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1. General Principles & Basic Science MCQs

Practice Set 685 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 13681

Topic: Surgical Anatomy & Approaches

A 24-year-old elite volleyball player presents with vague posterior shoulder pain and weakness in external rotation. MRI reveals isolated atrophy of the teres minor. Compression of the neurovascular bundle in the quadrilateral space is suspected. Which of the following forms the inferior boundary of this anatomical space?

. Teres minor
. Long head of the triceps
. Surgical neck of the humerus
. Teres major
. Latissimus dorsi

Correct Answer & Explanation

. Teres minor

Explanation

The quadrilateral space is bordered superiorly by the teres minor, inferiorly by the teres major, medially by the long head of the triceps, and laterally by the surgical neck of the humerus. It contains the axillary nerve and the posterior circumflex humeral artery. Compression here leads to quadrilateral space syndrome.

Question 13682

Topic: Biology, Genetics & Bone Healing

A 4-year-old boy presents with severe bowing of the lower extremities. Laboratory tests reveal normal serum calcium, abnormally low serum phosphate, and elevated alkaline phosphatase. Genetic testing identifies a mutation in the PHEX gene. Which of the following best describes the pathophysiology of this condition?

. Decreased renal production of 1,25-dihydroxyvitamin D
. Impaired intestinal absorption of calcium
. Overproduction of FGF23 leading to renal phosphate wasting
. Defect in type I collagen synthesis
. Deficient mineralization of the growth plate due to PTH resistance

Correct Answer & Explanation

. Overproduction of FGF23 leading to renal phosphate wasting

Explanation

The patient has X-linked hypophosphatemic rickets (XLHR), the most common form of heritable rickets. A mutation in the PHEX gene leads to decreased degradation and subsequent overproduction of Fibroblast Growth Factor 23 (FGF23). Elevated FGF23 decreases renal tubular reabsorption of phosphate (causing phosphate wasting) and inhibits 1-alpha-hydroxylase, decreasing active Vitamin D levels.

Question 13683

Topic: Biomechanics & Biomaterials

A surgeon is revising a femur fracture nonunion originally fixed with a 316L stainless steel plate. The surgeon plans to add a lag screw across the nonunion site using a titanium screw. Which of the following types of corrosion is most likely to occur if these two distinct metals are in direct contact in vivo?

. Fretting corrosion
. Crevice corrosion
. Galvanic corrosion
. Pitting corrosion
. Intergranular corrosion

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two electrochemically dissimilar metals (such as stainless steel and titanium) are placed in direct physical contact within a conductive fluid medium (like body fluids). The less noble metal becomes the anode and corrodes at an accelerated rate. Mixing metals in orthopedic implants should generally be avoided to prevent this complication.

Question 13684

Topic: 1. General Principles & Basic Science

A 14-year-old boy from Connecticut presents with an atraumatic, swollen, and mildly painful right knee. Aspiration yields a WBC count of 45,000 cells/mm3 with 85% neutrophils. Serology and PCR confirm Lyme arthritis. Assuming no neurological or cardiac involvement, what is the recommended initial management?

. Intravenous ceftriaxone for 14 days
. Oral doxycycline for 28 days
. Arthroscopic irrigation and debridement
. Intra-articular corticosteroid injection
. Serial arthrocentesis and observation

Correct Answer & Explanation

. Oral doxycycline for 28 days

Explanation

Lyme arthritis (late-stage disease caused by Borrelia burgdorferi) presenting without neurologic involvement is treated medically, not surgically. The first-line treatment is a 28-day course of oral doxycycline or amoxicillin. IV ceftriaxone is reserved for cases that are refractory to oral antibiotics or those presenting with neurological or severe cardiac manifestations.

Question 13685

Topic: Biomechanics & Biomaterials
Increasing the radiation dose during the cross-linking process of ultra-high molecular weight polyethylene (UHMWPE) for total joint arthroplasty has which of the following mechanical effects?
. Increases oxidative degradation
. Decreases adhesive wear but increases abrasive wear
. Decreases volumetric wear but decreases fatigue strength
. Increases ultimate tensile strength
. Decreases the risk of impingement

Correct Answer & Explanation

. Decreases volumetric wear but decreases fatigue strength

Explanation

Highly cross-linked polyethylene is manufactured by exposing UHMWPE to radiation. While this significantly increases wear resistance (decreasing volumetric wear and lowering the risk of osteolysis), it concomitantly alters the mechanical properties, leading to a reduction in ultimate tensile strength, yield strength, and fatigue strength. Remelting is subsequently used to reduce free radicals and prevent oxidative degradation.

Question 13686

Topic: 1. General Principles & Basic Science

Which of the following lower extremity amputations is associated with the highest percentage increase in metabolic energy expenditure during walking compared to a normal physiological baseline?

. Traumatic transtibial amputation
. Dysvascular transtibial amputation
. Traumatic transfemoral amputation
. Dysvascular transfemoral amputation
. Bilateral traumatic transtibial amputations

Correct Answer & Explanation

. Dysvascular transfemoral amputation

Explanation

Energy expenditure during gait increases significantly as the level of amputation moves proximally, and is higher in dysvascular patients than traumatic patients. A dysvascular transfemoral amputation increases energy expenditure by up to 100-120%, whereas traumatic transfemoral is ~60%, dysvascular transtibial is ~40-50%, and traumatic transtibial is ~25%.

Question 13687

Topic: Surgical Anatomy & Approaches

During the proximal extension of the volar (Henry) approach to the forearm, the surgeon develops an internervous plane. Which of the following correctly identifies the muscles and their respective nerve supplies forming this proximal interval?

. Brachioradialis (Radial) and Flexor Carpi Radialis (Median)
. Brachioradialis (Radial) and Pronator Teres (Median)
. Flexor Carpi Ulnaris (Ulnar) and Flexor Digitorum Superficialis (Median)
. Extensor Carpi Radialis Brevis (Radial) and Extensor Digitorum Communis (PIN)
. Pronator Teres (Median) and Flexor Carpi Radialis (Median)

Correct Answer & Explanation

. Brachioradialis (Radial) and Pronator Teres (Median)

Explanation

The proximal portion of the volar (Henry) approach to the radius utilizes the true internervous plane between the brachioradialis (innervated by the radial nerve) and the pronator teres (innervated by the median nerve). Distally, the plane shifts between the brachioradialis and the flexor carpi radialis.

Question 13688

Topic: Biology, Genetics & Bone Healing

A 32-year-old female presents with knee pain. Radiographs reveal an eccentric, purely lytic lesion in the distal femoral epiphysis extending to the subchondral bone. Biopsy shows multinucleated giant cells in a stroma of mononuclear cells. The mononuclear cells express which of the following crucial factors driving the pathogenesis of this tumor?

. RANKL
. Osteoprotegerin (OPG)
. FGF-23
. PTHrP
. Vascular Endothelial Growth Factor (VEGF)

Correct Answer & Explanation

. RANKL

Explanation

In a giant cell tumor (GCT) of bone, the true neoplastic cells are the mononuclear stromal cells. These cells overexpress Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL), which binds to RANK on osteoclast precursors, recruiting and activating the multinucleated, reactive osteoclast-like giant cells that cause bone resorption. This is the mechanism targeted by Denosumab.

Question 13689

Topic: Biomechanics & Biomaterials

In the biomechanics of articular cartilage, which lubrication mechanism is characterized by the elastic deformation of the articular surfaces under load, thereby increasing the surface area and trapping a pressurized fluid film between them?

. Boundary lubrication
. Elastohydrodynamic lubrication
. Weeping lubrication
. Hydrodynamic lubrication
. Boosted lubrication

Correct Answer & Explanation

. Elastohydrodynamic lubrication

Explanation

Elastohydrodynamic lubrication occurs when the load is high enough to cause elastic deformation of the compliant articular cartilage. This deformation increases the contact surface area and alters the geometry, trapping a fluid film (synovial fluid) that separates the surfaces, thereby protecting them during rapid motion under high loads.

Question 13690

Topic: Biology, Genetics & Bone Healing

A 68-year-old male presents with deep, aching thigh pain and reports that his hat size has increased over the past few years. Radiographs of the femur reveal cortical thickening, coarsened trabeculae, and a prominent bowing deformity. Histological examination of the affected bone would most likely demonstrate which of the following findings?

. A mosaic pattern of lamellar bone with prominent cement lines
. Extensive replacement of bone marrow by uniform plasma cells
. Disorganized whorls of fibroblastic stroma with woven bone lacking osteoblastic rimming
. Sheets of Langerhans cells with eosinophils
. Empty lacunae with surrounding necrotic marrow

Correct Answer & Explanation

. A mosaic pattern of lamellar bone with prominent cement lines

Explanation

The clinical picture is classic for Paget's disease of bone (osteitis deformans). The hallmark histological finding is a 'mosaic' or 'jigsaw puzzle' pattern of lamellar bone created by prominent, haphazard cement lines due to erratic and uncoupled osteoclast resorption and subsequent frantic osteoblast bone formation.

Question 13691

Topic: 1. General Principles & Basic Science

Tranexamic acid (TXA) is widely used in total joint arthroplasty to reduce perioperative blood loss. Which of the following best describes the molecular mechanism of action of TXA?

. Irreversibly inhibits cyclooxygenase to prevent platelet aggregation
. Competitively inhibits the binding of plasminogen to fibrin by mimicking a lysine residue
. Activates factor Xa to promote the conversion of prothrombin to thrombin
. Inhibits the synthesis of vitamin K-dependent clotting factors
. Directly cross-links fibrin polymers to stabilize the platelet plug

Correct Answer & Explanation

. Competitively inhibits the binding of plasminogen to fibrin by mimicking a lysine residue

Explanation

Tranexamic acid is a synthetic analog of the amino acid lysine. It functions as an antifibrinolytic by reversibly and competitively binding to the lysine receptor sites on plasminogen. This prevents plasminogen from binding to fibrin, thereby inhibiting its activation to plasmin and preventing the degradation of fibrin clots.

Question 13692

Topic: Biomechanics & Biomaterials

A 45-year-old patient undergoes an Achilles tendon repair. During rehabilitation, the physical therapist applies a constant load to the tendon over time, resulting in continued progressive deformation of the tendon. Which biomechanical property is described?

. Creep
. Stress relaxation
. Hysteresis
. Isotropy
. Anisotropy

Correct Answer & Explanation

. Creep

Explanation

Creep is defined as the progressive deformation of a material under a constant load over time. Viscoelastic materials, such as tendons and ligaments, exhibit this property. Stress relaxation, conversely, is the decrease in stress over time when a material is held at a constant strain (deformation).

Question 13693

Topic: Surgical Anatomy & Approaches
Following a closed humerus fracture, a patient develops a radial nerve palsy. An EMG performed at 4 weeks shows fibrillation potentials but no motor unit action potentials. The nerve injury involves disruption of the axons and myelin sheath, but the endoneurium, perineurium, and epineurium remain intact. Which Sunderland grade corresponds to this injury?
. Grade I
. Grade II
. Grade III
. Grade IV
. Grade V

Correct Answer & Explanation

. Grade II

Explanation

A Sunderland Grade II injury corresponds to axonotmesis where the axon and myelin sheath are disrupted, but the endoneurial tubes, perineurium, and epineurium are intact. Recovery is generally complete as the intact endoneurium guides regenerating axons at a rate of 1 mm/day.

Question 13694

Topic: Biology, Genetics & Bone Healing

A 72-year-old man presents with increasing head size (reports needing a larger hat) and deep aching bone pain in his right femur. Laboratory testing shows a significantly elevated alkaline phosphatase but normal calcium and phosphorus. Histological examination of the bone is most likely to show:

. Woven bone with empty lacunae
. Mosaic pattern of lamellar bone with prominent cement lines
. Thickened osteoid seams with poor mineralization
. Sheets of plasma cells with clock-face nuclei
. Proliferation of giant cells in a stroma of mononuclear cells

Correct Answer & Explanation

. Mosaic pattern of lamellar bone with prominent cement lines

Explanation

Paget disease (osteitis deformans) is characterized by increased, disorganized bone turnover. The hallmark histologic finding is a 'mosaic pattern' of lamellar bone with irregular, prominent cement lines indicating haphazard bone resorption and formation.

Question 13695

Topic: Biology, Genetics & Bone Healing

A 70-year-old female with severe osteoporosis and multiple vertebral compression fractures is started on teriparatide. Which of the following best describes the mechanism of action of this medication?

. Inhibits osteoclast activity via RANKL binding
. Stimulates bone formation through intermittent PTH receptor activation
. Inhibits farnesyl pyrophosphate synthase in osteoclasts
. Selective estrogen receptor modulation
. Neutralizes sclerostin to promote Wnt signaling

Correct Answer & Explanation

. Stimulates bone formation through intermittent PTH receptor activation

Explanation

Teriparatide is a recombinant parathyroid hormone (PTH 1-34) analog. When administered intermittently (daily subcutaneous injection), it has an anabolic effect, directly stimulating osteoblastic bone formation. Continuous endogenous PTH, however, leads to bone resorption. Bisphosphonates inhibit farnesyl pyrophosphate synthase, and denosumab binds RANKL.

Question 13696

Topic: Biomechanics & Biomaterials

In the context of orthopedic implants, which of the following material properties describes the ability to absorb energy and plastically deform prior to failure?

. Stiffness
. Elasticity
. Toughness
. Fatigue limit
. Ductility

Correct Answer & Explanation

. Toughness

Explanation

Toughness is represented by the total area under the stress-strain curve and indicates the amount of energy a material can absorb before failure. It incorporates both the strength and ductility of the material.

Question 13697

Topic: 1. General Principles & Basic Science

The formation of a biofilm on an orthopedic implant makes eradication of infection extremely difficult. Which of the following is the primary constituent of the extracellular polymeric substance (EPS) matrix that protects the bacteria in a biofilm?

. Hyaluronic acid
. Polysaccharides
. Collagen type I
. Fibrinogen
. Fibronectin

Correct Answer & Explanation

. Polysaccharides

Explanation

The extracellular polymeric substance (EPS) or 'slime' layer of a biofilm is composed primarily of polysaccharides (along with proteins, extracellular DNA, and lipids). This matrix protects the bacteria from host immune responses and restricts the penetration of antimicrobial agents.

Question 13698

Topic: Biology, Genetics & Bone Healing

A 45-year-old female presents with diffuse bone pain and muscle weakness. Laboratory studies reveal low serum calcium, low serum phosphate, elevated alkaline phosphatase, and elevated parathyroid hormone (PTH). Radiographs show pseudofractures (Looser zones) in the femoral neck. What is the most likely diagnosis?

. Primary hyperparathyroidism
. Osteoporosis
. Osteomalacia
. Paget disease
. Renal osteodystrophy

Correct Answer & Explanation

. Osteomalacia

Explanation

The clinical and laboratory profile (low calcium, low phosphate, high ALP, high PTH) along with Looser zones (pseudofractures) is classic for osteomalacia, typically due to severe vitamin D deficiency. Secondary hyperparathyroidism occurs as a physiological response to the hypocalcemia.

Question 13699

Topic: Infection, Pharmacology & VTE
Following a total knee arthroplasty, a patient is prescribed rivaroxaban for deep vein thrombosis prophylaxis. What is the specific mechanism of action of this medication?
. Direct thrombin (Factor IIa) inhibitor
. Direct Factor Xa inhibitor
. Vitamin K epoxide reductase inhibitor
. Enhances antithrombin III activity
. Irreversible cyclooxygenase inhibitor

Correct Answer & Explanation

. Direct Factor Xa inhibitor

Explanation

Rivaroxaban and apixaban are direct Factor Xa inhibitors. Dabigatran is a direct thrombin (IIa) inhibitor. Warfarin inhibits vitamin K epoxide reductase. Heparin and LMWH enhance antithrombin III activity.

Question 13700

Topic: Biomechanics & Biomaterials

A surgeon revises a femur fracture initially treated with a stainless steel plate and screws. Due to stripped screw holes, the surgeon replaces several screws with titanium screws, leaving the stainless steel plate in place. What biomechanical complication is most likely to occur?

. Fretting corrosion
. Galvanic corrosion
. Crevice corrosion
. Stress shielding
. Pitting corrosion

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals (e.g., stainless steel and titanium) are placed in contact within a conductive medium (body fluid). The electrochemical potential difference causes the less noble metal to corrode rapidly. Mixing distinct metal types in implants should be avoided.

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