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1. General Principles & Basic Science MCQs

Practice Set 595 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 11881

Topic: Biology, Genetics & Bone Healing

A 6-year-old child presents with bowed legs and a waddling gait. Radiographs show widening and cupping of the physes. Laboratory studies reveal low serum calcium, low phosphorus, and elevated parathyroid hormone (PTH). The primary defect in the most common heritable form of this disease involves:

. Inability to 1-alpha-hydroxylate 25-hydroxyvitamin D
. Defect in the PHEX gene leading to renal phosphate wasting
. Defect in the calcium-sensing receptor
. Vitamin C deficiency affecting collagen cross-linking
. Autoantibodies against the parathyroid gland

Correct Answer & Explanation

. Inability to 1-alpha-hydroxylate 25-hydroxyvitamin D

Explanation

The presentation describes rickets. The most common heritable form is X-linked hypophosphatemic rickets, caused by a mutation in the PHEX gene leading to excess FGF23 and subsequent renal phosphate wasting.

Question 11882

Topic: Surgical Anatomy & Approaches

A patient sustains a mid-shaft humerus fracture resulting in a radial nerve palsy. Electromyography at 4 weeks shows fibrillation potentials in the brachioradialis, but the nerve sheath is presumed intact. According to Seddon's classification, this injury is best categorized as:

. Neuropraxia
. Axonotmesis
. Neurotmesis
. Sunderland Grade I
. Sunderland Grade V

Correct Answer & Explanation

. Neuropraxia

Explanation

Axonotmesis involves disruption of the axon and myelin sheath but preservation of the epineurium, perineurium, and endoneurium. Fibrillation potentials on EMG indicate denervation (axonal disruption), distinguishing it from neuropraxia.

Question 11883

Topic: Biology, Genetics & Bone Healing

Denosumab is a monoclonal antibody used in the treatment of osteoporosis and giant cell tumor of bone. Which of the following is its specific molecular target?

. Cathepsin K
. RANK receptor
. RANKL (Receptor Activator of Nuclear factor Kappa-B Ligand)
. Osteoprotegerin (OPG)
. Sclerostin

Correct Answer & Explanation

. Cathepsin K

Explanation

Denosumab specifically binds to and inhibits RANKL, preventing it from interacting with the RANK receptor on osteoclasts and their precursors. This directly inhibits osteoclast formation, function, and survival.

Question 11884

Topic: 1. General Principles & Basic Science
A 22-year-old tall, thin male presents with a spontaneous pneumothorax and is noted to have arachnodactyly and a high arched palate. If he requires orthopedic surgery, he is at highest risk for complications related to a mutation affecting which of the following proteins?
. Type I collagen
. Type III collagen
. Fibrillin-1
. Elastin
. Fibroblast growth factor receptor 3

Correct Answer & Explanation

. Fibrillin-1

Explanation

The patient exhibits classic signs of Marfan syndrome, an autosomal dominant disorder caused by a mutation in the FBN1 gene encoding fibrillin-1. This structural defect leads to abnormalities in connective tissue elasticity and strength, particularly affecting the aorta and skeletal system.

Question 11885

Topic: Physiology & Rehabilitation

During a drop jump exercise, the quadriceps muscle actively lengthens as the person lands to absorb the impact. What type of muscle contraction is this?

. Isotonic concentric
. Isometric
. Isokinetic
. Eccentric
. Tetanic

Correct Answer & Explanation

. Isotonic concentric

Explanation

An eccentric contraction occurs when a muscle generates force while actively lengthening. This is commonly seen in decelerating movements, such as the quadriceps acting to control knee flexion upon landing.

Question 11886

Topic: 1. General Principles & Basic Science

Calcium sulfate (Plaster of Paris) is sometimes used as a bone void filler. Which of the following describes its most significant limitation in orthopedic applications?

. Severe exothermic reaction upon curing
. High risk of eliciting an immune response
. Rapid resorption rate exceeding the rate of new bone formation
. High incidence of disease transmission
. Inhibits osteoclastic activity

Correct Answer & Explanation

. Severe exothermic reaction upon curing

Explanation

Calcium sulfate is highly osteoconductive but has a very rapid resorption rate, often dissolving within 4 to 8 weeks. This rapid dissolution can outpace the ingrowth of new bone and may lead to persistent serous wound drainage.

Question 11887

Topic: Biomechanics & Biomaterials

When evaluating the material properties of a new orthopedic implant, the provided load-deformation data is converted to a stress-strain curve.

The slope of the linear portion of this curve is a measure of the material's:

. Toughness
. Yield strength
. Ultimate tensile strength
. Stiffness (Modulus of Elasticity)
. Ductility

Correct Answer & Explanation

. Toughness

Explanation

The slope of the linear (elastic) portion of the stress-strain curve represents the Modulus of Elasticity (Young's modulus), which is a measure of the material's stiffness. Toughness is the total area under the curve. Ductility is the amount of plastic deformation a material can undergo before failure.

Question 11888

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of fracture healing, primary bone healing (direct Haversian remodeling) can only occur if the interfragmentary strain is below which of the following thresholds?

. 2%
. 10%
. 20%
. 50%
. 100%

Correct Answer & Explanation

. 2%

Explanation

Perren's strain theory states that the type of tissue that forms in a fracture gap is dictated by the interfragmentary strain. Tissues cannot form if the strain exceeds their tolerance. Woven bone can form at strains up to 2% (allowing for primary bone healing if stable). Fibrocartilage can tolerate up to 10% strain (secondary healing), and granulation tissue can tolerate up to 100% strain.

Question 11889

Topic: 1. General Principles & Basic Science

Osteoclasts resorb bone by creating a highly acidic microenvironment at the ruffled border. Which of the following enzymes or transporters is directly responsible for actively secreting hydrogen ions into the resorption pit?

. Carbonic anhydrase II
. Vacuolar H+-ATPase
. Cathepsin K
. Tartrate-resistant acid phosphatase
. Chloride channel 7 (ClC-7)

Correct Answer & Explanation

. Carbonic anhydrase II

Explanation

The vacuolar H+-ATPase (V-ATPase) pump actively transports hydrogen ions across the ruffled border into the resorption lacuna, lowering the pH to dissolve hydroxyapatite. Carbonic anhydrase II generates the intracellular protons from CO2 and water. Cathepsin K and TRAP degrade the organic matrix after demineralization has occurred.

Question 11890

Topic: Biomechanics & Biomaterials

A 65-year-old patient undergoes revision of a total hip arthroplasty due to local tissue reaction. The retrieved implant demonstrates significant surface degradation at the modular head-neck junction.

Which of the following corrosion mechanisms is primarily driven by micro-motion between two closely apposed metal surfaces disrupting the protective oxide layer?

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

Correct Answer & Explanation

. Galvanic corrosion

Explanation

Fretting corrosion occurs when small-amplitude, oscillatory motion (micro-motion) between two loaded surfaces mechanically disrupts the protective oxide (passivation) layer, leading to accelerated chemical degradation. It is a classic failure mode for modular junctions, such as head-neck tapers in total hip arthroplasty.

Question 11891

Topic: 1. General Principles & Basic Science

In normal articular hyaline cartilage, the structural arrangement of collagen fibers and proteoglycan content vary by depth. Which of the following best describes the properties of the superficial (tangential) zone compared to the deeper zones?

. Highest proteoglycan content, collagen fibers oriented perpendicular to the surface
. Lowest proteoglycan content, collagen fibers oriented parallel to the surface
. Highest water content, collagen fibers oriented perpendicular to the surface
. Lowest water content, collagen fibers oriented parallel to the surface
. Highest proteoglycan content, collagen fibers randomly oriented

Correct Answer & Explanation

. Highest proteoglycan content, collagen fibers oriented perpendicular to the surface

Explanation

The superficial (tangential) zone of articular cartilage has the highest water content and the lowest proteoglycan content. The densely packed collagen fibers in this zone are oriented parallel to the joint surface to maximally resist shear stresses during joint motion.

Question 11892

Topic: 1. General Principles & Basic Science

Which of the following biochemical changes accurately differentiates the normal physiological aging of articular cartilage from the pathologic changes seen in early osteoarthritis?

. Water content increases in aging but decreases in osteoarthritis.
. Keratan sulfate concentration decreases in aging but increases in osteoarthritis.
. Chondroitin sulfate concentration increases in aging but decreases in osteoarthritis.
. Water content decreases in normal aging, whereas it significantly increases in early osteoarthritis.
. Collagen content significantly increases in osteoarthritis compared to normal aging.

Correct Answer & Explanation

. Water content increases in aging but decreases in osteoarthritis.

Explanation

In normal aging of articular cartilage, the tissue becomes somewhat dehydrated (decreased water content), total proteoglycans decrease slightly, chondroitin sulfate decreases, and keratan sulfate increases. In early osteoarthritis, the collagen meshwork is damaged, allowing the tissue to swell, resulting in an increased water content, accompanied by a significant drop in total proteoglycan content.

Question 11893

Topic: Biology, Genetics & Bone Healing

Bone Morphogenetic Proteins (BMPs) play a critical role in osteoinduction during fracture healing. BMPs bind to serine/threonine kinase receptors on the cell membrane. Which intracellular signaling molecules are directly phosphorylated to transduce this osteogenic signal to the nucleus?

. JAK/STAT
. Smad 1, 5, and 8
. Smad 2 and 3
. Beta-catenin
. MAPK/ERK

Correct Answer & Explanation

. JAK/STAT

Explanation

BMPs signal primarily through the canonical Smad pathway. Binding to the receptor leads to phosphorylation of receptor-regulated Smads (R-Smads) 1, 5, and 8. These form a complex with the common-partner Smad (Co-Smad 4) and translocate to the nucleus to regulate target genes like RUNX2. Smad 2 and 3 are primarily activated by the TGF-beta pathway.

Question 11894

Topic: Biology, Genetics & Bone Healing

The normal direct insertion of a tendon into bone (enthesis) demonstrates a gradual transition of tissue types to minimize stress risers and dissipate load. From the tendon substance advancing toward the bone, what is the correct sequence of these histological zones?

. Tendon, mineralized fibrocartilage, unmineralized fibrocartilage, bone
. Tendon, unmineralized fibrocartilage, mineralized fibrocartilage, bone
. Tendon, loose connective tissue, mineralized fibrocartilage, bone
. Tendon, unmineralized fibrocartilage, tidemark, woven bone
. Tendon, Sharpey's fibers, hyaline cartilage, bone

Correct Answer & Explanation

. Tendon, mineralized fibrocartilage, unmineralized fibrocartilage, bone

Explanation

A direct enthesis consists of four distinct transitional zones: 1) Tendon proper, 2) Unmineralized fibrocartilage, 3) Mineralized fibrocartilage, and 4) Bone. A basophilic line called the tidemark separates the unmineralized and mineralized fibrocartilage zones.

Question 11895

Topic: Infection, Pharmacology & VTE
A patient is placed on oral apixaban for VTE prophylaxis following a total knee arthroplasty. At which specific point in the coagulation cascade does this medication exert its primary inhibitory effect?
. Inhibition of thrombin (Factor IIa) directly
. Inhibition of the activation of Factor X to Factor Xa
. Direct inhibition of Factor Xa
. Binding to antithrombin III to enhance its activity
. Vitamin K antagonism preventing carboxylation of Factors II, VII, IX, and X

Correct Answer & Explanation

. Direct inhibition of Factor Xa

Explanation

Apixaban and rivaroxaban are direct, highly selective inhibitors of Factor Xa. They do not require antithrombin III for their antithrombotic activity. Dabigatran is a direct thrombin (IIa) inhibitor. Warfarin acts as a Vitamin K antagonist. Heparin and LMWH exert their effects by enhancing antithrombin III activity.

Question 11896

Topic: 1. General Principles & Basic Science
Osteolysis following total joint arthroplasty is initiated by the generation of particulate wear debris. Which cell type is primarily responsible for phagocytosing ultra-high-molecular-weight polyethylene (UHMWPE) wear particles and subsequently releasing pro-inflammatory cytokines like TNF-alpha and IL-1?
. Osteoblast
. Osteoclast
. Macrophage
. Polymorphonuclear leukocyte
. T-lymphocyte

Correct Answer & Explanation

. Macrophage

Explanation

Macrophages are the primary effector cells in particle-induced osteolysis. They phagocytose submicron UHMWPE wear particles and become activated, releasing a cascade of inflammatory cytokines (TNF-alpha, IL-1, IL-6). These cytokines stimulate osteoclast differentiation and activity (via the RANK/RANKL pathway), ultimately leading to periprosthetic bone resorption.

Question 11897

Topic: Biology, Genetics & Bone Healing

During cortical bone remodeling, basic multicellular units (BMUs) form a characteristic 'cutting cone'. Which of the following best describes the spatial cellular arrangement and function within this advancing BMU?

. Osteoblasts at the leading edge resorbing bone, followed by osteoclasts forming new lamellar bone
. Osteoclasts at the leading edge resorbing bone, followed by osteoblasts forming new lamellar bone
. Osteocytes releasing enzymes to resorb bone, followed by mesenchymal stem cells forming woven bone
. Macrophages removing dead bone, followed by chondrocytes forming a cartilage template
. Osteoblasts at the leading edge, followed directly by mineralization via matrix vesicles

Correct Answer & Explanation

. Osteoblasts at the leading edge resorbing bone, followed by osteoclasts forming new lamellar bone

Explanation

In cortical bone remodeling, the basic multicellular unit (BMU) travels longitudinally through the bone. Osteoclasts are situated at the leading edge (the 'cutting cone'), resorbing bone to create a tunnel. They are trailed by a capillary loop and osteoblasts in the 'closing cone', which lay down new osteoid in concentric layers to form a new Haversian system (osteon).

Question 11898

Topic: Biomechanics & Biomaterials

A surgeon places a self-retaining retractor to spread the soft tissues during an exposure. Initially, a high force is required to hold the tissues apart, but over time, the force required to maintain the retractor at that exact same width decreases. This phenomenon is a biomechanical property of viscoelastic materials known as:

. Creep
. Stress relaxation
. Hysteresis
. Fatigue failure
. Isotropy

Correct Answer & Explanation

. Creep

Explanation

Stress relaxation is the property of a viscoelastic material where the internal stress (force) decreases over time when the material is held at a constant strain (constant deformation). Creep, conversely, is the continuous deformation (increasing strain) of a material over time when subjected to a constant load (constant stress).

Question 11899

Topic: Biology, Genetics & Bone Healing

A 4-year-old child presents with gingival bleeding, petechiae, and metaphyseal changes on radiographs. The underlying biochemical defect involves impaired hydroxylation of proline and lysine residues in collagen. The deficient essential cofactor in this condition is required for the function of which enzyme?

. Lysyl oxidase
. Procollagen peptidase
. Prolyl hydroxylase
. Collagenase
. Elastase

Correct Answer & Explanation

. Lysyl oxidase

Explanation

Scurvy is caused by a deficiency in Vitamin C (ascorbic acid), which acts as an essential electron donor (cofactor) for prolyl hydroxylase and lysyl hydroxylase. These enzymes are required for the intracellular hydroxylation of proline and lysine, a critical step that allows the formation of stable collagen triple helices. Lysyl oxidase (which cross-links extracellular collagen) requires copper as a cofactor.

Question 11900

Topic: Biomechanics & Biomaterials

The torsional rigidity of a long bone or an intramedullary nail is heavily dependent on its cross-sectional geometry. For a hollow cylinder, the polar moment of inertia is proportional to which of the following geometric parameters?

. The difference between the outer radius squared and inner radius squared
. The outer radius cubed
. The difference between the outer radius cubed and inner radius cubed
. The difference between the outer radius to the fourth power and inner radius to the fourth power
. The square root of the outer radius

Correct Answer & Explanation

. The difference between the outer radius squared and inner radius squared

Explanation

The polar moment of inertia (J) dictates a structure's resistance to torsional deformation. For a hollow cylinder (such as the diaphysis of a long bone or a cannulated nail), J is proportional to the difference between the outer radius to the fourth power and the inner radius to the fourth power (r_outer^4 - r_inner^4). Therefore, even a small increase in the outer diameter exponentially increases torsional strength.

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