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1. General Principles & Basic Science MCQs

Practice Set 21 of 789

This practice set contains high-yield board review questions covering key concepts in 1. General Principles & Basic Science. Each clinical scenario is designed to test your diagnostic and management skills relevant to this subspecialty.

Question 401

Topic: Infection, Pharmacology & VTE

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:

. Vancomycin
. C lindamycin
. Rifampin
. Tetracycline
. Linezolid

Correct Answer & Explanation

. C lindamycin

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 402

Topic: Biology, Genetics & Bone Healing

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

. Nutritional deficiency of calcium
. Hypophosphatasia
. Vitamin D-dependent rickets (1-alpha-hydroxylase deficiency)
. Fanconi syndrome
. X-linked hypophosphatemic rickets

Correct Answer & Explanation

. X-linked hypophosphatemic rickets

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 403

Topic: Biology, Genetics & Bone Healing

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

. PHEX
. FGF23
. SLC 34A3
. EXT1
. C OL1A1

Correct Answer & Explanation

. PHEX

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 404

Topic: Biology, Genetics & Bone Healing

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

. An affected father and son in the family
. More females relatives affected than male relatives
. A negative family history for the disorder
. An equal number of affected male relatives and females relatives
. Affected siblings but no prior ancestors or relatives

Correct Answer & Explanation

. More females relatives affected than male relatives

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 405

Topic: Biology, Genetics & Bone Healing

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

. Low alkaline phosphatase
. High PO4
. Low calcium
. Low 25-(OH)2D3
. Low 1,25-(OH)2D3

Correct Answer & Explanation

. Low 1,25-(OH)2D3

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 406

Topic: Biology, Genetics & Bone Healing

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:

. Below average stature
. Exaggerated genu varus
. Dental abscesses or caries
. Delayed motor milestones
. Alopecia

Correct Answer & Explanation

. Alopecia

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 407

Topic: Biology, Genetics & Bone Healing
A 23-month-old patient is evaluated for intoeing. Based on his radiograph (Slide), what is the most likely diagnosis:
. Osteogenesis imperfecta
. Tibial torsion
. Physiologic bowing
. Rickets
. Fibrous dysplasia

Correct Answer & Explanation

. Rickets

Explanation

This patient has rickets. Note the diffuse osteopenia, 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 408

Topic: 1. General Principles & Basic Science

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?

. E-test
. D-zone test
. C-reactive protein assay
. Schlichter test
. Polymerase chain reaction (PCR) for mecA

Correct Answer & Explanation

. D-zone test

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 409

Topic: Infection, Pharmacology & VTE
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?
. Fungal cultures
. Acid-fast bacilli smear
. Inoculation of synovial fluid into blood culture vials (BACTEC)
. Synovial biopsy
. Dark-field microscopy

Correct Answer & Explanation

. Inoculation of synovial fluid into blood culture vials (BACTEC)

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 410

Topic: Infection, Pharmacology & VTE

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?

. Immediate surgical debridement of the pelvis
. Intravenous antibiotics
. Emergent arthrotomy of the hip joint
. Spica cast immobilization
. CT-guided core biopsy of the bone

Correct Answer & Explanation

. Intravenous antibiotics

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 411

Topic: Infection, Pharmacology & VTE

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

. Upregulation of efflux pumps
. Overproduction of beta-lactamase
. Alteration of penicillin-binding protein 2a (PBP2a)
. Mutation of DNA gyrase
. Decreased outer membrane permeability

Correct Answer & Explanation

. Alteration of penicillin-binding protein 2a (PBP2a)

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 412

Topic: Infection, Pharmacology & VTE

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?

. Toxic shock syndrome toxin-1 (TSST-1)
. Exfoliative toxin A
. Panton-Valentine leukocidin (PVL)
. Alpha-hemolysin
. Enterotoxin B

Correct Answer & Explanation

. Panton-Valentine leukocidin (PVL)

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 413

Topic: Infection, Pharmacology & VTE

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

. Cefazolin alone
. Cefotaxime and Vancomycin
. Clindamycin and Gentamicin
. Amoxicillin and Clavulanate
. Erythromycin and Rifampin

Correct Answer & Explanation

. Cefotaxime and Vancomycin

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 414

Topic: Infection, Pharmacology & VTE

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?

. Staphylococcus aureus
. Salmonella species
. Streptococcus pneumoniae
. Haemophilus influenzae
. Kingella kingae

Correct Answer & Explanation

. Salmonella species

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 415

Topic: Infection, Pharmacology & VTE

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?

. Less than 5%
. 10 to 20%
. 40 to 50%
. 93 to 99%
. 100%

Correct Answer & Explanation

. 93 to 99%

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 416

Topic: Infection, Pharmacology & VTE

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?

. Escalate to broader-spectrum antibiotics
. Initiate a scheduled trial of nonsteroidal anti-inflammatory drugs (NSAIDs)
. Perform a radical surgical debridement
. Start systemic oral corticosteroids
. Refer for low-dose radiation therapy

Correct Answer & Explanation

. Initiate a scheduled trial of nonsteroidal anti-inflammatory drugs (NSAIDs)

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 417

Topic: 1. General Principles & Basic Science

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?

. Anterior (Smith-Petersen)
. Medial (Ludloff)
. Posterior (Kocher-Langenbeck)
. Lateral (Hardinge)
. Trochanteric osteotomy

Correct Answer & Explanation

. Anterior (Smith-Petersen)

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 418

Topic: Infection, Pharmacology & VTE
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?
. Direct bacterial invasion of the myocardium
. Endotoxin release from Gram-negative bacterial cell walls
. Superantigen cross-linking of MHC class II and T-cell receptors
. Type III immune complex hypersensitivity reaction
. Intravascular coagulation induced directly by PVL toxin

Correct Answer & Explanation

. Superantigen cross-linking of MHC class II and T-cell receptors

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 419

Topic: Infection, Pharmacology & VTE

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)?

. 10 days
. 3 to 4 weeks
. 6 to 8 weeks
. 12 weeks
. 6 months

Correct Answer & Explanation

. 3 to 4 weeks

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 420

Topic: Infection, Pharmacology & VTE

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?

. Trimethoprim-sulfamethoxazole
. Linezolid
. Tetracycline
. Ciprofloxacin
. Cefdinir

Correct Answer & Explanation

. Trimethoprim-sulfamethoxazole

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.

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