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1. General Principles & Basic Science MCQs

Practice Set 773 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 15441

Topic: Infection, Pharmacology & VTE

In the setting of a chronic periprosthetic joint infection, organisms within a biofilm are highly resistant to systemic antibiotics. Which of the following best describes the metabolic state of bacteria in the deepest layers of a mature biofilm?

. Rapidly dividing
. Planktonic
. Senescent and metabolically dormant
. Actively secreting matrix metalloproteinases
. Highly susceptible to beta-lactams

Correct Answer & Explanation

. Senescent and metabolically dormant

Explanation

Bacteria deep within a mature biofilm exist in a stationary, metabolically dormant phase, which makes them highly resistant to antibiotics that target cell wall synthesis or active division. This necessitates surgical debridement or mechanical removal of the biofilm for eradication.

Question 15442

Topic: Infection, Pharmacology & VTE

A 45-year-old immigrant presents with chronic back pain, night sweats, and progressive lower extremity weakness. MRI demonstrates destruction of two adjacent thoracic vertebral bodies with disc space preservation and a large paraspinal abscess. What is the most appropriate initial management?

. Immediate anterior corpectomy and fusion
. CT-guided biopsy and initiation of multi-drug antitubercular therapy
. Empiric intravenous vancomycin and ceftriaxone
. Posterior spinal decompression and fusion
. Oral rifampin monotherapy

Correct Answer & Explanation

. CT-guided biopsy and initiation of multi-drug antitubercular therapy

Explanation

Spinal tuberculosis (Pott's disease) typically presents with anterior vertebral body destruction, relative disc preservation early on, and a cold abscess. Diagnosis via biopsy and medical management with multi-drug therapy is the primary treatment, reserving surgery for severe or progressive neurologic deficit or gross instability.

Question 15443

Topic: Biology, Genetics & Bone Healing

A 6-year-old girl with multiple prior fractures, blue sclerae, and dentinogenesis imperfecta is scheduled for rodding of bilateral femurs. Which of the following defects is the primary cause of her underlying condition?

. Defective osteoclast chloride channels
. Defective synthesis of Type II collagen
. Quantitative or qualitative defect in Type I collagen
. Impaired endochondral ossification
. Defective mineralization of osteoid

Correct Answer & Explanation

. Quantitative or qualitative defect in Type I collagen

Explanation

Osteogenesis imperfecta is predominantly an autosomal dominant disorder caused by mutations in the COL1A1 or COL1A2 genes, leading to qualitative or quantitative defects in Type I collagen. This results in brittle bones, blue sclerae, and dentinogenesis imperfecta.

Question 15444

Topic: Infection, Pharmacology & VTE
A 2-year-old girl presents with knee swelling and refusal to bear weight. Laboratory tests show an ESR of 45 and CRP of 3.2. Joint aspiration yields purulent fluid, but standard Gram stain and culture are negative after 48 hours. What is the most likely fastidious organism?
. Staphylococcus aureus
. Haemophilus influenzae
. Kingella kingae
. Streptococcus pneumoniae
. Neisseria gonorrhoeae

Correct Answer & Explanation

. Kingella kingae

Explanation

Kingella kingae is a fastidious Gram-negative organism and a leading cause of septic arthritis in children under 4 years of age. It often requires PCR or inoculation into BACTEC blood culture bottles for successful identification.

Question 15445

Topic: 1. General Principles & Basic Science
A 55-year-old diabetic male presents with rapidly spreading leg erythema, severe pain out of proportion to exam, and hemorrhagic bullae. Laboratory values include a CRP of 200, WBC of 22, and Na of 130. What is the most critical next step in management?
. MRI with IV contrast to evaluate deep spaces
. Broad-spectrum IV antibiotics and observation
. Urgent surgical exploration and radical debridement
. Hyperbaric oxygen therapy
. Aspiration of the bullae for Gram stain

Correct Answer & Explanation

. Urgent surgical exploration and radical debridement

Explanation

This patient exhibits classic signs of necrotizing fasciitis (pain out of proportion, hemorrhagic bullae, high LRINEC score parameters). The definitive and life-saving intervention is immediate, aggressive surgical debridement; delaying for advanced imaging is contraindicated.

Question 15446

Topic: Biology, Genetics & Bone Healing

A 12-year-old boy presents with a prominent forehead, delayed tooth eruption, and an unusually wide range of shoulder motion, easily approximating his shoulders anteriorly. A mutation in which transcription factor is responsible for this condition?

. SOX9
. RUNX2 (CBFA1)
. COMP
. FGFR3
. GLI3

Correct Answer & Explanation

. RUNX2 (CBFA1)

Explanation

Cleidocranial dysplasia is an autosomal dominant condition caused by a mutation in the RUNX2 (CBFA1) gene, which is essential for osteoblast differentiation. Key features include hypoplastic or absent clavicles, delayed closure of cranial sutures, and supernumerary teeth.

Question 15447

Topic: Biology, Genetics & Bone Healing
A 10-month-old infant exclusively fed boiled cow's milk presents with irritability, bleeding gums, and a pseudoparalysis of the lower extremities. Radiographs show a dense zone of provisional calcification and a lucent metaphyseal band. This condition is caused by a deficiency that directly impairs which biochemical process?
. Cross-linking of elastin
. Hydroxylation of proline and lysine residues in collagen
. Cleavage of procollagen C-terminals
. Mineralization of osteoid matrix
. Osteoclast attachment to the bone surface

Correct Answer & Explanation

. Hydroxylation of proline and lysine residues in collagen

Explanation

Scurvy is caused by Vitamin C deficiency, leading to impaired hydroxylation of proline and lysine residues, which is necessary for stable collagen triple helix formation. Radiographic hallmarks include the white line of Frankel (dense calcification) and the Trümmerfeld zone (lucent band).

Question 15448

Topic: Biology, Genetics & Bone Healing

A 65-year-old man presents with progressive bowing of his right femur and increasing hat size. Laboratory tests reveal an isolated, significantly elevated serum alkaline phosphatase. Histological examination of the affected bone is most likely to show:

. Defective osteoid mineralization with widened osteoid seams
. A mosaic pattern of lamellar bone with prominent cement lines
. Thin cortical trabeculae with normal mineralization ratios
. Multinucleated giant cells in a background of spindle cells
. Subperiosteal bone resorption with dissecting osteitis

Correct Answer & Explanation

. A mosaic pattern of lamellar bone with prominent cement lines

Explanation

The patient's presentation and isolated elevated alkaline phosphatase are classic for Paget's disease of bone. Histology demonstrates a characteristic mosaic pattern of lamellar bone with prominent, haphazard cement lines due to unregulated cycles of bone resorption and formation.

Question 15449

Topic: Infection, Pharmacology & VTE

A 72-year-old woman is diagnosed with an acute hematogenous staphylococcal periprosthetic joint infection of her total knee arthroplasty. She undergoes irrigation and debridement with modular exchange. Which of the following antibiotic properties makes it essential to include as part of her post-operative oral suppressive regimen?

. Inhibition of cell wall synthesis in the planktonic phase
. Irreversible binding to the 30S ribosomal subunit
. Penetration into biofilms and bactericidal activity against stationary-phase organisms
. Disruption of the bacterial cell membrane through depolarization
. Inhibition of DNA gyrase preventing replication

Correct Answer & Explanation

. Penetration into biofilms and bactericidal activity against stationary-phase organisms

Explanation

Rifampin is crucial in the management of retained hardware staphylococcal infections. It is highly lipid-soluble, allowing it to penetrate biofilms and kill slow-growing, stationary-phase bacteria.

Question 15450

Topic: Biology, Genetics & Bone Healing

A 55-year-old patient with end-stage renal disease presents with diffuse bone pain. Radiographs demonstrate subperiosteal resorption of the phalanges and a "rugger jersey" spine. The patient's condition is primarily driven by which of the following initial metabolic derangements?

. Decreased serum phosphate and elevated active vitamin D
. Elevated serum phosphate and decreased active vitamin D
. Elevated serum calcium and decreased active vitamin D
. Decreased serum PTH and elevated serum calcium
. Autoantibody formation against calcium-sensing receptors

Correct Answer & Explanation

. Elevated serum phosphate and decreased active vitamin D

Explanation

Renal osteodystrophy is driven by failing kidneys that retain phosphate and fail to convert 25(OH)D to active 1,25(OH)2D. The resulting hyperphosphatemia and hypocalcemia stimulate secondary hyperparathyroidism, leading to severe bone resorption.

Question 15451

Topic: Infection, Pharmacology & VTE

A 14-month-old child presents with a limp and refusal to bear weight. Laboratory work reveals a normal WBC count and mildly elevated CRP. Joint aspiration yields synovial fluid with 65,000 WBCs/mm3. Routine cultures are negative at 48 hours, but PCR is positive. Which of the following is the most likely pathogen?

. Staphylococcus aureus
. Salmonella typhimurium
. Kingella kingae
. Haemophilus influenzae
. Pseudomonas aeruginosa

Correct Answer & Explanation

. Kingella kingae

Explanation

Kingella kingae is a fastidious, Gram-negative organism that is now recognized as a leading cause of septic arthritis and osteomyelitis in children aged 6 to 36 months. It often fails to grow on routine culture media and is best detected using PCR.

Question 15452

Topic: Biology, Genetics & Bone Healing

A 4-year-old boy with recurrent fractures, blue sclerae, and dentinogenesis imperfecta is treated with intravenous pamidronate. The primary mechanism of action of this medication in this patient's disease is:

. Stimulation of osteoblastic collagen synthesis
. Inhibition of farnesyl pyrophosphate synthase in osteoclasts
. Binding to RANKL to prevent osteoclast activation
. Direct enhancement of intestinal calcium absorption
. Substitution of defective type 1 collagen fibrils

Correct Answer & Explanation

. Inhibition of farnesyl pyrophosphate synthase in osteoclasts

Explanation

Pamidronate is a nitrogen-containing bisphosphonate used to reduce fracture burden in Osteogenesis Imperfecta. It works by inhibiting farnesyl pyrophosphate synthase, which disrupts osteoclast function and promotes osteoclast apoptosis.

Question 15453

Topic: Biology, Genetics & Bone Healing

A 3-year-old boy presents with progressive varus bowing of the lower extremities. Labs show normal serum calcium, low serum phosphate, normal parathyroid hormone, and normal 25-hydroxyvitamin D. 1,25-dihydroxyvitamin D is inappropriately normal-low. Which of the following is the most likely mediator of this condition?

. Deficient dietary calcium intake
. Excess circulating fibroblast growth factor 23 (FGF23)
. Autoimmune destruction of parathyroid glands
. Impaired 1-alpha-hydroxylase activity in the kidney
. Defective intestinal vitamin D receptors

Correct Answer & Explanation

. Excess circulating fibroblast growth factor 23 (FGF23)

Explanation

These findings are classic for X-linked hypophosphatemic rickets, caused by a PHEX gene mutation. This results in excess FGF23, which profoundly inhibits renal phosphate reabsorption and 1-alpha-hydroxylase activity, leading to isolated hypophosphatemia.

Question 15454

Topic: 1. General Principles & Basic Science
A 60-year-old man with uncontrolled diabetes presents with severe leg pain, erythema, and swelling out of proportion to physical findings. Which of the following laboratory findings is a component of the Laboratory Risk Indicator for Necrotizing Fasciitis (LRINEC) score?
. Hyperkalemia
. Hyponatremia
. Hypocalcemia
. Thrombocytopenia
. Elevated serum albumin

Correct Answer & Explanation

. Hyponatremia

Explanation

The LRINEC score includes CRP, WBC count, hemoglobin, serum sodium (hyponatremia < 135 mmol/L scores points), serum creatinine, and serum glucose. Hyponatremia is a classic indicator of severe systemic toxicity in necrotizing soft tissue infections.

Question 15455

Topic: Biology, Genetics & Bone Healing

A 12-year-old boy presents to the orthopedic clinic with a remarkably broad forehead and the ability to approximate his shoulders in the anterior midline. Radiographs reveal delayed ossification of the cranial sutures and complete absence of the clavicles. The mutated gene responsible for this condition is a critical transcription factor for:

. Chondrocyte hypertrophy
. Osteoblast differentiation
. Osteoclast activation
. Type II collagen synthesis
. Sulfation of proteoglycans

Correct Answer & Explanation

. Osteoblast differentiation

Explanation

Cleidocranial dysplasia is caused by a mutation in RUNX2 (CBFA1). RUNX2 is a master transcription factor essential for the differentiation of mesenchymal stem cells into functional osteoblasts.

Question 15456

Topic: Biology, Genetics & Bone Healing

A young child presents with frequent fractures, hepatosplenomegaly, and pancytopenia. Radiographs reveal generalized, densely sclerotic bones with an "Erlenmeyer flask" deformity of the distal femora. A defect in which of the following enzymes is classically associated with the autosomal recessive form of this disease?

. Alkaline phosphatase
. Cathepsin K
. Tartrate-resistant acid phosphatase
. Carbonic anhydrase II
. Lysyl hydroxylase

Correct Answer & Explanation

. Carbonic anhydrase II

Explanation

Malignant infantile osteopetrosis is an autosomal recessive disorder characterized by non-functional osteoclasts. It is frequently caused by mutations affecting carbonic anhydrase II (or the TCIRG1 gene), preventing osteoclasts from generating the acidic environment necessary for bone resorption.

Question 15457

Topic: Infection, Pharmacology & VTE

A 22-year-old healthy female presents with acute pain and swelling in her right knee, along with a papulovesicular rash on her trunk and tenosynovitis of her wrists. Synovial fluid aspiration of the knee yields a white blood cell count of 45,000 cells/mm3. Gram stain is negative. Which of the following is the most appropriate initial management step?

. Immediate open arthrotomy and debridement of the knee
. Intravenous vancomycin and observation
. Intravenous ceftriaxone and azithromycin
. Intra-articular corticosteroid injection
. Oral doxycycline monotherapy for 7 days

Correct Answer & Explanation

. Intravenous ceftriaxone and azithromycin

Explanation

This is a classic presentation of disseminated gonococcal infection. Treatment is primarily medical with IV ceftriaxone (plus azithromycin for potential chlamydial coinfection) and serial aspirations; surgical drainage is usually reserved for refractory cases.

Question 15458

Topic: Biology, Genetics & Bone Healing

A 4-year-old boy presents with severe genu varum and short stature. Lab evaluation reveals normal serum calcium, low serum phosphate, and elevated alkaline phosphatase. Genetic testing shows a PHEX mutation. Which medical therapy is indicated?

. Oral calcium and Vitamin D3
. Bisphosphonates
. Calcitriol and oral phosphate
. Cinacalcet
. Recombinant PTH

Correct Answer & Explanation

. Calcitriol and oral phosphate

Explanation

X-linked hypophosphatemic rickets is caused by a PHEX mutation leading to elevated FGF23, renal phosphate wasting, and impaired vitamin D activation. Standard treatment includes active vitamin D (calcitriol) and oral phosphate supplementation.

Question 15459

Topic: Biology, Genetics & Bone Healing

A neonate presents with multiple fractures, blue sclerae, and dentinogenesis imperfecta. A defect in which of the following is the primary cause of this condition?

. Type II collagen
. Type I collagen
. Fibroblast growth factor receptor 3
. Runx2/Cbfa1 transcription factor
. Cartilage oligomeric matrix protein (COMP)

Correct Answer & Explanation

. Type I collagen

Explanation

Osteogenesis imperfecta is primarily caused by mutations in the COL1A1 or COL1A2 genes, which lead to qualitative or quantitative defects in Type I collagen. This results in bone fragility, blue sclerae, and dentinogenesis imperfecta.

Question 15460

Topic: Biology, Genetics & Bone Healing

A 9-year-old patient exhibits delayed closure of cranial sutures, absent clavicles, and delayed eruption of secondary teeth. The gene responsible for this condition plays a crucial role in which cellular process?

. Osteoclast differentiation
. Osteoblast differentiation
. Chondrocyte hypertrophy
. Collagen cross-linking
. Mineralization of osteoid

Correct Answer & Explanation

. Osteoblast differentiation

Explanation

Cleidocranial dysplasia is caused by a mutation in the RUNX2 (CBFA1) gene, which is an essential transcription factor for osteoblast differentiation. This defect severely impacts membranous bone formation, leading to absent clavicles and delayed cranial suture closure.

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