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1. General Principles & Basic Science MCQs

Practice Set 691 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 13801

Topic: Biology, Genetics & Bone Healing

Which of the following molecules is secreted by osteoblasts to act as a decoy receptor, preventing RANKL from binding to its target and thereby decreasing osteoclastogenesis?

. Macrophage colony-stimulating factor (M-CSF)
. Osteoprotegerin (OPG)
. Cathepsin K
. Sclerostin
. Runt-related transcription factor 2 (Runx2)

Correct Answer & Explanation

. Osteoprotegerin (OPG)

Explanation

Osteoprotegerin (OPG) is a decoy receptor produced by osteoblasts. It binds directly to RANKL (Receptor Activator of Nuclear factor Kappa-B Ligand), thereby preventing RANKL from interacting with the RANK receptor on osteoclast precursors. This prevents the differentiation and activation of osteoclasts, effectively inhibiting bone resorption. M-CSF is required for early osteoclast differentiation. Sclerostin inhibits the Wnt signaling pathway, decreasing bone formation. Cathepsin K is an enzyme secreted by active osteoclasts to degrade bone matrix.

Question 13802

Topic: Biomechanics & Biomaterials
A surgeon is evaluating implant options for a total hip arthroplasty. Which method of processing ultra-high molecular weight polyethylene (UHMWPE) maximizes its wear resistance while minimizing the risk of long-term in vivo oxidative degradation?
. Gamma irradiation in air without thermal treatment
. Ethylene oxide gas sterilization
. Gamma irradiation in a vacuum followed by remelting
. Autoclaving at 120°C
. Gamma irradiation in an inert gas followed by annealing

Correct Answer & Explanation

. Gamma irradiation in a vacuum followed by remelting

Explanation

Gamma irradiation creates cross-links in UHMWPE, which significantly improves wear resistance. However, irradiation also produces free radicals. If these free radicals are exposed to oxygen in vivo, they cause oxidative degradation, leading to embrittlement and catastrophic failure of the polyethylene. Heating the polyethylene above its melting point (remelting) after irradiation quenches these free radicals, preventing oxidation, although it slightly decreases the material's yield strength and fatigue resistance compared to annealing. Annealing (heating below the melting point) leaves residual free radicals.

Question 13803

Topic: Biomechanics & Biomaterials

When optimizing a cortical screw for fixation in diaphyseal bone, which of the following design modifications will most significantly increase its pullout strength?

. Increasing the inner (root) diameter
. Changing the material from titanium alloy to 316L stainless steel
. Decreasing the thread pitch
. Decreasing the outer (thread) diameter
. Increasing the screw head diameter

Correct Answer & Explanation

. Decreasing the thread pitch

Explanation

Screw pullout strength in bone is directly proportional to the outer (thread) diameter and the length of thread engagement, and it is inversely proportional to the thread pitch. Decreasing the thread pitch means there are more threads per unit length, which increases the amount of bone engaged and therefore increases pullout strength. Increasing the inner (root) diameter increases the screw's bending stiffness and resistance to fatigue failure, but does not increase pullout strength.

Question 13804

Topic: 1. General Principles & Basic Science

In normal articular cartilage, which zone is characterized by the highest water content, the lowest proteoglycan concentration, and collagen fibers oriented parallel to the joint surface?

. Superficial (tangential) zone
. Middle (transitional) zone
. Deep (radial) zone
. Calcified cartilage zone
. Tidemark

Correct Answer & Explanation

. Superficial (tangential) zone

Explanation

The superficial (tangential) zone of articular cartilage contains the highest proportion of water (up to 80%) and the highest concentration of collagen. The Type II collagen fibers are arranged parallel to the articular surface to resist shear forces. This zone also contains the lowest concentration of proteoglycans. Conversely, the deep zone has the lowest water content, highest proteoglycan content, and collagen fibers oriented perpendicular to the surface.

Question 13805

Topic: Biology, Genetics & Bone Healing

A 45-year-old woman with a history of Roux-en-Y gastric bypass 10 years ago presents with diffuse bone pain and bilateral groin aching. Radiographs reveal bilateral Looser zones in the femoral necks. Which of the following laboratory profiles is most consistent with her underlying metabolic bone disease?

. High Calcium, Low Phosphate, Elevated Alkaline Phosphatase, Elevated PTH
. Low Calcium, Low Phosphate, Elevated Alkaline Phosphatase, Elevated PTH
. Normal Calcium, Normal Phosphate, Elevated Alkaline Phosphatase, Normal PTH
. Low Calcium, High Phosphate, Elevated Alkaline Phosphatase, Elevated PTH
. Normal Calcium, High Phosphate, Normal Alkaline Phosphatase, Low PTH

Correct Answer & Explanation

. Low Calcium, Low Phosphate, Elevated Alkaline Phosphatase, Elevated PTH

Explanation

The patient has osteomalacia secondary to Vitamin D deficiency caused by malabsorption from her gastric bypass. Vitamin D deficiency leads to decreased intestinal calcium and phosphate absorption. This resulting hypocalcemia stimulates secondary hyperparathyroidism (elevated PTH). PTH acts to normalize calcium by increasing bone resorption and renal calcium reabsorption, but it causes renal phosphate wasting, leading to hypophosphatemia. Alkaline phosphatase is elevated due to increased, albeit defective, bone turnover (unmineralized osteoid).

Question 13806

Topic: Surgical Anatomy & Approaches

A patient sustained a closed midshaft humerus fracture 4 months ago and has a persistent radial nerve palsy. Electromyography (EMG) shows fibrillation potentials in the brachioradialis. Surgical exploration reveals the radial nerve is in continuity but contains a dense, focal neuroma. Resection of the neuroma and nerve grafting is planned. This injury best corresponds to which of the following Sunderland classifications?

. First-degree
. Second-degree
. Third-degree
. Fourth-degree
. Fifth-degree

Correct Answer & Explanation

. Fourth-degree

Explanation

A Sunderland fourth-degree nerve injury involves disruption of the axon, endoneurium, and perineurium; only the epineurium remains intact. Clinically, the nerve is in continuity but contains dense scar tissue (a neuroma-in-continuity) that mechanically blocks axonal regeneration. Because spontaneous recovery will not occur across this scar, excision and grafting are required. First-degree is neuropraxia. Second-degree is axonotmesis (intact endoneurium). Third-degree involves loss of endoneurium with intact perineurium. Fifth-degree is complete nerve transection.

Question 13807

Topic: Physiology & Rehabilitation

During the normal human gait cycle, at which phase does the tibialis anterior muscle reach its absolute peak of electrical activity?

. Initial contact (heel strike) to loading response
. Mid-stance
. Terminal stance (heel off)
. Pre-swing (toe off)
. Mid-swing

Correct Answer & Explanation

. Initial contact (heel strike) to loading response

Explanation

The tibialis anterior has two peaks of activity during the gait cycle. Its maximum peak occurs during initial contact (heel strike) to loading response, where it contracts eccentrically to decelerate plantar flexion, preventing the foot from slapping against the ground. The second, smaller peak occurs during the initial and mid-swing phases, where it contracts concentrically to dorsiflex the ankle and clear the toes.

Question 13808

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of bone healing, what is the maximum tissue strain environment under which primary lamellar bone formation can occur across a fracture gap?

. Less than 2%
. 2% to 10%
. 10% to 30%
. 30% to 50%
. Greater than 50%

Correct Answer & Explanation

. Less than 2%

Explanation

Perren's strain theory states that a tissue can only form under strain conditions that are less than its ultimate elongation to failure. Lamellar bone is extremely rigid and can only form when the strain across the fracture gap is less than 2%. Woven bone can form in strain environments up to roughly 10%. Fibrocartilage and cartilage can tolerate strains up to 10-30%, and granulation tissue can tolerate strains up to 100%.

Question 13809

Topic: Biomechanics & Biomaterials

A ligament is subjected to a constant, sustained load over a period of time, resulting in a progressive increase in deformation. Which of the following biomechanical properties does this describe?

. Stress relaxation
. Creep
. Hysteresis
. Fatigue
. Isotropy

Correct Answer & Explanation

. Creep

Explanation

Creep is the progressive deformation of a viscoelastic material under a constant load over time. Stress relaxation, in contrast, is the decrease in stress over time when the material is held at a constant strain.

Question 13810

Topic: Biology, Genetics & Bone Healing

A 32-year-old female presents with an expansile, eccentric lytic lesion in the distal femur epimetaphysis. Biopsy confirms a giant cell tumor of bone. If medical therapy is initiated prior to surgery, the agent used most likely targets which of the following?

. Vascular endothelial growth factor (VEGF)
. RANK ligand (RANKL)
. Tyrosine kinase
. Osteoprotegerin (OPG)
. Transforming growth factor beta (TGF-b)

Correct Answer & Explanation

. RANK ligand (RANKL)

Explanation

Denosumab is a monoclonal antibody that targets RANKL, preventing the activation of osteoclasts and giant cells in giant cell tumor of bone. This therapy induces sclerosis of the lesion and is often used as neoadjuvant treatment.

Question 13811

Topic: 1. General Principles & Basic Science

Which zone of articular cartilage is characterized by the highest concentration of proteoglycans, the lowest concentration of water, and collagen fibers oriented perpendicular to the articular surface?

. Superficial (tangential) zone
. Middle (transitional) zone
. Deep (radial) zone
. Calcified zone
. Tidemark

Correct Answer & Explanation

. Deep (radial) zone

Explanation

The deep (radial) zone of articular cartilage contains the highest proteoglycan content and vertically oriented collagen fibers. The superficial zone has the highest water content and collagen fibers parallel to the joint surface.

Question 13812

Topic: Infection, Pharmacology & VTE

A patient develops a delayed periprosthetic joint infection 14 months after a total hip arthroplasty. Cultures grow Staphylococcus epidermidis. The pathogenesis of this infection relies primarily on the bacteria's ability to produce which of the following?

. Protein A
. Panton-Valentine leukocidin
. Glycocalyx
. Beta-lactamase
. Alpha toxin

Correct Answer & Explanation

. Glycocalyx

Explanation

Staphylococcus epidermidis is a leading cause of implant-associated infections due to its ability to produce a glycocalyx slime layer, which facilitates biofilm formation. Biofilms protect the bacteria from both host immune responses and antibiotics.

Question 13813

Topic: Biology, Genetics & Bone Healing

Bone morphogenetic proteins (BMPs) play a crucial role in bone healing and osteoinduction. Upon binding to their transmembrane serine/threonine kinase receptors, which intracellular signaling molecules are directly phosphorylated to translocate to the nucleus?

. JAK and STAT
. Smad 1, 5, and 8
. Beta-catenin
. MAP kinase
. c-Fos and c-Jun

Correct Answer & Explanation

. Smad 1, 5, and 8

Explanation

BMPs signal through serine/threonine kinase receptors that phosphorylate receptor-regulated Smads (Smad 1, 5, and 8). These phosphorylated Smads then form a complex with Smad 4 and translocate to the nucleus to regulate gene transcription.

Question 13814

Topic: 1. General Principles & Basic Science
During the remodeling phase of tendon healing, the tensile strength of the tendon gradually increases. This process is primarily driven by the replacement of which type of collagen with another?
. Type I with Type II
. Type II with Type I
. Type III with Type I
. Type I with Type III
. Type IV with Type I

Correct Answer & Explanation

. Type I with Type III

Explanation

Tendon healing begins with the deposition of disorganized Type III collagen during the proliferative phase. During the remodeling phase, this is gradually replaced by stronger, highly cross-linked Type I collagen, increasing the tissue's tensile strength.

Question 13815

Topic: Biology, Genetics & Bone Healing

A patient with a history of recurrent fractures and severe anemia demonstrates diffusely dense bones on radiographs with an "Erlenmeyer flask" deformity of the distal femurs. A deficiency in which of the following enzymes is most commonly associated with the autosomal recessive form of this disease?

. Alkaline phosphatase
. Tartrate-resistant acid phosphatase
. Carbonic anhydrase II
. Cathepsin K
. Matrix metalloproteinase

Correct Answer & Explanation

. Carbonic anhydrase II

Explanation

The patient has osteopetrosis, characterized by defective osteoclast function. The autosomal recessive (malignant) form is most commonly caused by a mutation in carbonic anhydrase II, impairing the osteoclast's ability to acidify the ruffled border and resorb bone.

Question 13816

Topic: Infection, Pharmacology & VTE

A patient with an infected tibial nonunion is found to have methicillin-resistant Staphylococcus aureus (MRSA) cultured from intraoperative bone biopsies. The resistance of this organism to beta-lactam antibiotics is primarily mediated by which of the following mechanisms?

. Efflux pumps
. Production of beta-lactamase
. Alteration of penicillin-binding protein (PBP2a)
. Mutation in DNA gyrase
. Decreased outer membrane permeability

Correct Answer & Explanation

. Alteration of penicillin-binding protein (PBP2a)

Explanation

MRSA resistance is mediated by the mecA gene, which encodes for a novel penicillin-binding protein, PBP2a. This altered protein has a very low affinity for beta-lactam antibiotics, rendering them ineffective.

Question 13817

Topic: Biology, Genetics & Bone Healing

Romosozumab is a bone-building medication used for severe osteoporosis. It exerts its anabolic effect on bone primarily by inhibiting which of the following molecules in the Wnt signaling pathway?

. Beta-catenin
. Sclerostin
. RANKL
. Osteoprotegerin
. DKK1

Correct Answer & Explanation

. Sclerostin

Explanation

Romosozumab is a monoclonal antibody that binds to and inhibits sclerostin. Sclerostin normally inhibits the Wnt/beta-catenin pathway; therefore, its inhibition leads to increased osteoblastogenesis and bone formation.

Question 13818

Topic: Biology, Genetics & Bone Healing

A 2-year-old child presents with bowing of the lower extremities and widening of the wrists. Laboratory testing shows normal serum calcium, significantly decreased serum phosphate, normal parathyroid hormone, and normal 25-hydroxyvitamin D. Which of the following is the most likely diagnosis?

. Vitamin D-dependent rickets type I
. Nutritional rickets
. Hypophosphatasia
. X-linked hypophosphatemic rickets
. Renal osteodystrophy

Correct Answer & Explanation

. X-linked hypophosphatemic rickets

Explanation

X-linked hypophosphatemic rickets is caused by a PHEX mutation leading to increased FGF23, which causes renal phosphate wasting. Patients typically present with isolated low phosphate and normal calcium, PTH, and Vitamin D levels.

Question 13819

Topic: Biology, Genetics & Bone Healing

Secondary bone healing relies on endochondral ossification and callus formation, which is driven by interfragmentary motion. According to Perren's strain theory, what type of tissue forms when the interfragmentary strain is between 2% and 10%?

. Granulation tissue
. Fibrous tissue
. Fibrocartilage
. Lamellar bone
. Woven bone

Correct Answer & Explanation

. Fibrocartilage

Explanation

Perren's strain theory posits that tissue formation in a fracture gap depends on local strain. Fibrocartilage forms when strain is between 2% and 10%, while woven bone forms at strains less than 2%, and granulation tissue persists at high strains (>10%).

Question 13820

Topic: Biology, Genetics & Bone Healing

Which of the following intracellular signaling proteins is primarily activated following the binding of Bone Morphogenetic Protein-2 (BMP-2) to its cell surface receptor?

. cAMP
. SMAD
. Wnt
. JAK/STAT
. NF-kB

Correct Answer & Explanation

. SMAD

Explanation

BMPs bind to serine/threonine kinase cell surface receptors, leading to the phosphorylation of intracellular SMAD proteins. These activated SMAD complexes then translocate to the nucleus to regulate the transcription of osteogenic genes.

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