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1. General Principles & Basic Science MCQs

Practice Set 553 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 11041

Topic: Biology, Genetics & Bone Healing

A 3-year-old child presents with severe genu varum and short stature. Laboratory tests

reveal normal serum calcium, significantly decreased serum phosphate, normal parathyroid hormone (PTH), and elevated alkaline phosphatase. Genetic testing reveals a PHEX mutation. The underlying pathophysiology involves an excess of which of the following?

. Parathyroid hormone-related peptide (PTHrP)
. Fibroblast growth factor 23 (FGF23)
. 1-alpha-hydroxylase
. Bone morphogenetic protein 2 (BMP-2)
. Osteoprotegerin

Correct Answer & Explanation

. Parathyroid hormone-related peptide (PTHrP)

Explanation

The clinical picture describes X-linked hypophosphatemic rickets (XLHR), the most common heritable form of rickets. It is caused by a mutation in the PHEX gene. The PHEX mutation leads to an inability to degrade FGF23, resulting in high levels of circulating FGF23. FGF23 acts on the kidneys to inhibit phosphate reabsorption (causing phosphaturia and hypophosphatemia) and inhibits 1-alpha-hydroxylase, thereby reducing the synthesis of active Vitamin D (1,25-dihydroxyvitamin D).

Question 11042

Topic: 1. General Principles & Basic Science

A 7-year-old ambulatory patient with spastic diplegic cerebral palsy undergoes 3-D computerized gait analysis. Kinematic data reveals decreased peak knee flexion during the swing phase, causing foot drag. Dynamic electromyography (EMG) demonstrates continuous, inappropriate firing of the rectus femoris throughout the swing phase. Which of the following surgical interventions is most specifically indicated for this gait abnormality?

. Hamstring lengthening
. Distal femoral extension osteotomy
. Rectus femoris transfer to the sartorius or gracilis
. Patellar tendon advancement
. Gastrocnemius recession

Correct Answer & Explanation

. Hamstring lengthening

Explanation

A 'stiff-knee gait' in cerebral palsy is characterized by reduced peak knee flexion during the swing phase, which impairs ground clearance (causing toe drag or circumduction). It is primarily caused by overactivity or spasticity of the rectus femoris muscle during swing. The standard treatment to address this dynamic contracture is a rectus femoris transfer (e.g., to the sartorius, gracilis, or semitendinosus), which converts the rectus from a knee extensor/inhibitor of flexion into a knee flexor, thereby improving swing-phase knee kinematics.

Question 11043

Topic: Infection, Pharmacology & VTE

A 2-week-old neonate presents with decreased spontaneous movement of the right lower extremity, swelling of the right thigh, and pain with passive hip motion. Ultrasound demonstrates a large hip effusion. What anatomical feature unique to infants younger than 18 months predisposes them to concurrent osteomyelitis and septic arthritis in this location?

. An incomplete physical barrier at the articular cartilage
. A highly cellular hypertrophic zone of the physis
. The presence of transphyseal blood vessels connecting the metaphysis and epiphysis
. A thick, non-yielding periosteum surrounding the femoral neck
. The absence of an intra-articular ligamentum teres

Correct Answer & Explanation

. An incomplete physical barrier at the articular cartilage

Explanation

In neonates and infants younger than 12-18 months, there are transphyseal blood vessels that cross the physis, establishing direct communication between the metaphysis and the epiphysis (and by extension, the intra-articular space). This unique vascular anatomy allows an infection that begins as metaphyseal osteomyelitis to easily cross into the joint space, resulting in concomitant septic arthritis, which rapidly destroys the chondroepiphysis and increases the risk of avascular necrosis of the femoral head.

Question 11044

Topic: Infection, Pharmacology & VTE
A 4-year-old boy presents to the emergency department with acute right hip pain, a severe limp, and a refusal to bear weight. His temperature is 38.6°C. Laboratory workup reveals a WBC count of 13,500/mm³ and an ESR of 45 mm/hr. According to the Kocher criteria, what is the approximate statistical probability that this child has septic arthritis of the hip?
. 3%
. 40%
. 71%
. 93%
. 99%

Correct Answer & Explanation

. 99%

Explanation

The Kocher criteria for differentiating septic arthritis from transient synovitis include four predictors: fever >38.5°C, non-weight bearing status, ESR >40 mm/hr, and WBC >12,000/mm³. This patient has all 4 criteria. According to the original study, the probability of septic arthritis is approximately: 1 criterion = 3%, 2 criteria = 40%, 3 criteria = 93%, and 4 criteria = 99%.

Question 11045

Topic: Biology, Genetics & Bone Healing

A 3-year-old boy presents with progressive bowing of the lower extremities, short stature, and a waddling gait. Laboratory studies reveal a normal serum calcium, profoundly low serum phosphate, elevated alkaline phosphatase, and normal parathyroid hormone levels. What is the primary pathophysiology underlying his condition?

. Vitamin D receptor mutation
. 1-alpha-hydroxylase deficiency
. PHEX gene mutation leading to excess FGF23
. FGFR3 gene gain-of-function mutation
. Defect in type II collagen synthesis

Correct Answer & Explanation

. Vitamin D receptor mutation

Explanation

The clinical presentation and laboratory findings (normal Ca, very low PO4, normal PTH) are diagnostic of X-linked hypophosphatemic rickets (familial hypophosphatemic rickets). This is caused by a mutation in the PHEX gene, which leads to an overproduction of Fibroblast Growth Factor 23 (FGF23). FGF23 causes profound renal phosphate wasting and inhibits 1-alpha-hydroxylase.

Question 11046

Topic: 1. General Principles & Basic Science

An 8-year-old Japanese girl presents with a painless, palpable 'clunk' on the lateral side of her left knee during terminal extension. MRI confirms a discoid lateral meniscus. Which specific variant of a discoid meniscus is characterized by an absent posterior meniscotibial attachment, leading to hypermobility and snapping?

. Incomplete variant
. Complete variant
. Wrisberg ligament variant
. Anterior horn hypermobile variant
. Medial discoid variant

Correct Answer & Explanation

. Incomplete variant

Explanation

The Wrisberg ligament variant of a discoid meniscus lacks normal posterior meniscotibial (coronary ligament) attachments. Its only posterior attachment is to the meniscofemoral ligament of Wrisberg. This lack of peripheral stability allows the meniscus to hypermobilize into the joint during extension, causing a mechanical 'clunk' or 'snap' even in the absence of a meniscal tear.

Question 11047

Topic: Infection, Pharmacology & VTE

A 3-year-old presents with a 2-day history of right hip pain, refusal to bear weight, and a temperature of 38.8°C. According to the updated Kocher criteria by Caird et al., which laboratory value was added as a strong independent predictor for septic arthritis of the hip?

. Procalcitonin > 0.5 ng/mL
. C-reactive protein (CRP) > 2.0 mg/dL
. Erythrocyte sedimentation rate (ESR) > 20 mm/hr
. Peripheral WBC count > 10,000 cells/mm3
. Synovial fluid WBC > 25,000 cells/mm3

Correct Answer & Explanation

. Procalcitonin > 0.5 ng/mL

Explanation

Caird et al. modified the original Kocher criteria by adding a C-reactive protein (CRP) level > 2.0 mg/dL. The presence of all five predictors yields a >97% probability of septic arthritis.

Question 11048

Topic: Biology, Genetics & Bone Healing
A 5-year-old boy with Osteogenesis Imperfecta (Type III) is being treated with intravenous bisphosphonates. By what specific mechanism do bisphosphonates decrease the fracture rate in these patients?
. Stimulation of osteoblast proliferation
. Direct cross-linking of Type 1 collagen molecules
. Inhibition of osteoclast function and induction of osteoclast apoptosis
. Enhanced renal reabsorption of calcium and phosphate
. Suppression of parathyroid hormone (PTH) secretion

Correct Answer & Explanation

. Inhibition of osteoclast function and induction of osteoclast apoptosis

Explanation

Bisphosphonates are analogues of inorganic pyrophosphate that bind to hydroxyapatite crystals in bone. They are ingested by osteoclasts during resorption, which disrupts their intracellular metabolism and induces apoptosis, thereby increasing bone density.

Question 11049

Topic: 1. General Principles & Basic Science

An 8-year-old girl presents with a painless "clunking" in her lateral knee during flexion and extension. MRI demonstrates a Wrisberg-variant discoid lateral meniscus. What anatomical structure is characteristically absent in this variant?

. Anterior meniscofemoral ligament (Ligament of Humphrey)
. Posterior meniscofemoral ligament (Ligament of Wrisberg)
. Transverse intermeniscal ligament
. Coronary ligaments (posterior meniscotibial attachments)
. Popliteofibular ligament

Correct Answer & Explanation

. Anterior meniscofemoral ligament (Ligament of Humphrey)

Explanation

The Wrisberg variant of a discoid meniscus lacks normal posterior meniscotibial (coronary) ligament attachments, making it hypermobile. It attaches only via the posterior meniscofemoral ligament (Wrisberg), leading to a snapping sensation as it subluxates during knee motion.

Question 11050

Topic: Biology, Genetics & Bone Healing

A 4-year-old boy presents with profound bilateral genu varum, frontal bossing, and short stature. Laboratory evaluation reveals a normal serum calcium, significantly decreased serum phosphate, elevated alkaline phosphatase, and normal parathyroid hormone levels. A mutation in the PHEX gene is confirmed. What is the primary pathophysiologic defect in this disorder?

. Deficiency of 1-alpha-hydroxylase in the kidney
. Decreased renal tubular reabsorption of phosphate due to excess FGF23
. Defective mineralization of type 1 collagen matrix
. Resistance of end organs to 1,25-dihydroxyvitamin D
. Impaired intestinal calcium absorption

Correct Answer & Explanation

. Deficiency of 1-alpha-hydroxylase in the kidney

Explanation

The presentation and lab findings are classic for X-linked hypophosphatemic rickets (mutated PHEX gene). This mutation leads to elevated levels of Fibroblast Growth Factor 23 (FGF23), which inhibits renal phosphate reabsorption and decreases phosphate levels, causing rickets.

Question 11051

Topic: Infection, Pharmacology & VTE

A 5-year-old boy presents with a 2-day history of right hip pain and inability to bear weight. His temperature is 38.8°C (101.8°F), WBC count is 14,000/mm3, ESR is 55 mm/hr, and CRP is 3.5 mg/dL. Radiographs of the hip are normal. What is the most appropriate next step in management?

. MRI of the pelvis with and without contrast
. Ultrasound-guided aspiration of the right hip
. Intravenous antibiotics and clinical observation
. Immediate hip arthrotomy and washout
. Bone scan of the lower extremities

Correct Answer & Explanation

. MRI of the pelvis with and without contrast

Explanation

This patient meets all four Kocher criteria (fever, non-weight-bearing, ESR >40, WBC >12,000), giving him a >93% probability of septic arthritis. The definitive diagnostic next step is an ultrasound-guided hip aspiration to confirm the diagnosis before proceeding to surgical washout.

Question 11052

Topic: 1. General Principles & Basic Science

A 58-year-old poorly controlled diabetic presents with a red, hot, swollen unilateral foot. Radiographs show periarticular fragmentation and debris at the midfoot. Which of the following tests is most specific for differentiating acute Charcot neuroarthropathy from osteomyelitis?

. Erythrocyte sedimentation rate (ESR)
. C-reactive protein (CRP)
. WBC count
. Bone marrow scan combined with tagged white blood cell scan
. Plain radiography

Correct Answer & Explanation

. Erythrocyte sedimentation rate (ESR)

Explanation

Differentiating acute Charcot from osteomyelitis is challenging. A combined Indium-111 tagged WBC scan and Technetium-99m sulfur colloid bone marrow scan has the highest specificity for diagnosing osteomyelitis in the setting of Charcot arthropathy.

Question 11053

Topic: 1. General Principles & Basic Science

A 55-year-old diabetic male has a chronic, full-thickness neuropathic ulcer beneath the first metatarsal head. The ulcer probes to bone. No retained hardware is present. Which imaging modality provides the highest sensitivity and specificity for diagnosing osteomyelitis in this setting?

. Plain weight-bearing radiography
. Three-phase bone scan
. Indium-111 labeled leukocyte scan
. Magnetic Resonance Imaging (MRI)
. Computed Tomography (CT) scan

Correct Answer & Explanation

. Plain weight-bearing radiography

Explanation

MRI is the modality of choice, offering the highest sensitivity and specificity for detecting osteomyelitis in the diabetic foot. It excels at demonstrating bone marrow edema and differentiating between soft tissue infection and true bone infection.

Question 11054

Topic: Infection, Pharmacology & VTE

A 55-year-old diabetic male presents with unrelenting, severe lower back pain and fevers. MRI with gadolinium contrast reveals increased T2 signal and enhancement in the L3-L4 disc space and adjacent vertebral endplates. Blood cultures are drawn. What is the most common causative organism for spontaneous pyogenic spondylodiscitis in the adult population?

. Streptococcus pneumoniae
. Staphylococcus aureus
. Pseudomonas aeruginosa
. Escherichia coli
. Mycobacterium tuberculosis

Correct Answer & Explanation

. Streptococcus pneumoniae

Explanation

Staphylococcus aureus is the single most common causative organism of pyogenic spondylodiscitis and vertebral osteomyelitis in the general adult population, accounting for more than 50% of cases. While IV drug users may have a higher risk of Pseudomonas or MRSA, and genitourinary sources may seed gram-negatives like E. coli, S. aureus remains the overall most prevalent pathogen.

Question 11055

Topic: 1. General Principles & Basic Science

In the context of spinal fusion biology, which of the following bone graft substitutes functions strictly via an osteoinductive mechanism without providing osteoconductive scaffolding or osteogenic cells?

. Cancellous allograft
. Demineralized bone matrix (DBM)
. Recombinant human Bone Morphogenetic Protein-2 (rhBMP-2)
. Calcium phosphate cement
. Hydroxyapatite

Correct Answer & Explanation

. Cancellous allograft

Explanation

Bone graft properties are classified into osteoconductive (providing a scaffold), osteoinductive (stimulating mesenchymal stem cells to differentiate into osteoblasts), and osteogenic (containing live cells). Recombinant human Bone Morphogenetic Protein-2 (rhBMP-2) is a pure, potent osteoinductive agent. It does not contain cells (not osteogenic) and is usually delivered on a collagen sponge which provides minimal structural support compared to structural grafts, making its primary role strictly osteoinductive. Cancellous allograft is primarily osteoconductive. DBM has weak osteoinductivity but is mostly a carrier/conductive element. Calcium phosphate and hydroxyapatite are purely osteoconductive.

Question 11056

Topic: Biomechanics & Biomaterials

Viscoelasticity is a fundamental biomechanical property of biologic tissues such as ligaments and tendons. When these tissues are subjected to a high strain rate (rapid loading) compared to a low strain rate, how do their mechanical properties change?

. Increased stiffness and higher energy absorption before failure
. Decreased stiffness and lower energy absorption before failure
. Increased stiffness and lower energy absorption before failure
. Decreased stiffness and higher energy absorption before failure
. Modulus of elasticity decreases while ultimate tensile strength remains unchanged

Correct Answer & Explanation

. Increased stiffness and higher energy absorption before failure

Explanation

Viscoelastic materials demonstrate rate-dependent mechanical properties. At higher rates of strain, biologic tissues such as ligaments and tendons become stiffer (higher modulus) and absorb more energy before failing. This is protective during sudden, rapid loading events.

Question 11057

Topic: 1. General Principles & Basic Science

In normal articular cartilage, which zone is characterized by having the lowest water content, the highest concentration of proteoglycans, and collagen fibers oriented perpendicular to the joint surface?

. Superficial (tangential) zone
. Middle (transitional) zone
. Deep (radial) zone
. Calcified zone
. Tidemark

Correct Answer & Explanation

. Superficial (tangential) zone

Explanation

The deep (radial) zone of articular cartilage contains the largest diameter collagen fibrils, which are oriented perpendicular to the joint surface. It has the highest concentration of proteoglycans and the lowest water content, optimizing it to resist compressive loads and transmit them to the underlying subchondral bone.

Question 11058

Topic: Biomechanics & Biomaterials

Mixing orthopaedic implant components made of 316L stainless steel and Ti-6Al-4V (titanium alloy) within the same fracture construct can precipitate galvanic corrosion. In this scenario, which metal serves as the anode and undergoes accelerated corrosion?

. Titanium alloy
. Cobalt-chromium-molybdenum alloy
. Stainless steel
. Tantalum
. Nitinol

Correct Answer & Explanation

. Titanium alloy

Explanation

When two dissimilar metals are in contact within an electrolytic environment (such as body fluid), the less noble metal acts as the anode and corrodes, while the more noble metal acts as the cathode. Stainless steel is less noble than titanium and therefore functions as the anode, undergoing galvanic corrosion.

Question 11059

Topic: Biomechanics & Biomaterials

Review the biomechanical concept illustrated by standard load-deformation curves.

On a stress-strain curve for a ductile material like stainless steel, the specific point beyond which the material will no longer return to its original shape and undergoes permanent, non-reversible plastic deformation is defined as the:

. Ultimate tensile strength
. Proportional limit
. Yield point
. Failure point
. Modulus of elasticity

Correct Answer & Explanation

. Ultimate tensile strength

Explanation

The yield point marks the transition from the elastic region to the plastic region on a stress-strain curve. Loading a material beyond its yield point results in permanent (plastic) deformation. The proportional limit is slightly before the yield point where the stress-strain relationship ceases to be perfectly linear, but for practical orthopaedic biomechanics, the yield point defines the start of plastic deformation.

Question 11060

Topic: Biology, Genetics & Bone Healing

Osteoclastogenesis is tightly regulated by the interaction between RANK, RANKL, and Osteoprotegerin (OPG). Which of the following systemic hormones exerts an anti-resorptive effect primarily by upregulating the production of OPG in osteoblasts?

. Parathyroid hormone (PTH)
. 1,25-dihydroxyvitamin D
. Estrogen
. Cortisol
. Thyroid hormone

Correct Answer & Explanation

. Parathyroid hormone (PTH)

Explanation

Estrogen protects against bone loss by decreasing osteoclast activity and lifespan. It achieves this primarily by stimulating osteoblasts to increase the production of Osteoprotegerin (OPG), which acts as a decoy receptor for RANKL, thereby preventing RANKL from binding to RANK on osteoclast precursors. Estrogen also decreases the production of M-CSF and RANKL.

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