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1. General Principles & Basic Science MCQs

Practice Set 535 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 10681

Topic: Physiology & Rehabilitation

An elite marathon runner undergoes a muscle biopsy for a research study. Compared to a sprinter, the runner's predominant muscle fiber type is expected to demonstrate which of the following characteristics?

. High reliance on glycolytic metabolism and low myoglobin content
. Fast twitch speed and high glycogen stores
. High mitochondrial density and high oxidative capacity
. Large motor neuron size and rapid fatigue rate
. Low capillary density and high ATPase activity

Correct Answer & Explanation

. High mitochondrial density and high oxidative capacity

Explanation

Endurance athletes have predominantly Type I (slow-twitch) muscle fibers. These fibers are characterized by high mitochondrial density, high myoglobin, high capillary density, and rely on oxidative phosphorylation, making them highly fatigue-resistant.

Question 10682

Topic: Biology, Genetics & Bone Healing

A surgeon plans to use a structural cortical bone allograft to reconstruct an intercalary defect. At approximately 6 months postoperatively, what is the primary reason the allograft experiences a significant transient reduction in biomechanical strength?

. Exhaustion of the native bone morphogenetic proteins
. Invasion of macrophages causing diffuse necrosis
. Osteoclastic resorption creating porosity prior to new bone formation
. Premature degradation of the collagen matrix by matrix metalloproteinases
. Lack of osteoconductive scaffolding in the central core

Correct Answer & Explanation

. Osteoclastic resorption creating porosity prior to new bone formation

Explanation

Cortical bone grafts incorporate via creeping substitution, initially beginning with osteoclastic resorption along Haversian canals. This creates significant porosity and a temporary nadir in mechanical strength around 6 months before new osteoblastic bone formation restores it.

Question 10683

Topic: Biology, Genetics & Bone Healing

A 68-year-old man presents with back pain, anemia, and hypercalcemia. Skeletal survey reveals multiple "punched-out" lytic lesions. The extensive bone destruction in this condition is primarily mediated by myeloma cells secreting which of the following factors?

. Macrophage inflammatory protein-1 alpha (MIP-1a) and DKK1
. Bone morphogenetic protein-2 (BMP-2)
. Transforming growth factor-beta (TGF-b)
. Insulin-like growth factor-1 (IGF-1)
. Fibroblast growth factor-23 (FGF-23)

Correct Answer & Explanation

. Macrophage inflammatory protein-1 alpha (MIP-1a) and DKK1

Explanation

Multiple myeloma causes lytic lesions via an uncoupling of bone remodeling. Myeloma cells secrete MIP-1alpha (stimulating osteoclasts) and DKK1 (inhibiting osteoblasts by antagonizing Wnt signaling).

Question 10684

Topic: Biology, Genetics & Bone Healing

A 5-year-old boy is diagnosed with X-linked hypophosphatemic rickets. Which of the following pathophysiologic sequences best explains his condition?

. PHEX mutation -> Decreased FGF23 -> Increased renal phosphate resorption
. PHEX mutation -> Increased FGF23 -> Decreased renal phosphate resorption
. FGF23 mutation -> Increased 1-alpha-hydroxylase activity -> Hypercalcemia
. Vitamin D receptor mutation -> End-organ resistance -> Hypocalcemia
. CASR mutation -> Resetting of the calcium set-point -> Hypercalciuria

Correct Answer & Explanation

. PHEX mutation -> Increased FGF23 -> Decreased renal phosphate resorption

Explanation

X-linked hypophosphatemic rickets is caused by a PHEX mutation leading to elevated FGF23 levels. High FGF23 causes profound renal phosphate wasting and inappropriately downregulates 1-alpha-hydroxylase, preventing calcitriol synthesis.

Question 10685

Topic: 1. General Principles & Basic Science

The lubricating properties of normal synovial fluid are crucial for protecting articular cartilage. While lubricin primarily handles boundary lubrication, which of the following molecules is predominantly responsible for the fluid-film lubrication and high viscosity of synovial fluid?

. Chondroitin sulfate
. Aggrecan
. Hyaluronic acid
. Keratan sulfate
. Fibronectin

Correct Answer & Explanation

. Hyaluronic acid

Explanation

Hyaluronic acid (hyaluronan) is a non-sulfated glycosaminoglycan synthesized by Type B synoviocytes. It is the primary molecule responsible for the high viscosity and fluid-film elastohydrodynamic lubrication of synovial fluid.

Question 10686

Topic: Biology, Genetics & Bone Healing

A patient with profound dietary restrictions presents with perifollicular hemorrhages, gingival bleeding, and poor wound healing. The underlying vitamin deficiency directly impairs which of the following steps in collagen synthesis?

. Cleavage of procollagen C- and N-terminals
. Hydroxylation of proline and lysine residues
. Glycosylation of hydroxylysine residues
. Extracellular cross-linking by lysyl oxidase
. Transcription of COL1A1 mRNA

Correct Answer & Explanation

. Hydroxylation of proline and lysine residues

Explanation

Scurvy is caused by Vitamin C deficiency. Vitamin C acts as a necessary cofactor for prolyl hydroxylase and lysyl hydroxylase, which are essential for stabilizing the collagen triple helix.

Question 10687

Topic: 1. General Principles & Basic Science

During a regional nerve block, a patient inadvertently receives an intravascular injection of bupivacaine, leading to cardiovascular collapse. Intravenous lipid emulsion therapy is initiated. What is the primary mechanism of action of the local anesthetic responsible for this toxicity?

. Activation of voltage-gated potassium channels
. Blockade of the intracellular portion of voltage-gated sodium channels
. Blockade of the extracellular portion of voltage-gated calcium channels
. Agonism at the GABA-A receptor
. Inhibition of acetylcholinesterase

Correct Answer & Explanation

. Blockade of the intracellular portion of voltage-gated sodium channels

Explanation

Local anesthetics like bupivacaine work by crossing the neuronal cell membrane and binding to the intracellular portion of voltage-gated sodium channels, preventing depolarization. Toxicity affects cardiac sodium channels similarly.

Question 10688

Topic: Biomechanics & Biomaterials

During revision hip arthroplasty, a surgeon considers placing a stainless steel cable around a titanium femoral stem. What is the primary concern regarding mixed-metal combinations in this orthopaedic setting?

. Titanium will act as the anode, rapidly corroding the stem
. Galvanic corrosion will occur, with stainless steel acting as the anode and dissolving
. Crevice corrosion will exclusively occur due to poor local oxygen tension
. The combination promotes an accelerated type IV hypersensitivity reaction to titanium
. The modulus of elasticity mismatch will cause immediate stem fracture

Correct Answer & Explanation

. Galvanic corrosion will occur, with stainless steel acting as the anode and dissolving

Explanation

Mixing dissimilar metals in an electrolytic environment (the human body) causes galvanic corrosion. The less noble metal (stainless steel) becomes the anode and undergoes accelerated dissolution, while titanium acts as the cathode.

Question 10689

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory, what type of bone healing occurs in a fracture environment with absolute stability (strain less than 2%) and no gap?

. Endochondral ossification with abundant callus
. Intramembranous ossification with peripheral callus
. Primary bone healing via osteoclastic cutting cones
. Chondrogenesis followed by appositional bone growth
. Fibrous nonunion due to lack of micromotion

Correct Answer & Explanation

. Primary bone healing via osteoclastic cutting cones

Explanation

Under conditions of absolute stability (strain < 2%) and anatomic reduction, primary bone healing occurs. This process is mediated by osteoclastic cutting cones crossing the fracture site, followed by osteoblastic bone deposition, without the formation of an intermediate fracture callus.

Question 10690

Topic: Biology, Genetics & Bone Healing

A 6-year-old child presents with multiple unexplained fractures and severe anemia. Radiographs display symmetrically thickened, hyperdense bones ('marble bone') with an absence of normal medullary cavities. The pathogenesis of this condition (osteopetrosis) most commonly involves a defect in Carbonic Anhydrase II. This primarily disrupts which cellular process?

. Osteoblast matrix production
. Osteoclast ruffled border formation and acidification
. Osteocyte mechanotransduction
. Chondrocyte hypertrophy in the physis
. Extracellular Type I collagen cross-linking

Correct Answer & Explanation

. Osteoclast ruffled border formation and acidification

Explanation

Osteopetrosis is caused by defective osteoclast function, specifically the inability to acidify Howship's lacuna at the ruffled border. Mutations in Carbonic Anhydrase II (CAII) or the TCIRG1 gene prevent the necessary generation and transport of protons (H+) to dissolve bone mineral, leading to dense, brittle bones and obliterated marrow spaces.

Question 10691

Topic: Surgical Anatomy & Approaches

A surgeon utilizes the Smith-Petersen (anterior) approach for an open reduction of a developmental hip dysplasia. The superficial internervous plane lies between the sartorius and the tensor fasciae latae (TFL). What muscles define the deep internervous plane of this approach?

. Gluteus medius and piriformis
. Rectus femoris and gluteus minimus
. Gluteus maximus and medius
. Adductor longus and pectineus
. Vastus lateralis and rectus femoris

Correct Answer & Explanation

. Rectus femoris and gluteus minimus

Explanation

The Smith-Petersen (anterior) approach uses a true internervous plane superficially between the sartorius (femoral nerve) and TFL (superior gluteal nerve), and deeply between the rectus femoris (femoral nerve) and gluteus minimus (superior gluteal nerve).

Question 10692

Topic: Biology, Genetics & Bone Healing

Articular cartilage is a highly specialized tissue divided into four distinct zones. Which zone is characterized by having the highest concentration of water, the lowest concentration of proteoglycans, and collagen fibers oriented parallel to the articular surface?

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

Correct Answer & Explanation

. Superficial (tangential) zone

Explanation

The superficial (tangential) zone comprises 10-20% of articular cartilage thickness. It has the highest water content (up to 80%), the lowest proteoglycan content, and densely packed collagen type II fibers aligned parallel to the joint surface to resist shear and tensile stresses.

Question 10693

Topic: 1. General Principles & Basic Science

A 35-year-old male undergoes a structural allograft reconstruction following tumor resection. Which of the following best describes the incorporation process of a massive cortical bone allograft compared to a cancellous bone autograft?

. It proceeds via early osteoblastic activity followed by delayed osteoclastic resorption.
. It relies primarily on osteoinduction rather than osteoconduction.
. It undergoes initial osteoclastic resorption of the Haversian canals, leading to temporary mechanical weakness.
. It incorporates more rapidly due to the dense structural matrix and lack of immune rejection.
. It does not undergo creeping substitution due to the lack of live cells in the allograft.

Correct Answer & Explanation

. It undergoes initial osteoclastic resorption of the Haversian canals, leading to temporary mechanical weakness.

Explanation

Cortical bone grafts incorporate via creeping substitution, which is initiated by osteoclastic resorption of the existing Haversian canals. This leads to a temporary decrease in the mechanical strength of the graft before osteoblastic new bone formation occurs. In contrast, cancellous autografts incorporate via early osteoblastic apposition on the trabeculae, initially increasing in strength.

Question 10694

Topic: Physiology & Rehabilitation

Which of the following descriptions accurately characterizes the metabolic and physiologic profile of Type I (slow-twitch) skeletal muscle fibers compared to Type II (fast-twitch) fibers?

. Higher glycolytic enzyme capacity and lower mitochondrial density
. Larger cross-sectional area and faster calcium reuptake by the sarcoplasmic reticulum
. Higher oxidative capacity, higher mitochondrial density, and greater resistance to fatigue
. Primary reliance on anaerobic glycolysis for rapid ATP production
. Paler macroscopic appearance due to lower overall myoglobin content

Correct Answer & Explanation

. Higher oxidative capacity, higher mitochondrial density, and greater resistance to fatigue

Explanation

Type I muscle fibers (slow-twitch) are oxidative fibers. They have a high mitochondrial density, high myoglobin content (giving them a red appearance), and high oxidative capacity, making them highly resistant to fatigue and ideal for sustained posture and endurance. Type II fibers are fast-twitch, highly glycolytic, fatigue easily, and appear pale.

Question 10695

Topic: Surgical Anatomy & Approaches

An orthopedic surgeon performing an ilioinguinal approach for open reduction of an anterior column acetabular fracture encounters brisk arterial bleeding while dissecting over the superior pubic ramus. This bleeding is most likely originating from the 'corona mortis', which is an anastomosis between which two vascular territories?

. External iliac and internal iliac systems
. External iliac and femoral systems
. Obturator and internal pudendal systems
. Superior gluteal and internal iliac systems
. Deep circumflex iliac and inferior epigastric systems

Correct Answer & Explanation

. External iliac and internal iliac systems

Explanation

The 'corona mortis' (crown of death) is a highly variable vascular anastomosis between the external iliac system (inferior epigastric artery/vein) and the internal iliac system (obturator artery/vein). It crosses over the superior pubic ramus and is highly susceptible to injury during anterior approaches to the pelvis and acetabulum.

Question 10696

Topic: Physiology & Rehabilitation

During the single-leg stance phase of gait, the hip abductor muscles must generate significant force to maintain a level pelvis. Based on standard free-body diagram principles of the hip, if the body's center of gravity is displaced further laterally away from the center of rotation of the hip joint, what is the effect on the required abductor force and the overall hip joint reaction force?

. Abductor force decreases; joint reaction force increases
. Abductor force increases; joint reaction force increases
. Abductor force increases; joint reaction force decreases
. Abductor force decreases; joint reaction force decreases
. Abductor force remains unchanged; joint reaction force increases

Correct Answer & Explanation

. Abductor force increases; joint reaction force increases

Explanation

The hip acts as a class 1 lever. The moment arm of body weight is counterbalanced by the moment arm of the abductor muscles. If the center of gravity moves further away from the hip (increasing the body weight moment arm), the abductors must generate more force to maintain equilibrium. This increased abductor muscle force, combined with the body weight, results in a substantially increased overall joint reaction force.

Question 10697

Topic: Biology, Genetics & Bone Healing

According to Perren's strain theory of fracture healing, which type of tissue can tolerate the highest amount of interfragmentary strain before failing, thereby acting as the necessary initial bridging tissue in secondary bone healing?

. Lamellar bone
. Woven bone
. Cartilage
. Granulation tissue
. Fibrous connective tissue

Correct Answer & Explanation

. Granulation tissue

Explanation

Granulation tissue can tolerate up to 100% strain before failing. Fibrous tissue tolerates ~17%, cartilage ~2-10%, and lamellar bone only 2%. Therefore, in the high-strain environment immediately following a fracture, only granulation tissue can form. As the callus builds, stiffness increases, strain decreases, and subsequent tissue types can form.

Question 10698

Topic: 1. General Principles & Basic Science
Low-molecular-weight heparin (LMWH) is frequently used for venous thromboembolism prophylaxis. What is its primary mechanism of action compared to unfractionated heparin?
. Direct inhibition of thrombin (Factor IIa) independent of antithrombin III
. Selective inhibition of Factor Xa with significantly less effect on thrombin
. Vitamin K antagonism leading to depletion of Factors II, VII, IX, and X
. Irreversible inhibition of cyclooxygenase in platelets
. Direct activation of plasminogen to plasmin

Correct Answer & Explanation

. Selective inhibition of Factor Xa with significantly less effect on thrombin

Explanation

LMWH complexes with antithrombin III, primarily inhibiting Factor Xa, with a much lower affinity for inhibiting thrombin (Factor IIa) compared to unfractionated heparin. This selective anti-Xa activity provides a more predictable anticoagulant dose-response and a lower risk of Heparin-Induced Thrombocytopenia (HIT).

Question 10699

Topic: 1. General Principles & Basic Science

In normal articular cartilage, which zone is characterized by chondrocytes flattened parallel to the joint surface and primarily functions to resist shear forces?

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

Correct Answer & Explanation

. Superficial (tangential) zone

Explanation

The superficial (tangential) zone contains flattened chondrocytes and collagen fibrils oriented parallel to the joint surface. It is specifically adapted to resist shear stresses and has the highest concentration of water and collagen.

Question 10700

Topic: Biomechanics & Biomaterials

A 75-year-old man undergoes a cemented hemiarthroplasty for a displaced femoral neck fracture. During cement pressurization and stem insertion, his blood pressure acutely drops, and his oxygen saturation falls to 82%. Which of the following is the primary pathophysiological mechanism responsible for this acute intraoperative event?

. Anaphylactic reaction to the polymethylmethacrylate (PMMA) monomer
. Thermal necrosis of the endosteal blood vessels causing vasospasm
. Embolization of fat and marrow contents into the pulmonary circulation
. Acute systemic toxicity from the release of unreacted barium sulfate
. Rapid systemic absorption of the liquid monomer causing profound bradycardia

Correct Answer & Explanation

. Embolization of fat and marrow contents into the pulmonary circulation

Explanation

Bone cement implantation syndrome is primarily caused by the embolization of fat, marrow, and air into the pulmonary circulation during cement pressurization. While monomer toxicity was historically suspected, transesophageal echocardiography has repeatedly demonstrated massive embolic showers during stem insertion.

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