explain the delayed walking, short stature, and intoeing.

Question 8

The most common form of rickets in the United States is:

Explanation

Although rickets due to deficiency of vitamin D is common due to inadequate sunlight or prolonged breast feeding, nutritional deficiency of calcium is uncommon in the US. The most common cause of rickets in the US is X-linked hypophosphatemic rickets. Hypophosphatasia, caused by alkaline phosphatase deficiency, is a rare recessive condition. Fanconi syndrome is a failure of tubular reabsorption of small molecules and resembles X-linked hypophosphatemic rickets, although it is more rare. Vitamin Ddependent rickets is also rare.

Question 9

The gene most likely to be associated with inherited rickets is:

Explanation

PHEX is a gene whose product regulates transport of renal phosphates that leads to isolated renal phosphate wasting and consequent hypophosphatemia. This is the syndrome known as X-linked hypophosphatemic rickets, which is the most common cause of inherited rickets.

Question 10

Genetic testing performed for a patient reveals a defect in PHEX. The family history provided by the patient is most likely to include:

Explanation

PHEX is the gene for a product that regulates renal tubular phosphate reabsorption. A defect in this gene causes X-linked hypophosphatemic rickets, which is the most common inherited rickets. It is an X-linked dominant condition; therefore, male-tomale transmission cannot occur. An excess of affected female relatives to male relatives is approximately 2:1. Spontaneous mutation can occur but is less common than inheritance of the disorder.

Question 11

A toddler with bowing is suspected of X-linked hypophosphatemic rickets (XLH). The most likely laboratory abnormalities will include:

Explanation

The most common laboratory abnormalities in XLH include high alkaline phosphatase (in attempt to mobilize phosphate from the bone); low serum phosphate; normal calcium and 25-(OH)2D3; and low 1,25-(OH)2D3, presumably due to defective renal hydroxylation.

Question 12

An 18-month-old patient is suspected of X-linked hypophosphatemic rickets. The clinical findings are most likely to include all of the following except:

Explanation

Patients with X-linked hypophosphatemic rickets may present with below average stature, exaggerated genu varus, dental abscesses or caries, and delayed motor milestones. Alopecia is not common in patients with this condition but is characteristic of the rare condition of end-organ insensitivity to vitamin D (vitamin D-resistant rickets).

Question 13

A newborn is brought in for consultation due to a foot deformity. The left foot is dorsiflexed at the ankle, and the dorsum of the foot is able to be placed against the tibia. Neurologic and vascular examination is normal. Recommended treatment includes:

Explanation

The newborn is presenting with a calcaneovalgus foot. This condition resolves spontaneously, therefore, no additional work-up is needed.

Question 14

In a newborn, how much angulation is acceptable in a fracture of the humeral diaphysis:

Explanation

In the humeral diaphysis of a newborn, one can accept virtually any alignment because of the tremendous remodeling capacity of the child. Complete remodeling of 66° within 1 year has been documented.

Question 15

Which of the following radiographic parameters remains essentially fixed throughout life:

Explanation

Pelvic incidence is a measure of the relationship between the superior sacral endplate and the hips. It does not change significantly during life unless there is a major pelvic fracture. Pelvic incidence is defined as the angle between a line connecting the hip center and the midpoint of the S1 endplate, and a perpendicular to the midpoint of the S1 endplate. This angle is fixed in bone. By contrast, all of the other angles vary with posture and age.

Question 16

A 6-year-old boy presents with a linear area of swelling in the region of the posterior paraspinous muscles. He has no history of weight loss or fevers. He also has bilateral great toe deformities consisting of valgus and shortening. The most likely diagnosis is:

Explanation

Fibrodysplasia ossificans progressiva is characterized by linear swelling and progressive ossification from a posterior-central origin. Patients also have a characteristic shortened and valgus great toe. The natural history of this disorder is one of progressive ossification, and it eventually impairs nutrition.

Question 17

Based on the presented radiographs (Slide 1, Slide 2), what is the proper diagnosis of this 12-month-old female patient:

Explanation

This patient had a physeal fracture of the right proximal femur, which was the result of a non-accidental injury. Notice the displacement of the right femoral metaphysic anteriorly on the frog view despite comparable degrees of rotation. A metaphyseal calcification is apparent posterior to the femoral neck. Treatment involved spica cast application.

Question 18

A 23-month-old patient is evaluated for intoeing. Based on his radiograph (Slide), what is the most likely diagnosis:

Explanation

This patient has rickets. Note the diffuse osteopenia, â grainy nature, thinning of the tibial and femoral diaphyseal cortex, varus of both the proximal and distal tibial metaphyses, and widened and irregular physes at all levels.

Question 19

A 13-year-old boy has been experiencing hip pain for 1 month. The pain is worse with activity. The patient has no history of fever or weight loss, and both of his hips flex to 90Â°. Based on his radiograph (Slide 1) and magnetic resonance image (Slide 2), which of the following is the most likely diagnosis:

Explanation

This patient has a subtle grade 1 slipped capital femoral epiphysis. He is a male Risser 0 and is overweight. He has decreased internal rotation of the involved hip in flexion. The plain pelvis radiograph shows slightly less epiphyseal height on the left side, slight posterior metaphyseal overlap, and increased irregularity of the physis. MRI confirms a lucent plane in the physis and surrounding edema. A plain film (Slide 3) confirms the diagnosis. In situ fixation was performed on this patient. Orthopedic Prometric Exam Chapter 3 Image

Orthopedic Prometric Exam Chapter 3 Image

Question 20

A 13-year-old boy has been experiencing hip pain for 1 month. The pain is worse with activity. The patient has no history of fever or weight loss, and both of his hips flex to 90°. , which of the following is the most likely diagnosis:

Explanation

This patient has a subtle grade 1 slipped capital femoral epiphysis. He is a male Risser 0 and is overweight. He has decreased internal rotation of the involved hip in flexion. The plain pelvis radiograph shows slightly less epiphyseal height on the left side, slight posterior metaphyseal overlap, and increased irregularity of the physis. MRI confirms a lucent plane in the physis and surrounding edema. A plain film (Slide 3) confirms the diagnosis. In situ fixation was performed on this patient.

Question 21

An 8-year-old boy has acute hematogenous osteomyelitis caused by community-acquired MRSA (CA-MRSA). Sensitivity testing shows the isolate is susceptible to clindamycin but resistant to erythromycin. Which test must be performed before initiating clindamycin therapy?

Explanation

A D-zone test identifies inducible clindamycin resistance in erythromycin-resistant Staphylococcus strains. A positive test indicates the isolate harbors the erm gene, contraindicating the use of clindamycin.

Question 22

A 2-year-old child presents with a swollen, erythematous knee. Joint aspiration yields purulent fluid, but standard agar cultures are negative at 48 hours. What is the most appropriate method to identify the most likely atypical causative organism in this age group?

Explanation

Kingella kingae is a very common cause of septic arthritis in toddlers but is fastidious on solid media. Inoculation of synovial fluid directly into liquid blood culture vials significantly improves the diagnostic yield.

Question 23

A 7-year-old boy presents with a limp, fever, and inability to bear weight. Hip range of motion is slightly limited at the extremes but relatively painless in the mid-arc. Inflammatory markers are elevated. MRI reveals abnormal marrow signal in the ilium.

What is the most appropriate initial management for uncomplicated acute pelvic osteomyelitis?

Explanation

Acute pelvic osteomyelitis frequently mimics a septic hip but usually preserves functional, painless hip motion in the mid-arc. First-line treatment for uncomplicated cases without a large drainable abscess is intravenous antibiotics.

Question 24

Methicillin resistance in Staphylococcus aureus (MRSA) is primarily mediated by which of the following molecular mechanisms?

Explanation

MRSA strains acquire the mecA gene, which encodes for a variant penicillin-binding protein called PBP2a. This altered protein has a severely reduced affinity for beta-lactam antibiotics, rendering them ineffective.

Question 25

Community-acquired MRSA (CA-MRSA) strains frequently produce a specific toxin that is highly associated with severe deep tissue infections and necrotizing pneumonia. Which toxin is this?

Explanation

Panton-Valentine leukocidin (PVL) is a pore-forming cytotoxin that destroys leukocytes and causes severe tissue necrosis. It is a key virulence factor in many highly aggressive CA-MRSA infections.

Question 26

A 3-week-old neonate presents with a septic hip. Pending culture results, which of the following empiric intravenous antibiotic regimens is most appropriate?

Explanation

Empiric coverage for neonatal septic arthritis must target S. aureus, Group B Streptococcus, and Gram-negative bacilli. A combination of vancomycin (for MRSA) and a 3rd-generation cephalosporin (like cefotaxime) provides the necessary broad-spectrum coverage.

Question 27

A 6-year-old child with sickle cell disease presents with femoral osteomyelitis. While Staphylococcus aureus is highly prevalent, which organism is uniquely responsible for a disproportionately high percentage of osteomyelitis cases in this specific patient population?

Explanation

Due to hyposplenism and episodic bowel ischemia, patients with sickle hemoglobinopathies are uniquely susceptible to systemic infection and subsequent osteomyelitis caused by Salmonella species.

Question 28

According to the Kocher criteria, what is the predictive probability of septic arthritis in a child who presents with a fever of 38.6 degrees C, inability to bear weight, ESR of 45 mm/hr, and a serum WBC count of 13,000 cells/mm3?

Explanation

The patient meets all four Kocher criteria: fever > 38.5 C, non-weight-bearing, ESR > 40, and WBC > 12,000. The presence of all four criteria yields a 93-99% probability of septic arthritis.

Question 29

A 9-year-old girl presents with recurrent episodes of clavicular and tibial pain. Radiographs show sclerotic and lytic lesions. Multiple bone biopsies have yielded no bacterial growth, and she has not responded to prolonged IV antibiotics. What is the most appropriate next step in management?

Explanation

The clinical presentation is classic for Chronic Recurrent Multifocal Osteomyelitis (CRMO), an autoinflammatory disorder rather than an infection. First-line therapy for CRMO is the scheduled use of NSAIDs.

Question 30

A 7-year-old boy from Massachusetts presents with a massive, painless right knee effusion. He is afebrile and active. Joint aspiration yields a WBC count of 45,000 cells/mm3 (predominantly neutrophils), but Gram stain is negative. What is the most appropriate diagnostic test?

Explanation

Lyme arthritis typically presents as a large, relatively painless effusion in an endemic area. Two-tiered serum testing (EIA/IFA followed by Western blot) is the recommended diagnostic approach, as synovial fluid PCR often has lower sensitivity.

Question 31

An infant is diagnosed with septic arthritis of the hip. An emergent open arthrotomy is planned. Which surgical approach is most commonly utilized to provide safe and direct access to the pediatric hip joint?

Explanation

The anterior (Smith-Petersen) approach is the most widely utilized and safest approach for arthrotomy in a pediatric septic hip. It provides excellent capsular exposure while avoiding the precarious femoral head blood supply.

Question 32

A 14-year-old boy with a localized MRSA abscess develops sudden onset high fever, hypotension, and a diffuse macular erythroderma. Labs reveal acute kidney injury. This shock syndrome is mediated by which of the following mechanisms?

Explanation

Staphylococcal Toxic Shock Syndrome is caused by superantigens (such as TSST-1). These proteins cross-link MHC class II molecules directly to T-cell receptors, triggering a massive, uncoordinated cytokine storm.

Question 33

A 12-year-old boy presents with chronic, mild lower leg pain that worsens at night. Radiographs demonstrate a 2 cm radiolucent lesion in the distal tibial metaphysis with a thick rim of reactive sclerosis. Inflammatory markers are normal. What is the most likely diagnosis?

Explanation

A Brodie's abscess is a subacute or chronic localized form of osteomyelitis, often presenting with mild symptoms and normal inflammatory markers. It classically appears as a well-defined lytic metaphyseal lesion with a sclerotic rim.

Question 34

A 5-year-old child is treated for uncomplicated acute hematogenous osteomyelitis of the femur. After a week of intravenous antibiotics, the child is afebrile, and the CRP has normalized. What is the currently recommended total duration of antibiotic therapy (IV plus oral step-down)?

Explanation

For uncomplicated acute hematogenous osteomyelitis in children with a rapid clinical and serologic response, a short course (typically 3 to 4 weeks total) of antibiotics is considered safe and effective.

Question 35

A patient with confirmed active pulmonary tuberculosis develops an acute, sterile polyarthritis primarily affecting the knees and ankles. Joint aspirates show no evidence of direct mycobacterial invasion. What is the term for this specific reactive arthritis?

Explanation

Poncet's disease is a rare reactive polyarthritis that occurs in the presence of active extra-articular tuberculosis. The joint inflammation is immune-mediated, and the synovial fluid remains sterile.

Question 36

An 8-year-old girl is ready for discharge following surgical drainage of a CA-MRSA soft tissue abscess. She is prescribed an oral step-down antibiotic. Which of the following is an appropriate oral agent for MRSA that does NOT require routine monitoring for myelosuppression?

Explanation

Trimethoprim-sulfamethoxazole (TMP-SMX) is highly effective for CA-MRSA skin and soft tissue infections. Linezolid requires monitoring for myelosuppression (especially for use >2 weeks), and tetracyclines are avoided under age 8 due to tooth discoloration.

Question 37

Which of the following is NOT one of the classic Kocher criteria used to differentiate septic arthritis of the hip from transient synovitis in children?

Explanation

The classic Kocher criteria include non-weight-bearing, ESR >40, fever >38.5°C, and WBC >12,000. While CRP >2.0 mg/dL was later identified by Caird et al. as an excellent independent predictor, it is not one of the original four Kocher criteria.

Question 38

A 2-year-old girl presents with a warm, swollen knee and refusal to walk. Aspiration yields purulent synovial fluid, but standard agar cultures remain sterile at 48 hours. What is the optimal method for isolating the most likely causative organism in this age group?

Explanation

Kingella kingae is a fastidious organism and a leading cause of septic arthritis in children aged 6 months to 4 years. Its isolation is significantly improved by inoculating synovial fluid directly into aerobic blood culture vials or by using PCR.

Question 39

A 9-year-old boy presents with severe right thigh pain, fever, and inability to bear weight.

MRI reveals a large subperiosteal abscess. If blood cultures grow Community-Acquired MRSA (CA-MRSA), which of the following complication screening protocols is highly recommended?

Explanation

CA-MRSA osteomyelitis in children is strongly associated with deep vein thrombosis (DVT) and septic pulmonary emboli. Routine screening with venous duplex ultrasound of the affected limb is recommended due to the high incidence of associated DVT.

Question 40

A 10-year-old boy develops osteomyelitis of the calcaneus after stepping on a rusty nail that penetrated his rubber-soled sneaker. Which organism must be specifically covered by empiric antibiotic therapy?

Explanation

Puncture wounds through rubber-soled shoes have a high association with Pseudomonas aeruginosa osteomyelitis. Empiric therapy must include anti-pseudomonal coverage, and surgical debridement is often required.

Question 41

A 6-year-old girl with sickle cell disease presents with high fever, severe right arm pain, and localized swelling over the humerus. Which of the following is the most appropriate empiric antibiotic regimen while awaiting cultures?

Explanation

Children with sickle cell disease are at high risk for osteomyelitis caused by both Salmonella species and Staphylococcus aureus. Empiric coverage should include a third-generation cephalosporin (for Salmonella) and vancomycin or clindamycin (for MRSA).

Question 42

A 2-week-old neonate is brought to the clinic for decreased spontaneous movement of the left arm. She is afebrile with normal inflammatory markers. Examination reveals crying with passive motion of the shoulder. What is the most likely diagnosis?

Explanation

Neonatal septic arthritis often presents subacutely with pseudoparalysis and pain on passive motion, without systemic signs like fever or elevated WBC. This presentation requires immediate ultrasound and joint aspiration to prevent rapid joint destruction.

Question 43

A 9-year-old girl has a 6-month history of recurrent multifocal bone pain. Radiographs show lytic lesions with sclerotic borders in her clavicle and distal femur. Multiple bone biopsies have shown only sterile acute and chronic inflammation. Which of the following is the recommended initial management?

Explanation

The clinical picture describes Chronic Recurrent Multifocal Osteomyelitis (CRMO), an autoinflammatory disorder. The first-line treatment is nonsteroidal anti-inflammatory drugs (NSAIDs), followed by bisphosphonates or biologics for refractory cases.

Question 44

In children, the blood supply to the epiphysis directly communicates with metaphyseal vessels across the physis. This transphyseal vascular connection, which allows metaphyseal osteomyelitis to easily spread into the joint, typically obliterates by what age?

Explanation

Transphyseal vessels exist in children up to approximately 18 months of age. After this age, the physis acts as a barrier to the spread of infection, making concurrent osteomyelitis and septic arthritis less common until the physis closes in late adolescence.

Question 45

A 3-year-old boy presents with refusal to walk and a recent history of irritability. He refuses to pick up toys from the floor and maintains a rigidly straight back. ESR is elevated, but WBC is normal. What is the most accurate diagnostic imaging modality for the suspected condition?

Explanation

The child's presentation (refusal to walk, loss of lumbar lordosis) is classic for pediatric discitis. MRI of the spine with and without contrast is the most sensitive and specific imaging modality for diagnosing discitis.

Question 46

The Panton-Valentine leukocidin (PVL) toxin is a major virulence factor in Community-Acquired MRSA. What is its primary pathogenic mechanism in orthopedic infections?

Explanation

The PVL toxin is a cytotoxin that forms pores in the membranes of leukocytes (neutrophils), causing rapid cell lysis. This massive leukocyte destruction releases damaging enzymes, causing the severe, necrotic inflammation typical of CA-MRSA infections.

Question 47

A 16-year-old female presents with acute knee swelling, migratory polyarthralgia, and painless vesiculopustular skin lesions on her extremities. Gram stain of the synovial fluid is likely to reveal which of the following?

Explanation

The clinical presentation is classic for disseminated gonococcal infection (DGI), caused by Neisseria gonorrhoeae. This organism is a Gram-negative intracellular diplococcus.

Question 48

A 13-year-old boy complains of dull, aching pain in his distal tibia, which frequently wakes him at night and is relieved by ibuprofen. MRI demonstrates a metaphyseal intraosseous abscess with a hyperintense inner ring on T1-weighted imaging. What is this MRI finding called?

Explanation

The "penumbra sign" is characteristic of a Brodie's abscess on MRI. It consists of a T1-hyperintense rim of granulation tissue lining the abscess cavity, separating it from the surrounding sclerotic bone.

Question 49

A 14-year-old is prescribed a prolonged 6-week course of oral linezolid for a complex MRSA osteomyelitis. Which of the following complications requires routine monitoring during the duration of therapy?

Explanation

Prolonged use of linezolid (>2 weeks) is associated with reversible myelosuppression, most commonly manifesting as thrombocytopenia. Regular complete blood counts (CBC) must be monitored; peripheral and optic neuropathy are also risks.

Question 50

A 7-year-old boy from Massachusetts presents with a swollen, warm right knee, but no fever or signs of systemic toxicity. He has full range of motion of the knee without severe pain. What is the most appropriate initial diagnostic test for the suspected etiology?

Explanation

The child likely has Lyme arthritis, characterized by large joint effusions (especially the knee) without the extreme pain or systemic toxicity seen in bacterial septic arthritis. The initial test is a screening ELISA, followed by a confirmatory Western blot if positive.

Question 51

A 10-year-old boy presents with fever, limp, and severe tenderness over his mid-thigh. MRI reveals a large, rim-enhancing fluid collection within the vastus intermedius, but the adjacent femoral marrow shows normal signal intensity. What is the most common causative organism for this condition in temperate climates?

Explanation

The diagnosis is primary pyomyositis (abscess in the muscle without underlying osteomyelitis). Even in temperate climates, Staphylococcus aureus is the most common causative organism for primary pyomyositis.

Question 52

In a child being treated for uncomplicated acute hematogenous osteomyelitis (AHO) with intravenous antibiotics, which of the following criteria is most widely accepted for safely transitioning to oral antibiotics?

Explanation

Transition to oral antibiotics in uncomplicated pediatric AHO is guided by clinical improvement, being afebrile for 24-48 hours, and a decreasing CRP trend (often a 50% drop from the peak). ESR normalizes too slowly to guide this transition.

Question 53

A 9-year-old girl presents with vague buttock pain, fever, and a noticeable limp. Hip range of motion is full but painful at the extremes, and the FABER test is markedly positive. Plain radiographs are normal. What is the most appropriate next step in management?

Explanation

The clinical picture suggests pelvic osteomyelitis or sacroiliac joint infection, which is notoriously difficult to diagnose clinically and typically shows normal early X-rays. MRI of the pelvis is the gold standard for definitive diagnosis.

Question 54

Kingella kingae is a common cause of pediatric septic arthritis. A thorough history from the parents of an affected child will most likely reveal which of the following antecedent events?

Explanation

Kingella kingae colonizes the posterior pharynx of young children. Infections are frequently preceded by viral upper respiratory tract infections or stomatitis, which facilitate the organism's entry into the bloodstream.

Question 55

Rapid destruction of articular cartilage in bacterial septic arthritis of the hip is primarily caused by which of the following mechanisms?

Explanation

Cartilage destruction in septic arthritis is largely mediated by the host's immune response. Neutrophils and hyperplastic synovial cells release massive amounts of proteolytic enzymes, lysozymes, and matrix metalloproteinases that rapidly degrade glycosaminoglycans and collagen.

Question 56

When interpreting culture susceptibilities for a methicillin-resistant Staphylococcus aureus (MRSA) isolate from a pediatric osteomyelitis case, a positive "D-zone test" indicates which of the following?

Explanation

The D-zone test identifies isolates that appear susceptible to clindamycin on standard testing but possess inducible macrolide-lincosamide-streptogramin B (iMLS_B) resistance. If the test is positive, clindamycin should not be used for treatment due to the risk of clinical failure.

Question 57

Community-acquired MRSA is characterized by the presence of Panton-Valentine leukocidin (PVL). What is the primary mechanism of action of this virulence factor?

Explanation

PVL is a cytotoxin that creates pores in leukocyte membranes, leading to their destruction and increased tissue necrosis. This virulence factor is strongly associated with the aggressive soft tissue infections and severe osteomyelitis seen in CA-MRSA.

Question 58

Community-acquired MRSA (CA-MRSA) typically contains which of the following staphylococcal cassette chromosome mec (SCCmec) elements compared to hospital-acquired MRSA (HA-MRSA)?

Explanation

CA-MRSA is most commonly associated with SCCmec type IV. This element is smaller, more mobile, and typically does not carry the multi-drug resistance genes found in HA-MRSA (Types I, II, and III).

Question 59

A 9-year-old boy with severe community-acquired MRSA (CA-MRSA) tibial osteomyelitis experiences persistent tachycardia and swelling in the affected limb despite adequate surgical debridement and IV vancomycin. What is the most likely associated complication?

Explanation

Severe CA-MRSA osteomyelitis in children is highly associated with adjacent deep venous thrombosis (DVT) and septic pulmonary emboli. Prompt venous duplex imaging is indicated when limb swelling or respiratory symptoms arise.

Question 60

A 2-year-old child presents with an acute monoarticular knee effusion, low-grade fever, and refusal to bear weight. Aspiration yields purulent fluid, but standard agar plates show no growth at 48 hours. Which of the following is the most appropriate method to isolate the most likely causative organism?

Explanation

Kingella kingae is a fastidious Gram-negative organism that is a leading cause of septic arthritis in children under 4. It requires inoculation into aerobic blood culture vials (BACTEC) to enhance recovery rates.

Question 61

A 15-year-old female develops rapid-onset fever, hypotension, and a diffuse macular erythroderma following knee arthroscopy. Blood cultures are sterile. The condition is mediated by a toxin that exerts its effect through which of the following mechanisms?

Explanation

Toxic shock syndrome is typically caused by TSST-1 (a superantigen) produced by S. aureus. It binds directly to MHC class II and the T-cell receptor outside the normal antigen-binding groove, causing massive non-specific T-cell activation and a cytokine storm.

Question 62

A 2-week-old neonate presents with irritability, poor feeding, and pseudoparalysis of the right lower extremity. Ultrasound shows a right hip effusion. Aspiration reveals purulent fluid. Empiric antibiotic coverage must include agents effective against Staphylococcus aureus and which of the following organisms?

Explanation

Neonatal (under 1 month) septic arthritis is most commonly caused by S. aureus, Group B Streptococcus, and Gram-negative bacilli. Empiric treatment typically includes an antistaphylococcal agent combined with a third-generation cephalosporin or gentamicin.

Question 63

Which of the following laboratory parameters is a component of the Laboratory Risk Indicator for Necrotizing Fasciitis (LRINEC) score?

Explanation

The LRINEC score helps distinguish necrotizing fasciitis from severe cellulitis. It incorporates C-reactive protein, total WBC count, hemoglobin, serum sodium, creatinine, and glucose.

Question 64

A 14-year-old boy is recovering from an uncomplicated CA-MRSA skin and soft tissue infection that was surgically drained. He is to be discharged on oral antibiotics. The isolate is sensitive to trimethoprim-sulfamethoxazole, clindamycin, and tetracyclines. Which of the following antibiotics is known to also inhibit bacterial toxin production?

Explanation

Clindamycin is a lincosamide antibiotic that binds to the 50S ribosomal subunit. In addition to its bacteriostatic effect, it powerfully inhibits the synthesis of bacterial toxins, such as PVL, making it highly effective for toxin-mediated infections.

Question 65

A 7-year-old boy presents with a 3-day history of fever, limp, and severe right thigh pain. Blood cultures are drawn.

Based on the likely diagnosis of deep infection, what is the most appropriate next step in management after initiating empiric antibiotics?

Explanation

The clinical presentation suggests acute osteomyelitis or a deep subperiosteal abscess. Once a focal fluid collection or abscess is identified, prompt surgical drainage and debridement are required for adequate source control.

Question 66

In chronic orthopedic implant infections, Staphylococcus epidermidis evades host defenses primarily through the production of a glycocalyx biofilm. Which of the following phases of biofilm formation involves "quorum sensing"?

Explanation

Quorum sensing is a cell-to-cell communication mechanism that coordinates gene expression based on bacterial density. It is crucial during the maturation phase of the biofilm, regulating the production of the protective extracellular polymeric substance (EPS).

Question 67

A 6-year-old child with sickle cell disease presents with bilateral tibial pain and fever. Ultrasound shows no subperiosteal fluid collections. Blood cultures are pending. Which of the following imaging modalities is most specific for differentiating acute osteomyelitis from bone infarction in this patient?

Explanation

Distinguishing osteomyelitis from bone infarction in sickle cell patients is challenging. A combination of a radiolabeled WBC scan and a bone marrow scan provides the highest specificity; osteomyelitis shows increased WBC uptake with discordant marrow uptake, whereas infarction shows decreased uptake on both.

Question 68

A sexually active 21-year-old female presents with migratory polyarthralgia, tenosynovitis of the wrist, and sparse painless pustular skin lesions. Joint aspiration of her knee yields a WBC count of 45,000 cells/mm3. Gram stain is negative. What is the most common underlying host defect associated with disseminated infection by this organism?

Explanation

The presentation is classic for disseminated gonococcal infection (DGI). Patients with a deficiency in the terminal complement cascade (C5-C9), which forms the membrane attack complex, are at significantly increased risk for recurrent Neisserial infections.

Question 69

A 9-year-old boy from Connecticut presents with a massive, minimally painful effusion of his right knee. He denies recent trauma or fever. He had a transient rash on his thigh six months ago. Which of the following is the standard two-tiered testing protocol to confirm the diagnosis?

Explanation

The patient has Lyme arthritis caused by Borrelia burgdorferi. The CDC recommends a two-tiered testing protocol: an initial sensitive enzyme immunoassay (EIA/ELISA) followed by a specific Western blot if the first test is positive or equivocal.

Question 70

A 25-year-old construction worker stepped on a rusty nail that penetrated deeply through the rubber sole of his work boot into his foot. Seven days later, he develops signs of osteomyelitis of the calcaneus. In addition to S. aureus, which organism must be covered by empiric antibiotics?

Explanation

Deep puncture wounds through rubber-soled footwear are classically associated with Pseudomonas aeruginosa osteomyelitis. The rubber sole provides a warm, moist environment where the bacteria thrive.

Question 71

Which of the following complications is significantly more common in pediatric osteomyelitis caused by community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) compared to methicillin-sensitive Staphylococcus aureus (MSSA)?

Explanation

CA-MRSA osteomyelitis in children has a known strong association with adjacent deep venous thrombosis (DVT) and septic emboli compared to MSSA. This hypercoagulability requires clinical vigilance and often necessitates anticoagulation.

Question 72

In acute hematogenous osteomyelitis in a young child, what anatomical feature of the metaphyseal blood vessels strongly predisposes the metaphysis to bacterial seeding?

Explanation

The metaphyseal venous sinusoids feature sluggish, turbulent blood flow and a local deficiency in phagocytic cells. This creates an ideal microenvironment for bloodborne bacteria to deposit and proliferate.

Question 73

A 6-year-old child presents with a 4-day history of right thigh pain and high fever. Plain radiographs are unremarkable. An MRI is obtained.

Blood cultures are pending. Which of the following strictly dictates the need for urgent surgical debridement in this patient's condition?

Explanation

Urgent surgical debridement in acute pediatric osteomyelitis is primarily indicated if there is a drainable fluid collection, such as a large subperiosteal abscess, or if the patient fails to improve after 48-72 hours of IV antibiotics.

Question 74

A 4-year-old boy presents with a limp and right hip pain. He is afebrile (37.2°C) but refuses to bear weight on the right leg. Laboratory tests show a WBC of 10,500/mm3 and an ESR of 22 mm/hr. What is the approximate probability of septic arthritis according to the Kocher criteria?

Explanation

The Kocher criteria for septic arthritis of the pediatric hip include fever > 38.5C, inability to bear weight, ESR > 40, and WBC > 12,000. Having only one positive criterion (non-weight bearing) correlates with a roughly 3% probability.

Question 75

An 8-year-old girl with homozygous sickle cell disease presents with fever and severe tibial pain. Blood cultures are drawn. Empiric antibiotic therapy should ideally include robust coverage for which two most common organisms in this specific population?

Explanation

While Staphylococcus aureus remains the most common overall cause of osteomyelitis in sickle cell disease, Salmonella species are disproportionately elevated in this population. Empiric therapy with a third-generation cephalosporin and an anti-staphylococcal agent is standard.

Question 76

A 14-month-old child presents with acute onset of a limp and a swollen left knee. Arthrocentesis yields purulent fluid with 65,000 WBCs, but routine Gram stain and standard cultures are negative at 48 hours. Which of the following fastidious organisms is the most likely culprit?

Explanation

Kingella kingae is a fastidious Gram-negative organism that is now a leading cause of septic arthritis and osteomyelitis in children aged 6 months to 4 years. It is notoriously difficult to culture and often requires inoculation into BACTEC blood culture bottles.

Question 77

A 10-year-old girl presents with recurrent episodes of multifocal bone pain. Radiographs reveal lytic lesions with sclerotic borders in the clavicle and distal tibia. Biopsy shows sterile, non-specific inflammation. Which of the following is the most appropriate initial treatment?

Explanation

Chronic recurrent multifocal osteomyelitis (CRMO) is an autoinflammatory, non-infectious bone disease. The first-line treatment is NSAIDs, which provide significant symptomatic relief and can induce remission in most patients.

Question 78

A 3-year-old boy presents with refusal to walk and crying when placed in a sitting position. He is afebrile. Examination shows loss of lumbar lordosis but no focal neurologic deficits. Radiographs of the spine are normal. What is the most appropriate next step in diagnostic imaging?

Explanation

The clinical presentation is classic for pediatric discitis. MRI is the most sensitive and specific modality for early detection, demonstrating narrowing of the disc space and signal changes in the adjacent vertebral endplates.

Question 79

A 2-week-old neonate in the NICU is noted to have a swollen, immobile right shoulder. Ultrasound confirms an effusion, and aspiration is planned. In addition to Staphylococcus aureus, which organism is a highly prominent cause of septic arthritis specifically in this age group?

Explanation

Group B Streptococcus (Streptococcus agalactiae) and Gram-negative bacilli (such as E. coli) are major causes of neonatal septic arthritis and osteomyelitis, heavily influencing the choice of empiric antibiotics in neonates.

Question 80

What is the primary pathophysiological mechanism leading to the development of avascular necrosis (AVN) of the femoral head following neglected septic arthritis of the infant hip?

Explanation

The infant hip capsule is thick and strong; as purulent fluid rapidly accumulates, intra-articular pressure rises drastically. This pressure tamponades the extraosseous retinacular blood vessels, leading to ischemia and subsequent AVN.

Question 81

In a 7-year-old child being treated for acute hematogenous osteomyelitis, which inflammatory marker is most useful for evaluating the response to antibiotic therapy and guiding the transition from intravenous to oral medications?

Explanation

CRP peaks within 48 hours of infection onset and drops rapidly with effective treatment, making it the most reliable dynamic marker for monitoring therapeutic response. ESR can remain elevated for weeks despite adequate treatment.

Question 82

A 7-year-old boy presents with a massive knee effusion and a slight limp, but he retains full range of motion with only minimal pain. He is afebrile. Which of the following findings most strongly differentiates Lyme arthritis from pyogenic septic arthritis?

Explanation

Lyme arthritis classically presents with a large joint effusion but remarkably little pain on active or passive range of motion compared to the extreme pain seen in acute pyogenic septic arthritis.

Question 83

A 12-year-old steps on a rusty nail that pierces his rubber-soled sneaker, subsequently developing osteomyelitis in the foot. Following surgical debridement, what is the most appropriate empiric oral antibiotic choice for the most likely organism?

Explanation

Puncture wounds occurring through rubber-soled shoes are strongly associated with Pseudomonas aeruginosa osteomyelitis. Ciprofloxacin provides excellent oral anti-pseudomonal coverage and bone penetration.

Question 84

A 16-year-old male presents with dull, aching pain in his distal tibia for several months, typically worse at night. Radiographs show a well-circumscribed, radiolucent lesion with a thick sclerotic margin in the metaphysis. What is the most likely diagnosis?

Explanation

A Brodie's abscess is a localized form of subacute or chronic osteomyelitis. It classically presents as a purely lytic metaphyseal lesion with a thick sclerotic rim on radiographs, most commonly affecting the tibia.

Question 85

When performing an ultrasound-guided anterior aspiration of the hip joint in a child for suspected septic arthritis, which important neurovascular structure lies immediately medial to the optimal needle trajectory?

Explanation

The standard anterior approach to the hip joint aims lateral to the femoral neurovascular bundle. The femoral artery lies directly medial to the correct needle tract and should be protected during insertion.

Question 86

A 5-year-old boy is brought to the ER with rapid onset of severe leg pain, fever, and lethargy following a minor skin abrasion. The skin is tense, erythematous, and exquisitely tender beyond the visible margins of redness. What is the most critical initial step in management?

Explanation

This presentation (pain out of proportion to exam, rapid progression, systemic toxicity) is highly suspicious for necrotizing fasciitis. Urgent surgical exploration is definitive for both diagnosis and lifesaving treatment; imaging should not delay surgery.

Question 87

A sexually active 17-year-old female presents with migratory polyarthralgias, pain along the flexor tendons of the wrist, and a few discrete pustular skin lesions on her extremities. She is afebrile. What is the most likely causative organism?

Explanation

Disseminated gonococcal infection (DGI) classically presents in sexually active adolescents with a triad of tenosynovitis, dermatitis (pustular lesions), and migratory polyarthralgia.

Orthopedic Prometric MCQs - Chapter 3 Part 1

Orthopedic Prometric MCQs - Chapter 3 Part 19

Orthopedic Prometric MCQs - Chapter 3 Part 19

Chapter Index