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1. General Principles & Basic Science MCQs

Practice Set 272 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 5421

Topic: Biomechanics & Biomaterials
In total joint arthroplasty, highly cross-linked ultra-high molecular weight polyethylene (UHMWPE) is commonly used. What is the primary intended effect of increasing the cross-linking of UHMWPE, and what is a known negative biomechanical consequence?
. Decreased wear rate; decreased fracture toughness
. Decreased wear rate; increased oxidation potential
. Increased fatigue strength; decreased wear rate
. Increased elastic modulus; increased fracture toughness
. Decreased oxidation potential; decreased yield strength

Correct Answer & Explanation

. Decreased wear rate; decreased fracture toughness

Explanation

Highly cross-linked polyethylene significantly reduces adhesive and abrasive wear rates in joint replacements. However, this cross-linking process reduces the material's fracture toughness and ductility, making it more susceptible to fatigue crack propagation.

Question 5422

Topic: Biology, Genetics & Bone Healing

A 68-year-old woman is prescribed alendronate for the treatment of severe osteoporosis. Nitrogen-containing bisphosphonates inhibit bone resorption primarily by blocking which of the following enzymes in the osteoclast?

. Cathepsin K
. Carbonic anhydrase II
. Farnesyl pyrophosphate synthase
. Tartrate-resistant acid phosphatase
. Matrix metalloproteinase-9

Correct Answer & Explanation

. Farnesyl pyrophosphate synthase

Explanation

Nitrogen-containing bisphosphonates inhibit farnesyl pyrophosphate synthase in the mevalonate pathway. This prevents the prenylation of small GTPases essential for osteoclast function and survival, ultimately leading to osteoclast apoptosis.

Question 5423

Topic: Biology, Genetics & Bone Healing

During the incorporation of a massive structural cortical bone allograft, the process of creeping substitution is initiated. Which of the following biological events must occur first to allow creeping substitution to proceed?

. Osteoclastic resorption of the superficial lamellae
. Osteoblastic deposition of woven bone
. Vascular invasion into the graft haversian canals
. Formation of a fibrocartilage intermediate
. Remodeling to secondary osteons

Correct Answer & Explanation

. Vascular invasion into the graft haversian canals

Explanation

The incorporation of a cortical bone allograft begins with host vascular invasion into the existing haversian systems of the graft. This revascularization is requisite for the subsequent influx of osteoclasts to resorb dead bone, followed by osteoblastic bone formation.

Question 5424

Topic: Biology, Genetics & Bone Healing

Sclerostin is a key regulatory protein in bone metabolism and is the target of the monoclonal antibody romosozumab. Sclerostin exerts its primary osteo-regulatory effect by inhibiting which of the following intracellular signaling pathways?

. RANKL/RANK
. Wnt/beta-catenin
. Bone morphogenetic protein (BMP)/Smad
. Fibroblast growth factor (FGF)
. Notch signaling

Correct Answer & Explanation

. Wnt/beta-catenin

Explanation

Sclerostin is produced by osteocytes and binds to LRP5/6 receptors on osteoblasts to inhibit the canonical Wnt/beta-catenin signaling pathway. Inhibition of this pathway decreases osteoblastic differentiation and bone formation.

Question 5425

Topic: Biology, Genetics & Bone Healing

Recombinant human bone morphogenetic protein-2 (rhBMP-2) is utilized to enhance bone healing in specific fusion environments. BMPs transmit their osteoinductive intracellular signals primarily through which of the following secondary messengers?

. Cyclic AMP (cAMP)
. Inositol triphosphate (IP3)
. Smad proteins
. Beta-catenin
. Protein kinase C (PKC)

Correct Answer & Explanation

. Smad proteins

Explanation

BMPs bind to cell surface serine/threonine kinase receptors, which subsequently phosphorylate intracellular Smad proteins (typically Smad 1, 5, and 8). The resulting Smad complex translocates to the nucleus to initiate the transcription of osteogenic genes.

Question 5426

Topic: Biomechanics & Biomaterials

On a stress-strain curve representing a typical human tendon, a non-linear "toe region" is observed at very low strains. This specific region primarily represents which of the following microscopic events?

. Failure of cross-links between collagen fibrils
. Microscopic tearing of individual collagen fibers
. Plastic deformation of the collagen matrix
. Uncrimping of the resting wavy collagen fibers
. Water exudation from the proteoglycan matrix

Correct Answer & Explanation

. Uncrimping of the resting wavy collagen fibers

Explanation

The "toe region" of the tendon stress-strain curve represents the initial straightening out, or "uncrimping," of the wavy collagen fibers as tension is applied. Once the fibers are fully straightened, the curve enters the steeper, linear elastic region.

Question 5427

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of fracture healing, the mechanical environment dictates the type of tissue that forms in a fracture gap. Which type of tissue will predominantly form if the interfragmentary strain is between 2% and 10%?

. Lamellar bone
. Woven bone
. Fibrocartilage
. Granulation tissue
. Hyaline cartilage

Correct Answer & Explanation

. Fibrocartilage

Explanation

According to Perren's strain theory, lamellar bone forms only under very low strain (<2%). When the interfragmentary strain is intermediate (between 2% and 10%), fibrocartilage (soft callus) formation is tolerated and promotes secondary fracture healing.

Question 5428

Topic: Biomechanics & Biomaterials

Polymethylmethacrylate (PMMA) bone cement is widely used in arthroplasty for implant fixation. The primary mechanism by which PMMA provides stability to the implant is best described as:

. Chemical covalent bonding to the titanium oxide layer
. Osteointegration into the micro-porous cement surface
. Mechanical interlock with the interstices of cancellous bone
. Cross-linking with host bone collagen fibers
. Adhesive chemical bonding to bone hydroxyapatite

Correct Answer & Explanation

. Mechanical interlock with the interstices of cancellous bone

Explanation

PMMA bone cement does not possess adhesive properties nor does it chemically bond to host bone or metal implants. It functions essentially as a grout, providing rigid fixation via an intricate mechanical interlock by penetrating the trabecular interstices of cancellous bone.

Question 5429

Topic: Biology, Genetics & Bone Healing

A 55-year-old patient with end-stage chronic kidney disease presents with hypocalcemia, osteomalacia, and secondary hyperparathyroidism. This metabolic bone disease is primarily driven by a deficiency in which of the following steps of vitamin D metabolism?

. Intestinal absorption of dietary cholecalciferol
. Hepatic conversion of vitamin D to 25-hydroxyvitamin D
. Renal conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D
. Cutaneous UV conversion of 7-dehydrocholesterol
. Nuclear binding of 1,25-dihydroxyvitamin D to the VDR receptor

Correct Answer & Explanation

. Renal conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D

Explanation

In severe chronic kidney disease, the kidneys have a drastically reduced capacity to produce the enzyme 1-alpha-hydroxylase. This leads to an inability to convert 25-hydroxyvitamin D into its active form, 1,25-dihydroxyvitamin D, resulting in poor calcium absorption and secondary hyperparathyroidism.

Question 5430

Topic: Biomechanics & Biomaterials

During a complex revision trauma surgery, a surgeon considers using a stainless steel screw to secure a cobalt-chromium plate. This practice is contraindicated due to the risk of accelerated degradation of the less noble metal when bathed in host body fluids. This phenomenon is termed:

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

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Galvanic corrosion occurs when two dissimilar metals are in direct physical contact within an electrolytic environment, such as human tissue fluid. The less noble metal acts as an anode and undergoes accelerated oxidative degradation.

Question 5431

Topic: Biomechanics & Biomaterials

Cortical bone exhibits distinct mechanical properties depending on the direction of the applied load. For instance, it is significantly stronger in longitudinal compression than in transverse tension. This structural biomechanical property is known as:

. Viscoelasticity
. Isotropy
. Anisotropy
. Creep
. Biphasic behavior

Correct Answer & Explanation

. Anisotropy

Explanation

Anisotropy describes a material that exhibits different mechanical properties depending on the direction of the applied load. Cortical bone is highly anisotropic because its osteonal structure aligns to optimally resist physiological longitudinal compressive loads.

Question 5432

Topic: Biology, Genetics & Bone Healing

During the reparative phase of secondary fracture healing, the soft fibrocartilaginous callus is gradually replaced by a hard, bony callus. This specific tissue transition occurs primarily via which of the following biological mechanisms?

. Intramembranous ossification
. Endochondral ossification
. Creeping substitution
. Appositional bone growth
. Osteonal remodeling

Correct Answer & Explanation

. Endochondral ossification

Explanation

Secondary fracture healing proceeds through the formation of a cartilage intermediate known as a soft callus. This cartilaginous soft callus is subsequently converted into a bony hard callus primarily through the process of endochondral ossification.

Question 5433

Topic: Biology, Genetics & Bone Healing

Osteoprotegerin (OPG) plays a critical role in maintaining bone mass and preventing excessive resorption. What is the primary molecular function of OPG within the bone remodeling microenvironment?

. It acts as a soluble decoy receptor for RANKL, preventing osteoclast activation
. It binds directly to the RANK receptor to stimulate osteoclast apoptosis
. It acts as a nuclear transcription factor upregulating osteocalcin production
. It enhances the targeted fusion of mononuclear osteoclast precursors
. It inhibits the paracellular transport of calcium across the gut epithelium

Correct Answer & Explanation

. It acts as a soluble decoy receptor for RANKL, preventing osteoclast activation

Explanation

Osteoprotegerin (OPG) is secreted by osteoblasts and functions as a soluble decoy receptor that binds to RANKL. By sequestering RANKL, OPG prevents it from interacting with the RANK receptor on osteoclast precursors, thereby halting osteoclast differentiation and activity.

Question 5434

Topic: 1. General Principles & Basic Science

Which of the following noncollagenous proteins is the most abundant in the mature extracellular bone matrix, is highly dependent on vitamin K for its synthesis, and regulates mineralization by binding directly to calcium?

. Osteopontin
. Bone sialoprotein
. Osteocalcin
. Fibronectin
. Decorin

Correct Answer & Explanation

. Osteocalcin

Explanation

Osteocalcin is the most abundant noncollagenous protein in bone and is synthesized exclusively by mature osteoblasts. Its production requires vitamin K-dependent gamma-carboxylation, which allows it to strongly bind calcium and regulate hydroxyapatite crystal growth.

Question 5435

Topic: Biology, Genetics & Bone Healing

A 72-year-old woman with severe osteoporosis and a recent vertebral compression fracture is being evaluated for teriparatide therapy. Which of the following elements of her medical history represents an absolute contraindication to the use of this medication?

. A history of deep vein thrombosis
. A prior history of radiation therapy to the skeleton
. Concurrent daily use of calcium and vitamin D supplements
. A history of an insufficiency fracture of the distal radius
. A calculated glomerular filtration rate of 65 mL/min

Correct Answer & Explanation

. A prior history of radiation therapy to the skeleton

Explanation

Teriparatide is an anabolic recombinant parathyroid hormone (PTH) analog. It carries a black box warning and is strictly contraindicated in patients with an increased baseline risk of osteosarcoma, which includes a history of prior skeletal radiation therapy or Paget's disease.

Question 5436

Topic: 1. General Principles & Basic Science
The remarkable compressive stiffness and load-bearing capacity of articular cartilage are primarily generated by which of the following molecular interactions within its extracellular matrix?
. Covalent cross-linking between adjacent type I collagen fibers
. Electrostatic repulsion of negatively charged glycosaminoglycans
. Hydrophobic interactions among randomly coiled elastin fibers
. Mechanical interlocking of type III collagen and matrix fibronectin
. Covalent cross-linking of aggrecan monomers by COMP molecules

Correct Answer & Explanation

. Electrostatic repulsion of negatively charged glycosaminoglycans

Explanation

Articular cartilage achieves its profound compressive stiffness largely due to the high density of negatively charged sulfate and carboxyl groups on glycosaminoglycans (GAGs). These fixed negative charges create strong electrostatic repulsion and a high Donnan osmotic swelling pressure that fiercely resists compression.

Question 5437

Topic: 1. General Principles & Basic Science

Under high-load, low-velocity conditions (such as sustained standing), boundary lubrication is the predominant mode of tribological protection in a healthy synovial joint. Which of the following molecules is primarily responsible for mediating boundary lubrication at the articular surface?

. Hyaluronic acid
. Lubricin (Proteoglycan 4)
. Aggrecan
. Type II collagen
. Chondroitin sulfate

Correct Answer & Explanation

. Lubricin (Proteoglycan 4)

Explanation

Lubricin, also known as proteoglycan 4 (PRG4), is a mucinous glycoprotein synthesized by superficial zone chondrocytes and synovial cells. It binds directly to the articular cartilage surface, providing essential boundary lubrication that dramatically reduces friction under high-load, low-speed conditions.

Question 5438

Topic: Biomechanics & Biomaterials
Highly cross-linked ultra-high molecular weight polyethylene (UHMWPE) is frequently used in total joint arthroplasty to reduce wear debris. What mechanical property is most significantly decreased as a direct result of increasing the cross-linking in UHMWPE?
. Young's modulus
. Yield strength
. Fatigue crack propagation resistance
. Surface hardness
. Compressive strength

Correct Answer & Explanation

. Fatigue crack propagation resistance

Explanation

While highly cross-linking UHMWPE significantly reduces adhesive and abrasive wear, it simultaneously decreases its ductility, toughness, and resistance to fatigue crack propagation. This makes it more susceptible to fracture in high-stress applications or thin liners.

Question 5439

Topic: Biology, Genetics & Bone Healing

A 65-year-old woman is prescribed denosumab for osteoporosis. This medication works by inhibiting bone resorption through a mechanism that most closely mimics which of the following endogenous molecules?

. Receptor activator of nuclear factor kappa-B (RANK)
. Osteoprotegerin (OPG)
. Receptor activator of nuclear factor kappa-B ligand (RANKL)
. Parathyroid hormone (PTH)
. Sclerostin

Correct Answer & Explanation

. Receptor activator of nuclear factor kappa-B ligand (RANKL)

Explanation

Denosumab is a monoclonal antibody that binds to RANKL, preventing it from interacting with the RANK receptor on osteoclasts. This mechanism effectively mimics the action of endogenous osteoprotegerin (OPG), which acts as a decoy receptor for RANKL.

Question 5440

Topic: 1. General Principles & Basic Science

A massive structural cortical allograft is used for reconstruction after tumor resection. During the process of creeping substitution, at what postoperative time frame does the graft typically experience its maximum mechanical weakness?

. 1 to 2 months
. 6 to 12 months
. 18 to 24 months
. 36 to 48 months
. It continuously strengthens from the time of implantation

Correct Answer & Explanation

. 6 to 12 months

Explanation

Structural cortical allografts undergo creeping substitution, where osteoclastic resorption precedes osteoblastic bone formation. This resorptive phase causes an increase in porosity, resulting in maximum mechanical weakness between 6 to 12 months post-implantation, increasing the risk of fracture.

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