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1. General Principles & Basic Science MCQs

Practice Set 732 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 14621

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 14622

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 14623

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 14624

Topic: Surgical Anatomy & Approaches

A 6-year-old boy falls from a swing and sustains an extension-type supracondylar humerus fracture. Radiographs show that the distal fracture fragment is displaced posterolaterally. Based on this specific displacement pattern, which nerve is at the highest risk of injury from the proximal fracture fragment?

. Anterior interosseous nerve
. Radial nerve
. Ulnar nerve
. Musculocutaneous nerve
. Axillary nerve

Correct Answer & Explanation

. Anterior interosseous nerve

Explanation

Correct Answer: Anterior interosseous nerveIn extension-type supracondylar humerus fractures, the direction of distal fragment displacement dictates which neurovascular structures are at risk from the sharp proximal fragment. When the distal fragment displaces posterolaterally, the proximal fragment is directed anteromedially. This anteromedial spike places the median nerve (specifically its anterior interosseous branch) and the brachial artery at the highest risk of injury. Conversely, if the distal fragment displaces posteromedially, the proximal fragment is directed anterolaterally, placing the radial nerve at risk. The anterior interosseous nerve (AIN) is the most commonly injured nerve overall in pediatric supracondylar humerus fractures.

Question 14625

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.

Question 14626

Topic: Infection, Pharmacology & VTE

A 9-year-old African American boy with homozygous sickle cell disease (HbSS) presents with a 4-day history of fever, chills, and severe pain in his right thigh. Blood cultures and a subperiosteal aspirate grow a non-lactose fermenting, Gram-negative motile bacillus that produces hydrogen sulfide. Which of the following pathophysiological mechanisms best explains this patient's unique susceptibility to this specific organism?

. Impaired splenic macrophage function and reticuloendothelial blockade
. Deficiency in the terminal complement cascade (C5-C9)
. Defective neutrophil oxidative burst
. Absence of secretory IgA in the gastrointestinal tract
. T-cell lymphopenia due to chronic hemolysis

Correct Answer & Explanation

. Impaired splenic macrophage function and reticuloendothelial blockade

Explanation

Correct Answer: A (Impaired splenic macrophage function and reticuloendothelial blockade)The organism described isSalmonellaspecies, which is uniquely a common cause of osteomyelitis in patients with sickle cell disease (thoughS. aureusremains the most common overall). Patients with sickle cell disease experience recurrent vaso-occlusive crises, leading to microinfarctions of the spleen and eventual autosplenectomy by early childhood. This loss of splenic function severely impairs the reticuloendothelial system's ability to clear encapsulated organisms and intravascular bacteria. Furthermore, microinfarctions in the bowel wall allowSalmonellato translocate into the bloodstream, where the defective splenic macrophages fail to clear the bacteremia, allowing the bacteria to seed areas of ischemic bone.

Question 14627

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of fracture healing, which type of tissue initially forms in a fracture gap that is subjected to a high strain environment (10% to 100% strain)?

. Woven bone
. Lamellar bone
. Hyaline cartilage
. Granulation tissue
. Fibrocartilage

Correct Answer & Explanation

. Granulation tissue

Explanation

Perren's strain theory posits that the type of tissue formed in a fracture gap depends on the local strain. Granulation tissue can tolerate up to 100% strain, whereas bone can only tolerate very low strain (<2%).

Question 14628

Topic: Surgical Anatomy & Approaches

A surgeon performs a total hip arthroplasty utilizing the direct anterior approach (Smith-Petersen interval). This surgical approach exploits a true internervous plane between muscles innervated by which two nerves?

. Femoral nerve and superior gluteal nerve
. Femoral nerve and obturator nerve
. Superior gluteal nerve and inferior gluteal nerve
. Sciatic nerve and inferior gluteal nerve
. Obturator nerve and sciatic nerve

Correct Answer & Explanation

. Femoral nerve and superior gluteal nerve

Explanation

The direct anterior approach uses the internervous plane between the tensor fasciae latae (superior gluteal nerve) and the sartorius/rectus femoris (femoral nerve). This allows exposure of the hip joint without denervating the abductor musculature.

Question 14629

Topic: Biology, Genetics & Bone Healing

Which of the following Bone Morphogenetic Proteins (BMPs) is FDA-approved as an alternative to autograft for acute open tibial shaft fractures, and utilizes the intracellular SMAD signaling pathway to promote osteoinduction?

. BMP-2
. BMP-7
. BMP-3
. TGF-beta 1
. IGF-1

Correct Answer & Explanation

. BMP-2

Explanation

Recombinant human BMP-2 (rhBMP-2) is FDA-approved for acute open tibial shaft fractures and anterior lumbar interbody fusions. BMPs belong to the TGF-beta superfamily and primarily mediate osteoinduction via the intracellular SMAD pathway.

Question 14630

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory, what is the maximum amount of interfragmentary strain that will permit primary bone healing through cutting cones without the formation of a fracture callus?

. Less than 2%
. Between 2% and 10%
. Between 10% and 30%
. Between 30% and 50%
. Greater than 50%

Correct Answer & Explanation

. Less than 2%

Explanation

Perren's strain theory dictates that primary bone healing (direct osteonal remodeling via cutting cones without callus formation) requires absolute stability. This corresponds to an interfragmentary strain of less than 2%, a biomechanical environment typically achieved with rigid compression plating.

Question 14631

Topic: 1. General Principles & Basic Science

A 22-year-old rugby player sustained a hyperextension injury to his actively flexing right ring finger while grabbing an opponent's jersey. He is unable to actively flex the distal interphalangeal (DIP) joint. Ultrasound reveals the avulsed flexor digitorum profundus (FDP) tendon is retracted into the palm. According to the Leddy-Packer classification, what is the recommended timeframe for surgical repair to prevent tendon necrosis?

. Within 24 hours
. Within 7 to 10 days
. Within 3 to 4 weeks
. Within 6 to 8 weeks
. Delayed reconstruction with a tendon graft at 3 months

Correct Answer & Explanation

. Within 7 to 10 days

Explanation

This is a Leddy-Packer Type I FDP avulsion, where the tendon retracts into the palm, disrupting both the long and short vincula. Surgical repair must be performed within 7 to 10 days to prevent permanent ischemic necrosis and severe tendon contracture.

Question 14632

Topic: Biology, Genetics & Bone Healing

Denosumab is increasingly used for unresectable or recurrent Giant Cell Tumor of bone (GCTB). What is its specific mechanism of action in this disease?

. Inhibits RANKL, directly inducing apoptosis of the neoplastic stromal cells
. Inhibits RANKL, preventing the recruitment and activation of osteoclast-like giant cells
. Directly binds to and inhibits osteoclast-like giant cells via the calcitonin receptor
. Promotes osteoblast differentiation via Wnt signaling
. Inhibits VEGF, reducing tumor angiogenesis

Correct Answer & Explanation

. Inhibits RANKL, preventing the recruitment and activation of osteoclast-like giant cells

Explanation

Denosumab is a monoclonal antibody that binds to RANKL, which is heavily overexpressed by the neoplastic stromal cells in GCTB. By neutralizing RANKL, it prevents the activation of RANK on the multinucleated osteoclast-like giant cells, halting bone destruction.

Question 14633

Topic: Biology, Genetics & Bone Healing

A patient presents with absent clavicles, delayed closure of cranial sutures, and supernumerary teeth. Which transcription factor gene is mutated in this autosomal dominant condition?

. SOX9
. RUNX2 (CBFA1)
. HOXD13
. LMX1B
. GLI3

Correct Answer & Explanation

. RUNX2 (CBFA1)

Explanation

The clinical presentation is classic for Cleidocranial Dysplasia. This condition is caused by a mutation in the RUNX2 (formerly CBFA1) gene, a master transcription factor essential for osteoblast differentiation and intramembranous bone formation.

Question 14634

Topic: Biology, Genetics & Bone Healing

A 2-year-old child presents with short-limbed dwarfism, hitchhiker thumbs, bilateral rigid clubfeet, and cystic swelling of the pinnae. What is the underlying genetic defect responsible for this condition?

. Type I collagen mutation
. Fibroblast growth factor receptor 3 mutation
. Sulfate transport protein mutation
. Cartilage oligomeric matrix protein mutation
. Runx2/Cbfa1 mutation

Correct Answer & Explanation

. Sulfate transport protein mutation

Explanation

Diastrophic dysplasia is characterized by hitchhiker thumbs, cauliflower ears, and severe clubfeet. It is caused by a mutation in the SLC26A2 gene, which encodes a sulfate transport protein, leading to undersulfated proteoglycans in the cartilage.

Question 14635

Topic: Biology, Genetics & Bone Healing

A 4-year-old child with a history of recurrent fractures, blue sclerae, and dentinogenesis imperfecta is started on intravenous pamidronate. What is the primary mechanism of action of this medication in treating this patient's underlying condition?

. Stimulation of osteoblast differentiation and matrix synthesis
. Inhibition of osteoclast-mediated bone resorption
. Direct cross-linking of abnormal type 1 collagen fibrils
. Upregulation of FGFR3 expression in the physis
. Enhancement of calcium and phosphate absorption in the gut

Correct Answer & Explanation

. Inhibition of osteoclast-mediated bone resorption

Explanation

Osteogenesis imperfecta is typically caused by mutations affecting Type 1 collagen. Bisphosphonates like pamidronate increase bone mineral density and reduce fracture rates by inhibiting osteoclast-mediated bone resorption, which preserves the existing abnormal bone matrix.

Question 14636

Topic: Biology, Genetics & Bone Healing

Denosumab is highly effective in the management of unresectable Giant Cell Tumors of bone. What is the exact mechanism of action of this pharmacological agent?

. It binds to RANKL, preventing activation of the RANK receptor on osteoclasts
. It binds to the RANK receptor on osteoblasts, stimulating apoptosis
. It directly inhibits osteoclast proton pumps (V-ATPase)
. It inhibits farnesyl pyrophosphate synthase, disrupting osteoclast cytoskeletal function
. It blocks the basic fibroblast growth factor (bFGF) signaling pathway

Correct Answer & Explanation

. It binds to RANKL, preventing activation of the RANK receptor on osteoclasts

Explanation

Denosumab is a human monoclonal antibody that binds to Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL), which is expressed by the neoplastic stromal cells in Giant Cell Tumor. By neutralizing RANKL, it prevents the recruitment and activation of osteoclast-like giant cells.

Question 14637

Topic: Biology, Genetics & Bone Healing
A 6-year-old girl with multiple recurrent long bone fractures, blue sclerae, and dentinogenesis imperfecta is diagnosed with osteogenesis imperfecta (Type III). Her orthopedic surgeon recommends the initiation of intravenous pamidronate. What is the primary mechanism by which this medication improves bone density in this patient?
. Stimulation of osteoblast proliferation
. Direct enhancement of Type 1 collagen cross-linking
. Inhibition of osteoclast-mediated bone resorption
. Promotion of endochondral ossification at the physis
. Upregulation of runx2 gene expression

Correct Answer & Explanation

. Inhibition of osteoclast-mediated bone resorption

Explanation

Pamidronate is a bisphosphonate, which works by inhibiting osteoclast-mediated bone resorption and inducing osteoclast apoptosis. This increases bone mineral density, reduces the incidence of fractures, and improves mobility in children with moderate-to-severe osteogenesis imperfecta.

Question 14638

Topic: Biology, Genetics & Bone Healing

A 32-year-old female presents with an eccentric, lytic lesion in the proximal tibia extending to the subchondral bone. Biopsy confirms a Giant Cell Tumor of Bone (GCTB). Due to the tumor's proximity to the joint, neoadjuvant denosumab is planned to consolidate the lesion before curettage. What is the specific mechanism of action of denosumab?

. It is a monoclonal antibody that binds to RANKL, preventing osteoclast activation.
. It binds directly to the RANK receptor on osteoblasts to stimulate bone formation.
. It inhibits the fusion of EWS-FLI1 transcripts.
. It is a selective estrogen receptor modulator that decreases bone turnover.
. It induces apoptosis directly in the neoplastic mononuclear cells via p53 activation.

Correct Answer & Explanation

. It is a monoclonal antibody that binds to RANKL, preventing osteoclast activation.

Explanation

Denosumab is a fully human monoclonal antibody that binds directly to the Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). This prevents RANKL from binding to the RANK receptor on osteoclasts and their precursors, thereby halting the osteoclast-mediated bone destruction characteristic of Giant Cell Tumors.

Question 14639

Topic: Biology, Genetics & Bone Healing

Which of the following conditions is characterized by progressive metaphyseal widening, bowing of long bones, and 'Erlenmeyer flask' deformity on radiographs, often associated with pancytopenia and hepatosplenomegaly?

. Osteogenesis imperfecta
. Rickets
. Gaucher disease
. Paget's disease of bone
. Fibrous dysplasia

Correct Answer & Explanation

. Gaucher disease

Explanation

Gaucher disease, an autosomal recessive lysosomal storage disorder, is classically associated with the 'Erlenmeyer flask' deformity (undermodeling of the distal femur), progressive metaphyseal widening, bowing of long bones, and osteopenia. Bone crises, avascular necrosis, and pathological fractures are also common. The systemic manifestations include hepatosplenomegaly and pancytopenia due to glucocerebroside accumulation in macrophages. Osteogenesis imperfecta is characterized by fragile bones, blue sclera, and hearing loss. Rickets involves defective mineralization of bone. Paget's disease involves disorganized bone remodeling. Fibrous dysplasia is a localized lesion.

Question 14640

Topic: Biology, Genetics & Bone Healing

Which metabolic bone disorder is characterized by elevated serum alkaline phosphatase, normal serum calcium and phosphate, and disorganized bone remodeling leading to bone pain, deformity, and increased fracture risk?

. Osteoporosis
. Osteomalacia
. Hyperparathyroidism
. Paget's disease of bone
. Rickets

Correct Answer & Explanation

. Paget's disease of bone

Explanation

Paget's disease of bone (osteitis deformans) is characterized by a focal disorder of bone remodeling, with abnormally high osteoclast activity followed by compensatory osteoblast activity, leading to disorganized, enlarged, and weak bone. This results in elevated serum alkaline phosphatase (a marker of osteoblast activity) while serum calcium and phosphate levels typically remain normal. Clinical features include bone pain, deformity (e.g., bowing of long bones), pathological fractures, and neurologic complications due to nerve compression. Osteoporosis involves decreased bone mass but normal bone histology. Osteomalacia and rickets (in children) are due to defective mineralization, often with low phosphate and/or calcium. Hyperparathyroidism involves elevated PTH and altered calcium/phosphate levels.

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