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1. General Principles & Basic Science MCQs

Practice Set 258 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 5141

Topic: Surgical Anatomy & Approaches

During an ilioinguinal approach for the fixation of an anterior column acetabular fracture, significant arterial bleeding is encountered posterior to the superior pubic ramus. Which of the following anatomical connections is the most likely source of this bleeding (Corona Mortis)?

. Superior gluteal artery and inferior gluteal artery
. Inferior epigastric artery and obturator artery
. Internal pudendal artery and obturator artery
. Deep circumflex iliac artery and femoral artery
. External pudendal artery and obturator artery

Correct Answer & Explanation

. Inferior epigastric artery and obturator artery

Explanation

The Corona Mortis (Crown of Death) represents a vascular anastomosis between the external iliac system (usually via the inferior epigastric artery or vein) and the internal iliac system (obturator artery or vein). It is located posterior to the superior pubic ramus at a variable distance (average 4-6 cm) from the symphysis pubis. Iatrogenic injury during the ilioinguinal approach or placement of superior pubic ramus screws can lead to life-threatening hemorrhage.

Question 5142

Topic: Biology, Genetics & Bone Healing

A 34-year-old female with a recurrent giant cell tumor of the distal radius is deemed unresectable and is started on denosumab therapy. What is the specific mechanism of action of this medication?

. Directly induces apoptosis of multinucleated giant cells
. Binds to RANKL, preventing its interaction with the RANK receptor
. Acts as a competitive antagonist at the OPG (osteoprotegerin) receptor
. Inhibits the farnesyl pyrophosphate synthase enzyme in the mevalonate pathway
. Stimulates osteoblastic differentiation by activating the Wnt/beta-catenin pathway

Correct Answer & Explanation

. Binds to RANKL, preventing its interaction with the RANK receptor

Explanation

Denosumab is a fully human monoclonal antibody that specifically binds to the Receptor Activator of Nuclear factor Kappa-B Ligand (RANKL). By binding to RANKL, it prevents RANKL from activating the RANK receptor on the surface of osteoclasts and their precursors. In giant cell tumors (GCT), the neoplastic stromal cells express RANKL, which recruits and activates the reactive multinucleated giant cells (osteoclast-like cells) that cause bone destruction. Denosumab inhibits this interaction, halting bone lysis.

Question 5143

Topic: Biology, Genetics & Bone Healing

According to Perren's Strain Theory, the local mechanical environment dictates the type of tissue that forms in a fracture gap. What is the maximum interfragmentary strain that permits the formation of primary lamellar bone?

. Less than 1%
. Less than 2%
. Between 2% and 10%
. Between 10% and 30%
. Greater than 30%

Correct Answer & Explanation

. Less than 2%

Explanation

Perren's Strain Theory describes how tissue differentiation at a fracture site is governed by interfragmentary strain. Lamellar bone (primary bone healing without a cartilaginous intermediate) can only tolerate an interfragmentary strain of less than 2%. Strains between 2% and 10% permit woven bone formation. Strains between 10% and 30% lead to cartilage formation (enchondral ossification), and strains >30% result in granulation tissue, leading to a nonunion if not stabilized.

Question 5144

Topic: 1. General Principles & Basic Science

During a surgical dislocation of the hip via a trochanteric flip approach, preserving the blood supply to the femoral head is paramount. The main deep branch of the medial circumflex femoral artery (MFCA) runs consistently between which two anatomic structures as it passes posterior to the hip joint?

. Piriformis and superior gemellus
. Superior gemellus and obturator internus
. Obturator internus and inferior gemellus
. Inferior gemellus and quadratus femoris
. Obturator externus and quadratus femoris

Correct Answer & Explanation

. Obturator externus and quadratus femoris

Explanation

Understanding the precise course of the medial circumflex femoral artery (MFCA) is crucial for joint-preserving hip surgery (e.g., Ganz approach). The deep branch of the MFCA passes superiorly after coursing between the pectineus and iliopsoas, then consistently travels posteriorly between the obturator externus and the quadratus femoris muscles. It then runs deep to the common tendon of the obturator internus and gemelli to supply the femoral head.

Question 5145

Topic: 1. General Principles & Basic Science

Articular cartilage is structurally organized into distinct zones, each with unique mechanical properties and biochemical compositions. Which zone contains the highest concentration of proteoglycans and features chondrocytes arranged in vertical columns perpendicular to the joint surface?

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

Correct Answer & Explanation

. Deep (radial) zone

Explanation

Articular cartilage consists of four zones: Superficial, Middle, Deep, and Calcified. The Deep (radial) zone provides the greatest resistance to compressive forces. It is characterized by having the highest concentration of proteoglycans, the lowest water content of the uncalcified zones, and chondrocytes that are arranged in vertical columns. The collagen fibers in this zone are thickest and run perpendicular to the articular surface, anchoring into the tidemark.

Question 5146

Topic: Biology, Genetics & Bone Healing

Bone morphogenetic proteins (BMPs) play a crucial role in inducing osteoblastic differentiation of mesenchymal stem cells. Which of the following intracellular signaling molecules is directly phosphorylated following BMP binding to its cell-surface serine/threonine kinase receptor?

. Wnt
. Beta-catenin
. Smad 1/5/8
. Smad 2/3
. Mitogen-activated protein (MAP) kinase

Correct Answer & Explanation

. Smad 1/5/8

Explanation

Bone morphogenetic proteins (BMPs) belong to the TGF-beta superfamily. When BMP binds to its serine/threonine kinase receptor, it phosphorylates receptor-regulated Smads (R-Smads), specifically Smad 1, 5, and 8. These phosphorylated Smads then form a complex with the common-partner Smad (Co-Smad), Smad 4, and translocate to the nucleus to regulate gene transcription. In contrast, TGF-beta primarily phosphorylates Smad 2 and 3.

Question 5147

Topic: Biomechanics & Biomaterials

A patient undergoes revision of a total hip arthroplasty composed of a titanium alloy stem and a modular cobalt-chromium femoral head. The revision is performed for an adverse local tissue reaction (ALTR) secondary to metallosis. The primary mechanism of metal degradation at the modular head-neck junction in this scenario is classified as:

. Galvanic corrosion
. Mechanically assisted crevice corrosion (fretting corrosion)
. Pitting corrosion
. Intergranular corrosion
. Stress corrosion cracking

Correct Answer & Explanation

. Mechanically assisted crevice corrosion (fretting corrosion)

Explanation

Corrosion at the modular head-neck taper junction (trunnionosis) is primarily caused by mechanically assisted crevice corrosion, commonly referred to as fretting corrosion. Micromotion between the head and the neck disrupts the protective passivation layer of the metals, exposing raw metal to the body fluid environment (crevice corrosion), which accelerates the release of metal ions and debris, leading to ALTR.

Question 5148

Topic: Biomechanics & Biomaterials

A surgeon revises a failed hip arthroplasty and notices significant corrosion at the modular head-neck junction. The femoral stem is titanium alloy and the femoral head is cobalt-chromium. Which of the following best describes the primary mechanism of galvanic corrosion in this setting?

. Fretting motion disrupting the passivation layer, exposing different metals to an electrolyte.
. Micro-organism induced acidic environment dissolving the titanium alloy.
. Excessive compressive loads causing plastic deformation of the cobalt-chromium head.
. Release of free radicals from the polyethylene liner interacting with the metal junction.
. High levels of systemic chromium leading to a hypersensitivity reaction at the junction.

Correct Answer & Explanation

. Fretting motion disrupting the passivation layer, exposing different metals to an electrolyte.

Explanation

Galvanic corrosion occurs when two dissimilar metals (e.g., Titanium and Cobalt-Chromium) are in contact within an electrolyte solution (body fluid). Fretting (micro-motion) disrupts the protective oxide (passivation) layer, accelerating the electrochemical dissolution of the less noble metal.

Question 5149

Topic: 1. General Principles & Basic Science

A patient undergoes primary repair of a flexor digitorum profundus (FDP) laceration in the middle finger. Postoperatively, the patient demonstrates an inability to make a full composite fist, specifically showing limited active flexion of the index, ring, and small fingers. What is the most likely cause?

. Lumbrical plus syndrome
. Quadrigia effect due to over-advancement of the FDP tendon
. Adhesions of the flexor digitorum superficialis (FDS) tendon
. Intrinsic tightness
. Extensor habitus

Correct Answer & Explanation

. Quadrigia effect due to over-advancement of the FDP tendon

Explanation

The Quadrigia effect occurs when the FDP tendon of one finger is advanced too far (typically >1 cm) during repair. Because the FDP tendons to the middle, ring, and small fingers share a common muscle belly, overtensioning one restricts the excursion of the others, leading to a decreased grip strength and incomplete active flexion of the uninjured digits.

Question 5150

Topic: Biology, Genetics & Bone Healing

Bone morphogenetic proteins (BMPs) are essential for bone healing. They initiate intracellular signaling primarily by binding to transmembrane receptors. Which of the following intracellular signaling molecules is directly phosphorylated following BMP receptor activation?

. JAK/STAT
. Smad 1/5/8
. c-Fos
. Wnt/beta-catenin
. NFATc1

Correct Answer & Explanation

. Smad 1/5/8

Explanation

BMPs belong to the TGF-beta superfamily. When they bind to their serine/threonine kinase cell surface receptors, they cause phosphorylation and activation of receptor-regulated Smads (R-Smads), specifically Smad 1, 5, and 8. These complex with Smad 4 to enter the nucleus and regulate gene transcription for osteoblast differentiation.

Question 5151

Topic: 1. General Principles & Basic Science

An orthopedic surgeon is performing an open repair of a severe medial-sided knee injury. The surgeon identifies the superficial medial collateral ligament (sMCL) and the posterior oblique ligament (POL). What is the primary functional difference in knee stability provided by these two structures?

. The sMCL is the primary restraint to valgus stress at 30 degrees of flexion, while the POL is tightest in full extension.
. The sMCL primarily restrains internal rotation, while the POL restrains valgus stress.
. The sMCL is tightest in full extension, while the POL is tightest at 90 degrees of flexion.
. The POL is the primary restraint to anterior tibial translation, while the sMCL restrains posterior translation.
. There is no biomechanical difference; both are tightest at 90 degrees of flexion.

Correct Answer & Explanation

. The sMCL is the primary restraint to valgus stress at 30 degrees of flexion, while the POL is tightest in full extension.

Explanation

The superficial MCL is the primary restraint to valgus stress and is best tested at 30 degrees of knee flexion, where it is taut. The posterior oblique ligament (POL), which is a thickening of the posteromedial capsule, is tightest in full extension and provides resistance against valgus and external rotation forces in extension.

Question 5152

Topic: 1. General Principles & Basic Science
Tendon healing progresses through overlapping phases: inflammatory, proliferative (fibroblastic), and remodeling. During the remodeling phase, what is the primary change in the composition of the extracellular matrix that contributes to increased tensile strength?
. Replacement of Type I collagen with Type II collagen
. Replacement of Type III collagen with Type I collagen
. Increased production of proteoglycans and glycosaminoglycans
. Conversion of highly cross-linked collagen to non-cross-linked collagen
. Rapid proliferation of tenocytes leading to hypercellularity

Correct Answer & Explanation

. Replacement of Type III collagen with Type I collagen

Explanation

During the early stages of tendon healing (proliferative phase), fibroblasts lay down predominantly Type III collagen, which is disorganized and mechanically weak. During the remodeling phase, Type III collagen is gradually replaced by Type I collagen, which organizes into parallel bundles along the axis of tension, significantly increasing the tendon's tensile strength.

Question 5153

Topic: 1. General Principles & Basic Science

In healthy articular cartilage, which zone is characterized by the highest concentration of proteoglycans, the lowest concentration of water, and chondrocytes aligned in vertical columns?

. Superficial (tangential) zone
. Middle (transitional) zone
. Deep (radial) zone
. Calcified cartilage zone
. Subchondral bone plate

Correct Answer & Explanation

. Deep (radial) zone

Explanation

The deep (radial) zone of articular cartilage contains the highest concentration of proteoglycans and the lowest water content. The chondrocytes in this layer are arranged in vertical columns parallel to the collagen fibers, which orient perpendicular to the joint surface to effectively resist and distribute compressive forces.

Question 5154

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of bone healing, what is the maximum amount of interfragmentary strain that can be tolerated for primary (osteonal) bone healing to occur without the formation of a visible 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 states that primary (direct) bone healing via osteonal cutting cones requires absolute stability. This occurs only when interfragmentary strain is less than 2%. Strains between 2% and 10% result in secondary bone healing (callus formation). Strains >10% typically lead to nonunion, as the strain exceeds the tolerance of bone and cartilage formation.

Question 5155

Topic: 1. General Principles & Basic Science

A surgeon uses a structural cortical allograft to reconstruct a massive diaphyseal defect following tumor resection. Which of the following best describes the initial phase of graft incorporation for this specific type of bone graft?

. Osteoblastic apposition of new bone
. Osteoclastic resorption via cutting cones
. Endochondral ossification
. Chondrogenesis and cartilaginous intermediate formation
. Intramembranous ossification

Correct Answer & Explanation

. Osteoclastic resorption via cutting cones

Explanation

Cortical bone grafts incorporate via creeping substitution, which initially involves osteoclastic resorption through cutting cones. This initial resorptive phase leads to a temporary decrease in the mechanical strength of the cortical graft. In contrast, cancellous bone grafts incorporate primarily through early osteoblastic apposition of new bone onto the existing dead trabecular framework without initial significant resorption.

Question 5156

Topic: Surgical Anatomy & Approaches

During the ilioinguinal approach to the acetabulum, severe bleeding is encountered while dissecting over the superior pubic ramus. This is likely due to injury to the 'corona mortis'. What is the correct anatomical description of this structure?

. An anastomosis between the external iliac vein and the obturator vein only
. An anastomosis between the internal iliac artery and the external pudendal artery
. An anastomosis between the obturator vessels and the external iliac or inferior epigastric vessels
. An anastomosis between the superior gluteal artery and the internal pudendal artery
. An anastomosis between the femoral artery and the obturator artery

Correct Answer & Explanation

. An anastomosis between the obturator vessels and the external iliac or inferior epigastric vessels

Explanation

The corona mortis ('crown of death') is a vascular anastomosis between the obturator system (internal iliac system) and the external iliac or inferior epigastric system. It typically crosses the superior pubic ramus and is highly vulnerable to injury during anterior approaches to the pelvis (e.g., ilioinguinal approach) or during placement of superior pubic ramus screws.

Question 5157

Topic: Physiology & Rehabilitation

According to Saunders et al., the determinants of normal human gait function to minimize the vertical and lateral displacement of the body's center of gravity. Which of the following movements is a primary determinant that specifically acts to reduce the peak of the vertical center of gravity trajectory?

. Pelvic tilt (Trendelenburg drop in swing phase)
. Foot pronation during initial contact
. Knee flexion in the stance phase
. Hip extension in the swing phase
. Ankle dorsiflexion in mid-stance

Correct Answer & Explanation

. Knee flexion in the stance phase

Explanation

Saunders et al. described six determinants of gait. Knee flexion in the stance phase (typically around 15 degrees) serves to lower the peak of the vertical displacement of the center of gravity, thereby reducing the metabolic energy required for walking.

Question 5158

Topic: Biomechanics & Biomaterials
Which of the following modifications to ultra-high molecular weight polyethylene (UHMWPE) most effectively increases its wear resistance in total hip arthroplasty?
. Decreasing overall crystallinity
. Gamma irradiation in air
. Electron beam or gamma irradiation followed by remelting
. Adding barium sulfate for radiopacity
. Sterilization with ethylene oxide gas

Correct Answer & Explanation

. Electron beam or gamma irradiation followed by remelting

Explanation

Highly cross-linked polyethylene is produced by exposing UHMWPE to irradiation (gamma or electron beam), which creates free radicals that form cross-links. This significantly improves wear resistance. Subsequent remelting or annealing eliminates residual free radicals to prevent long-term in vivo oxidation.

Question 5159

Topic: Biology, Genetics & Bone Healing
A patient presents with recurrent fractures, anemia, and hepatosplenomegaly. Radiographs demonstrate diffuse osteosclerosis and a 'bone-within-a-bone' appearance. The primary defect in this disorder is related to the dysfunction of which of the following?
. Osteoblasts
. Osteoclasts
. Osteocytes
. Chondrocytes
. Fibroblasts

Correct Answer & Explanation

. Osteoclasts

Explanation

The clinical and radiographic presentation is classic for osteopetrosis (Albers-Schönberg disease). It is caused by a genetic defect resulting in impaired osteoclast function (e.g., defective carbonic anhydrase II, TCIRG1 mutation, or CLCN7 mutation), leading to a failure of normal bone resorption, acidification, and remodeling.

Question 5160

Topic: Biology, Genetics & Bone Healing

Bone morphogenetic protein-2 (BMP-2) is heavily utilized in spine surgery for its potent osteoinductive properties. Through which of the following intracellular signaling pathways does BMP-2 primarily exert its osteogenic effect?

. Activation of the Wnt/beta-catenin pathway
. Phosphorylation of SMAD 1, 5, and 8
. Inhibition of RANKL expression
. Activation of the JAK/STAT signaling cascade
. Stimulation of fibroblast growth factor receptor 3 (FGFR3)

Correct Answer & Explanation

. Phosphorylation of SMAD 1, 5, and 8

Explanation

BMP-2 binds to a heterodimeric serine/threonine kinase receptor on the cell surface. This binding leads to the intracellular phosphorylation of receptor-regulated SMADs (SMAD 1, 5, and 8). These form a complex with the common-mediator SMAD 4, which then translocates to the nucleus to regulate the transcription of osteogenic genes such as Runx2.

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