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1. General Principles & Basic Science MCQs

Practice Set 335 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 6681

Topic: 1. General Principles & Basic Science

Parathyroid hormone (PTH) plays a critical role in maintaining serum calcium homeostasis by acting on bone, the kidneys, and the intestines. Which of the following represents a direct physiological effect of PTH on the kidneys?

. Decreased calcium reabsorption in the distal convoluted tubule
. Increased phosphate reabsorption in the proximal convoluted tubule
. Decreased 1-alpha-hydroxylase enzyme activity
. Increased phosphate excretion in the proximal convoluted tubule
. Increased active excretion of 1,25-dihydroxyvitamin D

Correct Answer & Explanation

. Decreased calcium reabsorption in the distal convoluted tubule

Explanation

In the kidney, PTH stimulates calcium reabsorption in the distal tubule and potently inhibits phosphate reabsorption in the proximal tubule, leading to phosphaturia. This prevents the formation of calcium-phosphate complexes in the blood, effectively raising ionized serum calcium levels.

Question 6682

Topic: Biomechanics & Biomaterials

In the stress-strain curve of cortical bone, the point at which the material begins to undergo permanent deformation without a significant increase in load is known as the:

. Proportional limit
. Yield point
. Ultimate failure point
. Elastic limit
. Fracture point

Correct Answer & Explanation

. Proportional limit

Explanation

The yield point marks the transition from elastic (reversible) to plastic (irreversible) deformation. Past this point, the bone undergoes permanent structural changes before ultimate failure.

Question 6683

Topic: Biomechanics & Biomaterials

Ligaments and tendons exhibit time-dependent mechanical properties. The phenomenon where a tissue experiences a decrease in stress over time when held at a constant strain is known as:

. Creep
. Hysteresis
. Stress relaxation
. Fatigue
. Isotropy

Correct Answer & Explanation

. Creep

Explanation

Stress relaxation is a viscoelastic property where the stress decreases over time when a material is held at a constant strain. Creep is the increase in strain over time under a constant load.

Question 6684

Topic: Biomechanics & Biomaterials

Which of the following best describes the mechanical difference between cortical and cancellous bone?

. Cortical bone is more ductile and fails at a higher strain.
. Cancellous bone has a higher modulus of elasticity.
. Cancellous bone can absorb more energy before failure.
. Cortical bone is more porous and tolerates higher strain.
. Cancellous bone undergoes brittle failure at low strain.

Correct Answer & Explanation

. Cortical bone is more ductile and fails at a higher strain.

Explanation

Cancellous bone is highly porous and less stiff (lower elastic modulus) than cortical bone, making it more compliant. It can sustain higher strains before failure and absorbs more energy before fracture compared to cortical bone.

Question 6685

Topic: Biology, Genetics & Bone Healing

Sclerostin is a protein that plays a key role in bone remodeling by inhibiting bone formation. Which of the following cells primarily secretes sclerostin?

. Osteoblasts
. Osteoclasts
. Osteocytes
. Chondrocytes
. Mesenchymal stem cells

Correct Answer & Explanation

. Osteoblasts

Explanation

Sclerostin is primarily secreted by mature osteocytes embedded in the bone matrix. It inhibits bone formation by antagonizing the Wnt/beta-catenin signaling pathway in osteoblasts.

Question 6686

Topic: Biology, Genetics & Bone Healing

Denosumab is used in the treatment of osteoporosis and giant cell tumors of bone. It acts by directly binding to and inhibiting which of the following?

. RANK
. RANKL
. Osteoprotegerin (OPG)
. Macrophage colony-stimulating factor (M-CSF)
. Cathepsin K

Correct Answer & Explanation

. RANK

Explanation

Denosumab is a monoclonal antibody that mimics OPG by directly binding to RANKL. This prevents RANKL from binding to RANK on osteoclasts, thereby inhibiting osteoclast maturation and function.

Question 6687

Topic: Biology, Genetics & Bone Healing

Which of the following local factors is considered the most critical initiator of the fracture healing cascade immediately following injury and hematoma formation?

. Bone morphogenetic protein-2 (BMP-2)
. Transforming growth factor-beta (TGF-beta)
. Platelet-derived growth factor (PDGF)
. Vascular endothelial growth factor (VEGF)
. Insulin-like growth factor (IGF)

Correct Answer & Explanation

. Bone morphogenetic protein-2 (BMP-2)

Explanation

Following the initial hematoma formation, PDGF is released by degranulating platelets. It acts as a potent early chemoattractant for macrophages and mesenchymal stem cells, initiating the inflammatory phase.

Question 6688

Topic: Biology, Genetics & Bone Healing

Demineralized bone matrix (DBM) primarily provides which of the following properties to aid in bone healing?

. Osteogenesis and osteoinduction
. Osteoinduction and osteoconduction
. Osteoconduction only
. Osteogenesis only
. Osteogenesis and osteoconduction

Correct Answer & Explanation

. Osteogenesis and osteoinduction

Explanation

DBM is prepared by acid extraction of allograft bone, removing the mineral phase but retaining the collagen matrix and growth factors like BMPs. Thus, it provides both osteoinduction and osteoconduction, but lacks live cells.

Question 6689

Topic: Biomechanics & Biomaterials

Mixing stainless steel and titanium implants in the same anatomical region is generally discouraged due to the risk of galvanic corrosion. Which of the following factors primarily drives this process?

. Differences in the modulus of elasticity
. Mechanical fretting between the metals
. Differences in their electrochemical potentials
. Rejection by the host immune system
. Incompatibility of their passivation layers

Correct Answer & Explanation

. Differences in the modulus of elasticity

Explanation

Galvanic corrosion occurs when two dissimilar metals with different electrochemical potentials are placed in contact within a conductive fluid. This leads to accelerated corrosion of the more anodic metal.

Question 6690

Topic: Biology, Genetics & Bone Healing

In the classical pathway of vitamin D metabolism, which of the following enzymes is responsible for the conversion of 25-hydroxyvitamin D to its most active form, 1,25-dihydroxyvitamin D?

. 25-hydroxylase in the liver
. 1-alpha-hydroxylase in the kidney
. 24-hydroxylase in the kidney
. Alkaline phosphatase in the bone
. 7-dehydrocholesterol reductase in the skin

Correct Answer & Explanation

. 25-hydroxylase in the liver

Explanation

The final step in the synthesis of the active form of vitamin D occurs in the kidneys. The enzyme 1-alpha-hydroxylase converts 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D, a process stimulated by PTH.

Question 6691

Topic: Biomechanics & Biomaterials

Cortical bone exhibits different mechanical properties depending on the direction of the applied load. This characteristic is known as:

. Viscoelasticity
. Isotropy
. Anisotropy
. Plasticity
. Ductility

Correct Answer & Explanation

. Viscoelasticity

Explanation

Anisotropy refers to the property of a material having different mechanical properties in different directions. Cortical bone is highly anisotropic, being stronger in compression along its longitudinal axis than in tension or shear.

Question 6692

Topic: Biomechanics & Biomaterials

In biomaterials science, the stress level below which an implant material can undergo an infinite number of loading cycles without failing is termed the:

. Yield strength
. Endurance limit
. Ultimate tensile strength
. Proportional limit
. Fracture toughness

Correct Answer & Explanation

. Yield strength

Explanation

The endurance limit (or fatigue limit) is the maximum stress amplitude below which a material can endure an infinite number of repeated load cycles without exhibiting fatigue failure.

Question 6693

Topic: 1. General Principles & Basic Science

A 10-year-old child presents with bleeding gums, petechiae, and bone pain. Radiographs reveal a Pelkan spur. The underlying pathophysiology involves a defect in the synthesis of which of the following?

. Type I collagen due to deficient hydroxylation of proline and lysine
. Type II collagen due to defective glycosylation
. Hydroxyapatite due to calcium deficiency
. Osteocalcin due to vitamin K deficiency
. Proteoglycans due to lack of sulfation

Correct Answer & Explanation

. Type I collagen due to deficient hydroxylation of proline and lysine

Explanation

Scurvy is caused by Vitamin C deficiency, which is a required cofactor for prolyl and lysyl hydroxylase. This leads to defective hydroxylation of proline and lysine residues, resulting in weak, unstable Type I collagen.

Question 6694

Topic: 1. General Principles & Basic Science

The pullout strength of a cortical screw is directly proportional to which of the following parameters?

. Outer (major) diameter of the screw
. Core (minor) diameter of the screw
. Length of the screw head
. Thread pitch
. Screw insertion speed

Correct Answer & Explanation

. Outer (major) diameter of the screw

Explanation

The pullout strength of a screw is directly proportional to the outer (major) diameter, the length of thread engagement, and the shear strength of the bone. It is inversely proportional to the thread pitch.

Question 6695

Topic: Biology, Genetics & Bone Healing

Parathyroid hormone (PTH) maintains serum calcium levels through several mechanisms. Which of the following is a direct physiologic effect of PTH?

. Increased intestinal absorption of calcium
. Decreased renal reabsorption of calcium
. Increased osteoclast activity via direct binding to osteoclasts
. Increased renal reabsorption of calcium and excretion of phosphate
. Inhibition of 1-alpha-hydroxylase in the kidney

Correct Answer & Explanation

. Increased intestinal absorption of calcium

Explanation

PTH directly increases renal tubular reabsorption of calcium and decreases reabsorption of phosphate. It stimulates osteoclasts indirectly by binding to osteoblasts, and increases intestinal calcium absorption indirectly via Vitamin D activation.

Question 6696

Topic: Biology, Genetics & Bone Healing

The primary defect in Paget's disease of bone is characterized by which of the following cellular abnormalities?

. Decreased osteoblast proliferation
. Abnormal, large, multinucleated osteoclasts
. Defective mineralization of osteoid by osteoblasts
. Absence of osteocytes within lacunae
. Overproduction of poorly cross-linked collagen

Correct Answer & Explanation

. Decreased osteoblast proliferation

Explanation

The initial phase of Paget's disease is characterized by intense osteoclastic bone resorption mediated by abnormal, giant, multinucleated osteoclasts. This is later followed by a disorganized, excessive bone formation phase.

Question 6697

Topic: Biology, Genetics & Bone Healing

Which of the following is the hallmark histological finding in osteomalacia?

. Woven bone interspersed with lamellar bone
. Thickened, unmineralized osteoid seams
. Empty osteocyte lacunae
. Howship's lacunae filled with osteoclasts
. Disorganized chondrocyte columns in the physis

Correct Answer & Explanation

. Woven bone interspersed with lamellar bone

Explanation

Osteomalacia is characterized by defective mineralization of newly formed osteoid. This leads to abnormally wide and thickened unmineralized osteoid seams on histological examination.

Question 6698

Topic: Biomechanics & Biomaterials

Which of the following best describes the process of fretting corrosion in orthopedic implants?

. Corrosion due to different electrochemical potentials of two metals
. Corrosion occurring in oxygen-depleted crevices, such as screw holes
. Micromotion between two metal surfaces disrupting the protective oxide layer
. Degradation of ultra-high-molecular-weight polyethylene due to oxidation
. Fatigue failure of a metal implant due to cyclic loading

Correct Answer & Explanation

. Corrosion due to different electrochemical potentials of two metals

Explanation

Fretting corrosion is the mechanical wearing away of a metal's passive oxide layer due to micromotion between two components (e.g., between a screw head and a plate). This exposes the reactive metal to the corrosive body fluid.

Question 6699

Topic: Biomechanics & Biomaterials

The compressive stiffness of articular cartilage is primarily provided by the interaction between which two matrix components?

. Type II collagen and proteoglycans
. Type I collagen and hydroxyapatite
. Elastin and aggrecan
. Fibronectin and integrins
. Type IX collagen and decorin

Correct Answer & Explanation

. Type II collagen and proteoglycans

Explanation

The biomechanical properties of articular cartilage rely on the interaction between the Type II collagen network (tensile strength) and hydrophilic proteoglycans like aggrecan (which swell with water to resist compressive loads).

Question 6700

Topic: Biology, Genetics & Bone Healing

Which of the following cytokines is secreted by osteoblasts to directly inhibit osteoclastogenesis?

. RANKL
. Osteoprotegerin (OPG)
. Macrophage colony-stimulating factor (M-CSF)
. Interleukin-1 (IL-1)
. Tumor necrosis factor-alpha (TNF-a)

Correct Answer & Explanation

. RANKL

Explanation

Osteoprotegerin (OPG) is a decoy receptor produced by osteoblasts that binds to RANKL, preventing it from interacting with RANK on osteoclast precursors, thereby inhibiting osteoclastogenesis.

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