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1. General Principles & Basic Science MCQs

Practice Set 397 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 7921

Topic: Biology, Genetics & Bone Healing

Recombinant human bone morphogenetic protein-2 (rhBMP-2) is heavily utilized in anterior lumbar interbody fusions. Upon binding to its serine/threonine kinase receptor on the cell membrane of an osteoprogenitor cell, which intracellular signaling molecules are directly phosphorylated to translocate to the nucleus and initiate transcription of osteogenic genes?

. Wnt and beta-catenin
. Smad 1, 5, and 8
. RANKL and osteoprotegerin
. MAPK and ERK
. JAK and STAT

Correct Answer & Explanation

. Smad 1, 5, and 8

Explanation

Bone morphogenetic proteins (BMPs) signal primarily through the canonical Smad pathway. Binding of the BMP ligand to the receptor complex causes phosphorylation of receptor-regulated Smads (R-Smads 1, 5, and 8). These phosphorylated Smads form a complex with the common-partner Smad (co-Smad 4), which then translocates to the nucleus to regulate the transcription of osteoblast-specific genes like Runx2.

Question 7922

Topic: 1. General Principles & Basic Science



Figure 4 illustrates the histologic zones of normal articular cartilage. In the superficial (tangential) zone (zone 1), how are the collagen fibrils primarily oriented relative to the articular surface, and what is the predominant collagen type within the matrix?

. Perpendicular; Type II
. Parallel; Type II
. Randomly oriented; Type II
. Parallel; Type I
. Perpendicular; Type I

Correct Answer & Explanation

. Parallel; Type II

Explanation

Hyaline articular cartilage is primarily composed of Type II collagen. In the superficial (tangential) zone, the collagen fibrils are closely packed and oriented parallel to the joint surface to maximally resist shear forces. In contrast, the deep zone features collagen fibers oriented perpendicularly to the subchondral bone to resist compressive forces.

Question 7923

Topic: Biology, Genetics & Bone Healing
A 2-year-old boy presents with multiple fractures after minimal trauma, blue sclerae, and dentinogenesis imperfecta. Genetic testing reveals a point mutation resulting in the substitution of glycine by a bulkier amino acid within the triple helix domain of type I collagen. Which of the following best describes this specific genetic mechanism and its associated disease severity?
. Quantitative defect (null allele) resulting in mild Osteogenesis Imperfecta (Type I)
. Qualitative defect (dominant negative) resulting in severe Osteogenesis Imperfecta (Type III)
. Defect in core binding factor alpha-1 (Cbfa1/Runx2) resulting in cleidocranial dysplasia
. Mutation in the COMP gene resulting in multiple epiphyseal dysplasia
. Defect in the RANKL gene resulting in osteopetrosis

Correct Answer & Explanation

. Qualitative defect (dominant negative) resulting in severe Osteogenesis Imperfecta (Type III)

Explanation

Osteogenesis imperfecta (OI) is caused by mutations in the COL1A1 or COL1A2 genes. The substitution of glycine (the smallest amino acid, strictly required at every third position of the alpha chain to allow triple helix coiling) with a bulkier amino acid disrupts the triple helix formation. This causes a 'dominant negative' (qualitative) defect, meaning abnormal collagen is incorporated into the matrix, which typically results in more severe, structurally compromised forms of OI (e.g., Types II, III, IV). Mild OI (Type I) is generally caused by a premature stop codon (null allele), resulting in a decreased amount of structurally normal collagen (quantitative defect).

Question 7924

Topic: Biology, Genetics & Bone Healing

Aseptic loosening in total joint arthroplasty is largely driven by a biologic reaction to wear debris. Which of the following is the most critical primary effector cell responsible for phagocytosing submicron polyethylene particles and subsequently releasing osteolytic cytokines such as TNF-alpha and IL-1?

. Osteoblasts
. Osteoclasts
. Macrophages
. T lymphocytes
. Fibroblasts

Correct Answer & Explanation

. Macrophages

Explanation

Macrophages are the primary cellular drivers of particle-induced osteolysis in aseptic loosening. They phagocytose wear debris particles (most reactively between 0.1 to 1.0 micrometers in size). Unable to digest the polyethylene, they become activated and secrete pro-inflammatory cytokines (TNF-alpha, IL-1, IL-6, PGE2), which upregulate RANKL expression and subsequently stimulate osteoclast-mediated periprosthetic bone resorption.

Question 7925

Topic: 1. General Principles & Basic Science
Tranexamic acid (TXA) is widely integrated into orthopedic surgery protocols to reduce perioperative blood loss and transfusion requirements. What is the specific mechanism of action of this pharmacological agent?
. Inhibits the cyclooxygenase (COX) enzyme
. Competitively blocks the lysine binding sites on plasminogen
. Activates the antithrombin III complex
. Directly inhibits Factor Xa
. Irreversibly binds to the ADP receptor on platelets

Correct Answer & Explanation

. Competitively blocks the lysine binding sites on plasminogen

Explanation

Tranexamic acid (TXA) is a synthetic derivative of the amino acid lysine. It acts as an antifibrinolytic agent by reversibly and competitively binding to the lysine binding sites on plasminogen. This prevents plasminogen from interacting with fibrin and converting into active plasmin, effectively halting the enzymatic degradation of fibrin clots.

Question 7926

Topic: Physiology & Rehabilitation

During rehabilitation after a prolonged lower extremity immobilization, a physical therapist aims to specifically recruit and rebuild Type I skeletal muscle fibers. Which of the following physiologic characteristics correctly defines Type I muscle fibers?

. They rely primarily on anaerobic glycolysis for ATP production.
. They have a remarkably low density of mitochondria.
. They are rich in myoglobin and highly fatigue-resistant.
. They are the first to atrophy after prolonged immobilization.
. They produce short, powerful bursts of high-tension contraction.

Correct Answer & Explanation

. They are rich in myoglobin and highly fatigue-resistant.

Explanation

Type I muscle fibers (slow-twitch, 'red' muscle) are characterized by their reliance on aerobic oxidative metabolism, extensive capillary supply, high mitochondrial density, and abundant myoglobin. They are highly fatigue-resistant and utilized for endurance and posture. Conversely, Type II fibers (fast-twitch) rely more on anaerobic glycolysis, produce rapid and powerful contractions, and are preferentially affected (atrophy first) during states of prolonged immobilization.

Question 7927

Topic: Surgical Anatomy & Approaches

A 35-year-old patient sustains a closed midshaft humerus fracture and presents with an immediate, complete radial nerve palsy. Electromyography (EMG) is performed at 3 weeks and reveals abundant fibrillation potentials in the brachioradialis and extensor carpi radialis longus. Based on Sunderland's classification, what is the minimum grade of nerve injury sustained?

. First-degree (Neurapraxia)
. Second-degree (Axonotmesis)
. Third-degree
. Fourth-degree
. Fifth-degree (Neurotmesis)

Correct Answer & Explanation

. Second-degree (Axonotmesis)

Explanation

Fibrillation potentials on EMG indicate active muscle denervation secondary to Wallerian degeneration, which takes about 2 to 3 weeks to become electrically evident. First-degree injuries (neurapraxia) represent a focal conduction block without axonal disruption; therefore, Wallerian degeneration does not occur and fibrillations are absent. The presence of fibrillations implies axonal death distal to the injury, making the injury at least a second-degree (axonotmesis), where the axon is disrupted but the endoneurium remains intact.

Question 7928

Topic: Biomechanics & Biomaterials

A surgeon performs an open reduction and internal fixation of a comminuted femur fracture using a 316L stainless steel plate and titanium alloy screws due to a shortage of matching stainless steel screws. Two years later, the patient presents with thigh pain and radiographic evidence of hardware failure and localized osteolysis.

What specific type of biomaterial degradation is predominantly driven by the electrochemical potential difference between these two intimately mixed metals in a physiological fluid medium?

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

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals are in direct electrical contact within a conductive fluid medium (such as human serum or interstitial fluid). The metal with the lower electrochemical potential (less noble) acts as an anode and preferentially corrodes, while the more noble metal acts as the cathode. Mixing stainless steel (less noble) with titanium (more noble) can lead to accelerated galvanic corrosion of the stainless steel. Crevice corrosion occurs in restricted spaces (like under a screw head) due to oxygen depletion. Fretting corrosion is mechanically driven by micromotion disrupting the passivation layer. Pitting is localized corrosion forming small holes, and intergranular corrosion occurs at grain boundaries, often related to manufacturing impurities.

Question 7929

Topic: Biology, Genetics & Bone Healing

A 68-year-old woman with a T-score of -3.2 at the femoral neck and a history of a distal radius fracture is prescribed romosozumab. This medication is a humanized monoclonal antibody designed to increase bone mineral density. What is the primary molecular mechanism of action of this pharmacological agent?

. Binding to RANKL to prevent osteoclastogenesis and induce osteoclast apoptosis
. Direct agonism of the calcium-sensing receptor on the parathyroid gland
. Inhibition of the Wnt/beta-catenin signaling pathway to prevent bone resorption
. Prevention of sclerostin-mediated inhibition of the Wnt/beta-catenin signaling pathway
. Acting as a decoy receptor for interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-a)

Correct Answer & Explanation

. Prevention of sclerostin-mediated inhibition of the Wnt/beta-catenin signaling pathway

Explanation

Romosozumab is a monoclonal antibody that binds to and neutralizes sclerostin. Sclerostin is a glycoprotein secreted primarily by osteocytes that naturally inhibits osteoblastogenesis by binding to LRP5/6 receptors, thereby blocking the Wnt/beta-catenin signaling pathway. By neutralizing sclerostin, romosozumab disinhibits (activates) the Wnt/beta-catenin pathway, leading to a profound anabolic effect (increased bone formation) and a secondary antiresorptive effect. Denosumab binds to RANKL (Option A). Cinacalcet modulates the calcium-sensing receptor (Option B).

Question 7930

Topic: Biology, Genetics & Bone Healing
During the pathogenesis of osteoarthritis, the degradation of the articular cartilage extracellular matrix is driven by a cascade of proteolytic enzymes. While aggrecanases (such as ADAMTS-4 and ADAMTS-5) are responsible for early proteoglycan loss, which of the following enzymes is considered the primary mediator responsible for the irreversible cleavage and destruction of Type II collagen?
. Matrix metalloproteinase-1 (MMP-1)
. Matrix metalloproteinase-13 (MMP-13)
. Matrix metalloproteinase-3 (MMP-3)
. Cathepsin K
. Tissue inhibitor of metalloproteinases-1 (TIMP-1)

Correct Answer & Explanation

. Matrix metalloproteinase-13 (MMP-13)

Explanation

Matrix metalloproteinase-13 (MMP-13), also known as collagenase-3, is the primary enzyme responsible for the degradation of Type II collagen in osteoarthritic articular cartilage. Although MMP-1 (collagenase-1) also degrades collagen, MMP-13 is significantly more efficient at cleaving Type II collagen and is highly overexpressed in osteoarthritic cartilage. MMP-3 (stromelysin-1) degrades non-collagenous matrix proteins and activates other MMPs. Cathepsin K is the primary enzyme used by osteoclasts to degrade Type I collagen in bone. TIMPs are inhibitors of MMPs, and their relative deficiency contributes to OA progression.

Question 7931

Topic: Biomechanics & Biomaterials
To improve the wear characteristics of total hip arthroplasty bearing surfaces, ultra-high-molecular-weight polyethylene (UHMWPE) is often subjected to gamma irradiation to induce cross-linking. While highly cross-linked polyethylene (HXLPE) demonstrates significantly reduced adhesive and abrasive wear, which of the following is an expected mechanical trade-off when compared to conventional UHMWPE?
. Increased ultimate tensile strength
. Decreased fatigue strength and fracture toughness
. Increased adhesive wear volume
. Decreased resistance to oxidative degradation prior to thermal stabilization
. Elimination of all free radicals without the need for thermal treatment

Correct Answer & Explanation

. Decreased fatigue strength and fracture toughness

Explanation

Highly cross-linked polyethylene (HXLPE) is manufactured by exposing UHMWPE to gamma or electron beam irradiation, which breaks carbon-hydrogen bonds and allows adjacent polymer chains to cross-link. This dramatically reduces adhesive and abrasive wear. However, the cross-linking process decreases the polymer's ductility, yield strength, ultimate tensile strength, fatigue strength, and fracture toughness. Additionally, irradiation creates free radicals that can lead to oxidative degradation over time; thus, thermal treatments (melting or annealing) or the addition of antioxidants (like Vitamin E) are required to quench these free radicals. Melting further reduces the mechanical properties.

Question 7932

Topic: 1. General Principles & Basic Science
Following a severe traction injury to the brachial plexus resulting in axonotmesis, the nerve segment distal to the zone of injury undergoes Wallerian degeneration. Which of the following cellular events is most critical for the successful clearance of myelin debris and the creation of an environment permissive to axonal regeneration in the peripheral nervous system?
. Phagocytosis exclusively by resident central microglia and astrocytes
. Proliferation and phagocytic activity of Schwann cells and recruited hematogenous macrophages
. Secretion of chondroitin sulfate proteoglycans by perineurial fibroblasts
. Apoptosis of oligodendrocytes and replacement by ependymal cells
. Endocytosis of myelin directly by the advancing axonal growth cone

Correct Answer & Explanation

. Proliferation and phagocytic activity of Schwann cells and recruited hematogenous macrophages

Explanation

In the peripheral nervous system (PNS), Wallerian degeneration involves the breakdown of the axon and myelin sheath distal to the site of injury. Schwann cells initially dedifferentiate, proliferate, and begin phagocytosing myelin debris. They also secrete chemokines that recruit hematogenous macrophages to assist in the rapid and efficient clearance of myelin. Once cleared, Schwann cells form the bands of Büngner to guide the regenerating axon. In contrast, in the central nervous system (CNS), myelin clearance is carried out slowly by microglia, and astrocytes form a glial scar (often secreting inhibitory chondroitin sulfate proteoglycans), which creates a non-permissive environment for regeneration. Oligodendrocytes are the myelin-producing cells of the CNS, not the PNS.

Question 7933

Topic: Biomechanics & Biomaterials
A 65-year-old active male undergoes a total hip arthroplasty using a highly cross-linked polyethylene (HXLPE) liner. While cross-linking polyethylene significantly improves its wear characteristics, the process of irradiation and subsequent thermal treatment alters other mechanical properties. Compared to conventional ultra-high-molecular-weight polyethylene (UHMWPE), which of the following is an expected biomechanical trade-off of using HXLPE?
. Decreased resistance to adhesive wear
. Increased yield strength
. Decreased ultimate tensile strength
. Increased oxidation potential
. Increased fatigue crack propagation resistance

Correct Answer & Explanation

. Decreased ultimate tensile strength

Explanation

Highly cross-linked polyethylene is manufactured by exposing UHMWPE to gamma or electron beam irradiation, creating free radicals that bond to form cross-links. While this significantly decreases adhesive and abrasive wear, it negatively impacts other mechanical properties. The necessary thermal treatments (melting or annealing) used to extinguish residual free radicals decrease the material's crystallinity. This results in decreased ultimate tensile strength, yield strength, and fracture toughness (fatigue crack propagation resistance).

Question 7934

Topic: Biology, Genetics & Bone Healing

A 72-year-old female with severe osteoporosis is treated with denosumab, a monoclonal antibody that targets RANKL. By blocking RANKL, this medication prevents the activation and maturation of osteoclasts. What natively produced physiological molecule performs an identical biological function by acting as a decoy receptor for RANKL?

. Osteocalcin
. Osteoprotegerin
. Macrophage colony-stimulating factor (M-CSF)
. Sclerostin
. Cathepsin K

Correct Answer & Explanation

. Osteoprotegerin

Explanation

Osteoprotegerin (OPG) is a glycoprotein naturally secreted by osteoblasts that acts as a soluble decoy receptor for RANKL. By binding to RANKL, OPG prevents RANKL from interacting with the RANK receptor on osteoclast precursors, thereby inhibiting osteoclastogenesis and reducing bone resorption. Denosumab mimics this exact mechanism therapeutically. Sclerostin is an inhibitor of the Wnt pathway. M-CSF promotes osteoclast precursor survival. Cathepsin K is an enzyme secreted by mature osteoclasts to degrade bone matrix.

Question 7935

Topic: Biomechanics & Biomaterials

Stress shielding and subsequent bone resorption around a femoral stem are related to the difference in the modulus of elasticity between the metallic implant and the host bone. Cortical bone has a modulus of elasticity of approximately 15-20 GPa. Which of the following standard orthopedic implant materials has a modulus of elasticity that most closely approximates that of native cortical bone?

. Alumina ceramic
. Cobalt-chromium alloy
. Stainless steel
. Titanium alloy
. Zirconia ceramic

Correct Answer & Explanation

. Titanium alloy

Explanation

The modulus of elasticity (Young's modulus) represents a material's stiffness. Cortical bone has a modulus of roughly 15-20 GPa. Titanium alloy is roughly 100-110 GPa, which is the closest of the commonly used bulk structural metals. Stainless steel has a modulus of roughly 200 GPa, and Cobalt-chromium alloy is approximately 220-240 GPa. Ceramics like alumina and zirconia are much stiffer (>300 GPa). Because titanium is closest to cortical bone, it typically generates less stress shielding than stainless steel or cobalt-chromium.

Question 7936

Topic: Biology, Genetics & Bone Healing

A surgeon utilizes recombinant human bone morphogenetic protein-2 (rhBMP-2) on an absorbable collagen sponge to augment an anterior lumbar interbody fusion. Upon BMP-2 binding to its specific cell surface serine/threonine kinase receptors on mesenchymal stem cells, which of the following intracellular signaling pathways is primarily activated to promote osteoblastic differentiation?

. Wnt/beta-catenin pathway
. JAK/STAT pathway
. SMAD 1/5/8 pathway
. MAP kinase pathway
. Cyclic AMP/Protein kinase A pathway

Correct Answer & Explanation

. SMAD 1/5/8 pathway

Explanation

Bone morphogenetic proteins (BMPs) belong to the TGF-beta superfamily. When a BMP ligand binds to its transmembrane serine/threonine kinase receptor complex, it phosphorylates receptor-regulated SMADs, specifically SMAD 1, 5, and 8. These phosphorylated SMADs then form a complex with the co-SMAD (SMAD 4) and translocate to the nucleus to regulate the transcription of osteogenic genes like Runx2. The Wnt pathway uses beta-catenin.

Question 7937

Topic: Biomechanics & Biomaterials

A neonate with a clubfoot deformity undergoes weekly manipulations and applications of a Ponseti cast. The cast holds the foot in a constantly deformed, corrected position. Over the course of the week, the tension felt within the contracted medial soft tissues gradually decreases without any further manipulation. Which biomechanical property of viscoelastic materials does this clinical phenomenon best represent?

. Creep
. Stress relaxation
. Hysteresis
. Fatigue failure
. Plastic deformation

Correct Answer & Explanation

. Stress relaxation

Explanation

Stress relaxation is a classic viscoelastic property defined as a decrease in stress (tension or internal force) over time when a material is held under a constant deformation (constant strain). This is exactly what happens when a cast is applied holding a joint in a static position. Creep, in contrast, is the progressive deformation of a material over time when subjected to a constant load (stress). Hysteresis is the loss of energy (usually as heat) between the loading and unloading cycles.

Question 7938

Topic: Biomechanics & Biomaterials

A patient who previously underwent total hip arthroplasty with a cobalt-chromium femoral head impacted onto a titanium alloy stem presents with groin pain and an expanding pseudotumor. Retrieval analysis of the explanted components demonstrates significant material degradation, pitting, and black debris exclusively at the modular head-neck taper junction. Which of the following is the primary initiating mode of corrosion responsible for this phenomenon?

. Galvanic corrosion
. Uniform attack corrosion
. Fretting corrosion
. Pitting corrosion
. Intergranular corrosion

Correct Answer & Explanation

. Fretting corrosion

Explanation

The failure at the modular head-neck taper in total hip arthroplasty (often termed trunnionosis) is primarily initiated by fretting corrosion. Micro-motion at the taper junction physically disrupts the protective passivation (oxide) layer on the metal surfaces (fretting). Once this layer is breached, local changes in the micro-environment (depletion of oxygen, drop in pH) lead to mechanically assisted crevice corrosion (MACC). While galvanic potential exists between differing metals, fretting is recognized as the necessary inciting mechanical event.

Question 7939

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory, the phenotype of tissue that differentiates within a fracture gap is strictly dependent on the local interfragmentary strain. In order for primary (direct) bone healing with Haversian remodeling to occur without the intermediate formation of a visible fracture callus, the absolute interfragmentary strain must be kept below what critical threshold?

. 2%
. 10%
. 15%
. 50%
. 100%

Correct Answer & Explanation

. 2%

Explanation

Perren's strain theory states that tissues will not form if the local strain exceeds the maximum strain that tissue can withstand before rupturing. Granulation tissue tolerates up to 100% strain, fibrous tissue up to 17%, and fibrocartilage up to 10%. Bone tissue is highly brittle and can only withstand <2% strain. Therefore, primary (direct) bone healing—which bypasses callus formation—requires rigid fixation that maintains interfragmentary strain below 2%.

Question 7940

Topic: 1. General Principles & Basic Science
Tranexamic acid (TXA) is routinely administered either intravenously or topically during total joint arthroplasty to reduce perioperative blood loss and the need for transfusions. Which of the following best describes the specific molecular mechanism of action of this pharmacological agent?
. Direct inhibition of Factor Xa
. Reversible binding to the antithrombin III receptor
. Competitive inhibition of plasminogen activation
. Irreversible inhibition of cyclooxygenase-1
. Activation of tissue factor pathway inhibitor

Correct Answer & Explanation

. Competitive inhibition of plasminogen activation

Explanation

Tranexamic acid (TXA) is a synthetic analog of the amino acid lysine. It functions as an antifibrinolytic by reversibly binding to the lysine receptor sites on plasminogen. This action competitively inhibits the activation of plasminogen into the active enzyme plasmin by tissue plasminogen activator (tPA). Because plasmin is responsible for the degradation of fibrin clots, TXA effectively stabilizes the fibrin matrix and prevents early clot breakdown.

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