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1. General Principles & Basic Science MCQs

Practice Set 380 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 7581

Topic: Physiology & Rehabilitation

During the rehabilitation of an anterior cruciate ligament reconstruction, closed kinetic chain exercises frequently emphasize eccentric muscle contractions. At the cellular level, what occurs within the skeletal muscle fibers during a purely eccentric contraction?

. They shorten while generating maximum tension.
. They actively lengthen while generating tension.
. They maintain a constant length while generating tension.
. They exhibit a decrease in tension as shortening velocity increases.
. The I bands shorten while the A bands remain the same length.

Correct Answer & Explanation

. They shorten while generating maximum tension.

Explanation

During an eccentric contraction, the muscle actively generates tension while the overall muscle-tendon unit is lengthening. This happens when the external force applied to the muscle exceeds the internal force generated by the cross-bridges. Eccentric contractions are capable of generating the highest amount of force compared to isometric or concentric contractions, but they are also most strongly associated with ultrastructural muscle damage and delayed-onset muscle soreness (DOMS). Concentric contraction involves shortening, while isometric involves no change in length.

Question 7582

Topic: Biology, Genetics & Bone Healing

Bone morphogenetic proteins (BMPs) are potent osteoinductive growth factors utilized in spine fusion and fracture nonunions. Upon binding to their specific cell-surface serine/threonine kinase receptors, which primary intracellular signaling pathway is activated to promote osteoblast differentiation?

. Wnt/beta-catenin
. JAK/STAT
. Smad
. Notch
. Hedgehog

Correct Answer & Explanation

. Wnt/beta-catenin

Explanation

BMPs are members of the transforming growth factor-beta (TGF-beta) superfamily. When BMP binds to its heterodimeric cell-surface serine/threonine kinase receptor, it triggers the phosphorylation of intracellular receptor-regulated Smad proteins (primarily Smad1, Smad5, and Smad8). These phosphorylated Smads then form a complex with the co-Smad (Smad4), translocate into the nucleus, and regulate the transcription of target genes essential for osteoblast differentiation, such as Runx2. The Wnt pathway uses beta-catenin.

Question 7583

Topic: 1. General Principles & Basic Science

A 45-year-old man undergoes open reduction and internal fixation of a tibial plateau fracture. He is prescribed high-dose nonsteroidal anti-inflammatory drugs (NSAIDs) for postoperative pain. If his fracture exhibits delayed healing, this is primarily mediated by the NSAID-induced suppression of which specific molecule critical to early bone repair?

. Thromboxane A2
. Prostaglandin E2 (PGE2)
. Interleukin-1 (IL-1)
. Tumor necrosis factor-alpha (TNF-alpha)
. Leukotriene B4

Correct Answer & Explanation

. Thromboxane A2

Explanation

NSAIDs exert their effects by inhibiting cyclooxygenase (COX-1 and COX-2) enzymes, thereby blocking the synthesis of prostaglandins from arachidonic acid. In bone healing, Prostaglandin E2 (PGE2) produced predominantly by COX-2 during the early inflammatory phase is crucial. PGE2 stimulates osteoblast differentiation, regulates osteoclast activity for remodeling, and promotes angiogenesis. Suppression of PGE2 by NSAIDs can disrupt this delicate balance and potentially impair or delay fracture healing.

Question 7584

Topic: 1. General Principles & Basic Science

A researcher is evaluating articular cartilage biopsies from a patient with early osteoarthritis. When examining normal, healthy articular cartilage as a control, which of the following best describes the collagen fibril orientation and chondrocyte morphology in the superficial (tangential) zone?

. Collagen fibrils perpendicular to the joint surface, spherical chondrocytes
. Randomly oriented collagen fibrils, hypertrophic chondrocytes
. Collagen fibrils parallel to the joint surface, flattened chondrocytes
. Collagen fibrils parallel to the joint surface, spherical chondrocytes
. Collagen fibrils perpendicular to the joint surface, flattened chondrocytes

Correct Answer & Explanation

. Collagen fibrils perpendicular to the joint surface, spherical chondrocytes

Explanation

The superficial (tangential) zone of articular cartilage comprises 10% to 20% of the articular cartilage thickness. In this zone, collagen fibrils (primarily type II collagen) are oriented parallel to the articular surface to resist shear forces. The chondrocytes are flattened and synthesize a high concentration of collagen and a low concentration of proteoglycans. The deep zone features collagen fibrils oriented perpendicular to the joint surface.

Question 7585

Topic: 1. General Principles & Basic Science

Tranexamic acid (TXA) is widely incorporated into protocols for total joint arthroplasty to reduce perioperative blood loss. Which of the following best describes the precise mechanism of action of systemic tranexamic acid?

. Direct inhibition of factor Xa in the coagulation cascade
. Activation of antithrombin III leading to indirect inhibition of thrombin
. Synthetic analogue of the amino acid lysine that reversibly binds to plasminogen
. Irreversible inhibition of cyclooxygenase-1 (COX-1) in platelets
. Irreversible binding to the adenosine diphosphate (ADP) receptor on platelets

Correct Answer & Explanation

. Direct inhibition of factor Xa in the coagulation cascade

Explanation

Tranexamic acid (TXA) is an antifibrinolytic agent. It is a synthetic analogue of the amino acid lysine. It functions by competitively and reversibly binding to the lysine receptor sites on plasminogen, preventing plasminogen from binding to fibrin and activating into plasmin. By blocking plasmin formation, TXA inhibits the breakdown of fibrin clots (fibrinolysis), significantly reducing surgical blood loss.

Question 7586

Topic: Biomechanics & Biomaterials

A 60-year-old active female is scheduled for a total hip arthroplasty. To minimize the risk of future particle-induced osteolysis, the surgeon selects a highly cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) liner. Which of the following is the most notable mechanical trade-off associated with the increased cross-linking of UHMWPE?

. Increased volumetric wear rate
. Increased susceptibility to oxidation in vivo
. Decreased elastic modulus
. Increased generation of submicron wear particles
. Decreased fatigue strength and fracture toughness

Correct Answer & Explanation

. Increased volumetric wear rate

Explanation

Highly cross-linked UHMWPE is manufactured by exposing the polymer to gamma or electron beam irradiation, creating chemical bonds (cross-links) between adjacent polymer chains. This markedly reduces adhesive and abrasive wear. However, the cross-linking process decreases the polymer's bulk mechanical properties, most notably reducing its fatigue strength, fracture toughness, and ultimate tensile strength, potentially making thin components more susceptible to rim failure or fracture.

Question 7587

Topic: Infection, Pharmacology & VTE

A 65-year-old man undergoes a two-stage revision for a periprosthetic joint infection of the hip. Operative tissue cultures grow Staphylococcus epidermidis. Which of the following virulence factors is primarily responsible for allowing this organism to persistently adhere to the orthopedic implant and evade host immune responses?

. Endotoxin production
. Spore formation
. Intracellular replication within macrophages
. Polysaccharide glycocalyx (biofilm) production
. Release of hyaluronidase

Correct Answer & Explanation

. Endotoxin production

Explanation

Staphylococcus epidermidis is a leading cause of implant-associated periprosthetic joint infections. Its primary virulence mechanism is the ability to form a dense biofilm on the surface of foreign biomaterials. This biofilm consists of an extracellular polymeric substance, primarily a polysaccharide glycocalyx, which physically shields the bacteria from systemic antibiotics and host immune cells (such as macrophages and neutrophils).

Question 7588

Topic: 1. General Principles & Basic Science

A 28-year-old carpenter lacerates his index finger flexor digitorum profundus (FDP) tendon in zone II. During the primary repair, the surgeon focuses on early active motion protocols. Which of the following biomechanical adjustments to the repair technique will provide the greatest increase in tensile strength to the repair site?

. Decreasing the core suture purchase length to less than 0.5 cm
. Increasing the number of core suture strands crossing the repair site
. Omitting the epitendinous suture
. Using a smaller caliber core suture
. Placing the knot on the palmar surface of the tendon

Correct Answer & Explanation

. Decreasing the core suture purchase length to less than 0.5 cm

Explanation

The ultimate tensile strength of a flexor tendon repair is directly proportional to the number of core suture strands that cross the repair site (e.g., a 4-strand repair is significantly stronger than a 2-strand repair). An epitendinous suture adds an additional 10% to 50% strength and improves the biomechanical profile by reducing gapping. A core suture purchase length of 0.7 to 1.0 cm is recommended for optimal pullout strength.

Question 7589

Topic: Biomechanics & Biomaterials

During a lecture on joint biomechanics, an attending asks a resident to describe the mechanism that protects the articular cartilage of the knee from wear when a patient stands up from a chair (a condition of high load and low relative motion). Which of the following forms of joint lubrication is primarily responsible for preventing cartilage-to-cartilage contact in this specific scenario?

. Hydrodynamic lubrication
. Boundary lubrication
. Elastohydrodynamic lubrication
. Weeping lubrication
. Squeeze-film lubrication

Correct Answer & Explanation

. Hydrodynamic lubrication

Explanation

Boundary lubrication is the primary protective mechanism under conditions of high compressive load and low relative sliding speeds (such as standing from a seated position). It relies on a monolayer of molecules, principally lubricin (PRG4) and hyaluronic acid, that adhere to the articular surfaces to reduce friction and prevent direct solid-to-solid contact. Hydrodynamic and elastohydrodynamic lubrication mechanisms are fluid-film based and require relative motion (speed) to drag fluid into the gap between surfaces.

Question 7590

Topic: Biology, Genetics & Bone Healing

A 65-year-old woman is prescribed romosozumab for severe osteoporosis. This medication primarily increases bone mineral density through which of the following cellular mechanisms?

. Inhibition of RANK ligand
. Activation of the Wnt/beta-catenin pathway
. Direct stimulation of parathyroid hormone receptors
. Inhibition of osteoclast proton pumps
. Neutralization of Dickkopf-1 (DKK1)

Correct Answer & Explanation

. Inhibition of RANK ligand

Explanation

Romosozumab is a monoclonal antibody that binds and inhibits sclerostin. Sclerostin is an extracellular Wnt antagonist produced by osteocytes. By inhibiting sclerostin, romosozumab promotes Wnt binding to LRP5/6 receptors, thereby activating the canonical Wnt/beta-catenin signaling pathway, which enhances osteoblast differentiation and bone formation.

Question 7591

Topic: Biomechanics & Biomaterials

In total hip arthroplasty, the use of highly cross-linked polyethylene (HXLPE) has significantly reduced the incidence of osteolysis compared to conventional ultra-high molecular weight polyethylene (UHMWPE). Which of the following manufacturing steps is primarily responsible for reducing the oxidative degradation of HXLPE?

. Gamma irradiation in a vacuum
. Electron beam irradiation at room temperature
. Remelting or annealing after irradiation
. Ethylene oxide sterilization
. Addition of barium sulfate

Correct Answer & Explanation

. Gamma irradiation in a vacuum

Explanation

The cross-linking process (via gamma or electron beam irradiation) breaks carbon-hydrogen bonds, creating free radicals. These free radicals can react with oxygen, leading to oxidative degradation and premature wear in vivo. To eliminate or reduce these free radicals, the polyethylene is thermally treated through either remelting (heating above the melting point) or annealing (heating below the melting point) after irradiation.

Question 7592

Topic: 1. General Principles & Basic Science

Which of the following modifications to a cortical screw design will most significantly increase its pullout strength?

. Increasing the core diameter
. Decreasing the thread pitch
. Decreasing the outer diameter
. Increasing the shaft length
. Switching from a buttress thread to a V-thread

Correct Answer & Explanation

. Increasing the core diameter

Explanation

The pullout strength of a screw is proportional to the outer (major) diameter, the length of thread engagement, and inversely proportional to the thread pitch. Decreasing the thread pitch (i.e., making the threads closer together) increases the number of threads engaged in the cortical bone, thereby increasing the pullout strength. Increasing the core diameter primarily increases the torsional and bending strength of the screw but decreases the thread depth (if outer diameter is constant), potentially reducing pullout strength.

Question 7593

Topic: 1. General Principles & Basic Science

Aggrecan is the most abundant proteoglycan in articular cartilage and provides the tissue with its ability to resist compressive loads. Which of the following glycosaminoglycans is the primary side chain attached to the aggrecan core protein?

. Chondroitin sulfate
. Hyaluronic acid
. Dermatan sulfate
. Heparan sulfate
. Keratan sulfate

Correct Answer & Explanation

. Chondroitin sulfate

Explanation

Aggrecan is a large aggregating proteoglycan characterized by a core protein with numerous covalently attached glycosaminoglycan (GAG) chains. The predominant GAG side chain is chondroitin sulfate (which makes up about 90% of the GAGs on aggrecan), along with a smaller amount of keratan sulfate. These aggrecan monomers non-covalently bind to a hyaluronic acid backbone via link proteins to form large proteoglycan aggregates that draw water into the cartilage extracellular matrix.

Question 7594

Topic: 1. General Principles & Basic Science

During skeletal muscle contraction, what is the primary role of the calcium released from the sarcoplasmic reticulum?

. It binds to tropomyosin to expose myosin-binding sites on actin
. It binds to troponin C, causing a conformational change that moves tropomyosin
. It activates myosin light chain kinase
. It binds to the myosin head, directly causing the power stroke
. It facilitates the reuptake of acetylcholine into the presynaptic terminal

Correct Answer & Explanation

. It binds to tropomyosin to expose myosin-binding sites on actin

Explanation

In skeletal muscle contraction, action potentials trigger the release of calcium from the sarcoplasmic reticulum. Calcium binds to troponin C, inducing a conformational change in the troponin complex. This change pulls tropomyosin away from the myosin-binding sites on the actin filament, allowing the myosin heads to bind to actin and initiate the cross-bridge cycle.

Question 7595

Topic: Biology, Genetics & Bone Healing

Bone morphogenetic proteins (BMPs) belong to the TGF-beta superfamily and play a critical role in osteoinduction. Which of the following intracellular signaling molecules is directly phosphorylated and activated by the BMP receptor upon ligand binding?

. STAT3
. MAPK
. Smad 1/5/8
. Beta-catenin
. NF-kB

Correct Answer & Explanation

. STAT3

Explanation

BMPs signal primarily through the canonical Smad pathway. When a BMP binds to its serine/threonine kinase receptor, the receptor phosphorylates and activates receptor-regulated Smads (R-Smads), specifically Smad 1, 5, and 8. These activated R-Smads then form a complex with the common-partner Smad (Co-Smad), Smad 4, and translocate to the nucleus to regulate the transcription of osteogenic target genes. In contrast, TGF-beta primarily signals through Smad 2 and 3.

Question 7596

Topic: Infection, Pharmacology & VTE

Staphylococcus aureus is the most common organism associated with periprosthetic joint infections. Which of the following phases of biofilm formation is characterized by the production of extracellular polymeric substances (EPS) and decreased metabolic activity of bacteria deep within the structure?

. Reversible attachment
. Irreversible attachment
. Maturation
. Dispersion
. Planktonic phase

Correct Answer & Explanation

. Reversible attachment

Explanation

Biofilm formation occurs in several distinct phases: 1) Reversible attachment of planktonic bacteria to the surface, 2) Irreversible attachment mediated by adhesins, 3) Maturation, characterized by cell proliferation, intercellular communication (quorum sensing), and the production of a protective extracellular polymeric substance (EPS) matrix. In the maturation phase, bacteria deep within the biofilm enter a stationary, low-metabolic dormant state, making them highly resistant to both antibiotics and the host immune system, and 4) Dispersion, where bacteria detach to colonize new sites.

Question 7597

Topic: Biology, Genetics & Bone Healing

A diaphyseal femur fracture is stabilized with a locked intramedullary nail. Which of the following modes of bone healing is expected to predominate?

. Primary bone healing through direct Haversian remodeling
. Secondary bone healing through callus formation
. Membranous ossification without an intermediate cartilage phase
. Creeping substitution
. Osteonal cutting cone formation

Correct Answer & Explanation

. Primary bone healing through direct Haversian remodeling

Explanation

Intramedullary nailing provides relative stability (as opposed to absolute stability achieved with compression plating). Relative stability allows for a small amount of interfragmentary motion (micromotion), which stimulates secondary bone healing. Secondary healing is characterized by the formation of a soft cartilaginous callus that eventually undergoes endochondral ossification to form a hard bony callus. Primary bone healing (direct Haversian remodeling with cutting cones) occurs only under conditions of absolute stability and anatomic reduction.

Question 7598

Topic: Surgical Anatomy & Approaches

During a surgical exposure of the anterior shoulder, a retractor is placed inferiorly and inadvertently compresses a nerve structure. Postoperatively, the patient demonstrates isolated weakness in internal rotation of the humerus with completely intact sensation in the upper extremity. Which of the following nerves was most likely injured?

. Musculocutaneous nerve
. Axillary nerve
. Lower subscapular nerve
. Thoracodorsal nerve
. Suprascapular nerve

Correct Answer & Explanation

. Musculocutaneous nerve

Explanation

The subscapularis muscle is innervated by the upper and lower subscapular nerves, both of which arise from the posterior cord of the brachial plexus. The lower subscapular nerve also innervates the teres major muscle. An injury to the subscapular nerves would result in isolated weakness of internal rotation of the humerus. Because these are purely motor nerves, sensation remains intact. Axillary nerve injury would result in deltoid/teres minor weakness and sensory deficits over the lateral shoulder.

Question 7599

Topic: Biomechanics & Biomaterials

A patient who underwent a revision total hip arthroplasty with a mixed-metal system (e.g., cobalt-chromium head on a titanium stem) is at risk for galvanic corrosion. Which of the following describes the primary mechanism of this process?

. Mechanical wear leading to particulate debris and third-body wear
. Release of free radicals from ultra-high-molecular-weight polyethylene
. Electrochemical potential difference between two dissimilar metals in an electrolytic solution
. Fatigue failure of the titanium stem due to mismatch in elastic modulus
. Biological reaction to metallic ions leading to aggressive local osteolysis

Correct Answer & Explanation

. Mechanical wear leading to particulate debris and third-body wear

Explanation

Galvanic corrosion occurs when two dissimilar metals are in direct physical contact within an electrolytic solution (such as body fluids). The metal with the lower electrochemical potential acts as the anode and corrodes, releasing ions, while the more noble metal acts as the cathode. This can lead to mechanically assisted crevice corrosion and adverse local tissue reactions.

Question 7600

Topic: Biology, Genetics & Bone Healing

Osteoclasts resorb bone by creating an acidic environment in the sealed zone. Which of the following enzymes is primarily responsible for generating the hydrogen ions required for this process?

. Alkaline phosphatase
. Tartrate-resistant acid phosphatase (TRAP)
. Carbonic anhydrase II
. Cathepsin K
. Matrix metalloproteinase-9 (MMP-9)

Correct Answer & Explanation

. Alkaline phosphatase

Explanation

Carbonic anhydrase II catalyzes the intracellular conversion of carbon dioxide and water into carbonic acid, which then dissociates into hydrogen ions and bicarbonate. The hydrogen ions are pumped into the resorption pit by a V-type H+-ATPase to dissolve the inorganic bone mineral. Cathepsin K and MMPs are subsequently responsible for degrading the remaining organic matrix.

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