Menu

1. General Principles & Basic Science MCQs

Practice Set 467 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 9321

Topic: Surgical Anatomy & Approaches

A 6-year-old non-ambulatory boy with SMA Type II is noted to have progressive right hip subluxation on routine surveillance radiographs. He is currently asymptomatic. What is the primary driving force behind this hip instability?

. Primary acetabular dysplasia inherent to the genetic defect.
. Ligamentous laxity of the iliofemoral ligament.
. Muscle imbalance with relatively stronger iliopsoas and adductors compared to abductors.
. Spasticity of the hip adductors and flexors.
. Avascular necrosis of the femoral head due to recurrent microtrauma.

Correct Answer & Explanation

. Muscle imbalance with relatively stronger iliopsoas and adductors compared to abductors.

Explanation

Correct Answer: CHip subluxation and dislocation are common orthopedic complications in non-ambulatory patients with SMA. The primary etiology is a muscle imbalance around the hip joint. Specifically, the hip flexors (iliopsoas) and adductors remain relatively stronger than the hip abductors and extensors. This imbalance gradually pulls the femoral head out of the acetabulum. Because SMA is a lower motor neuron disease, spasticity (an upper motor neuron sign) is absent. While ligamentous laxity and lack of weight-bearing contribute, the primary deforming force is the muscle imbalance.

Question 9322

Topic: Biology, Genetics & Bone Healing
A 5-year-old girl with Osteogenesis Imperfecta Type III is started on intravenous pamidronate. Her parents ask how this medication will help her bones. The orthopedic surgeon correctly explains that the drug primarily acts by:
. Stimulating osteoblast differentiation and collagen synthesis.
. Binding to hydroxyapatite and inducing osteoclast apoptosis.
. Enhancing intestinal calcium and phosphorus absorption.
. Inhibiting the production of abnormal type I collagen.
. Promoting the cross-linking of existing collagen fibrils.

Correct Answer & Explanation

. Binding to hydroxyapatite and inducing osteoclast apoptosis.

Explanation

Bisphosphonates, such as pamidronate and zoledronic acid, are antiresorptive agents used extensively in the management of OI. They have a high affinity for bone hydroxyapatite. Once bound, they are ingested by osteoclasts during the bone resorption process. Inside the osteoclast, they disrupt cellular metabolism and induce apoptosis (programmed cell death). By inhibiting osteoclast-mediated bone resorption, bisphosphonates decrease bone turnover, increase bone mineral density, and reduce the incidence of fractures. They do not directly stimulate osteoblasts or alter the underlying genetic defect in collagen synthesis.

Question 9323

Topic: Biology, Genetics & Bone Healing

A neonate born at 36 weeks gestation presents with severe micromelia, a soft calvarium, and multiple intrauterine fractures. The infant expires shortly after birth due to respiratory failure. Post-mortem electron microscopy of the bone would most likely reveal which of the following abnormalities?

. Absence of osteoclasts in the primary spongiosa.
. Broad, ribbon-like collagen fibrils.
. Accumulation of unmineralized osteoid seams.
. Disorganized, woven bone without lamellar conversion.
. Hypertrophic chondrocytes extending into the diaphysis.

Correct Answer & Explanation

. Broad, ribbon-like collagen fibrils.

Explanation

Correct Answer: BThe clinical presentation of severe micromelia, multiple prenatal fractures, a soft calvarium, and perinatal lethality is pathognomonic for Osteogenesis Imperfecta Type II. This severe phenotype is typically caused by a qualitative defect in type I collagen (often a dominant-negative mutation). On electron microscopy, this qualitative defect classically manifests as broad, ribbon-like collagen fibrils, reflecting the abnormal assembly of the collagen triple helix. Unmineralized osteoid seams are characteristic of rickets/osteomalacia, while absent osteoclasts suggest osteopetrosis.

Question 9324

Topic: Biology, Genetics & Bone Healing
A 6-year-old girl with Osteogenesis Imperfecta (OI) Type III is treated with cyclical intravenous pamidronate. She requires bilateral femoral osteotomies and Fassier-Duval rodding for progressive bowing. Regarding her bisphosphonate therapy in the perioperative period, what is the most widely accepted recommendation to optimize bone healing?
. Increase the dose immediately post-operatively to prevent disuse osteopenia.
. Discontinue permanently as the intramedullary rods provide sufficient stability.
. Delay the infusion for several months post-operatively to allow for initial callus formation and remodeling.
. Switch to an oral bisphosphonate to avoid systemic acute phase reactions.
. Administer the infusion intraoperatively to maximize local bone density at the osteotomy site.

Correct Answer & Explanation

. Delay the infusion for several months post-operatively to allow for initial callus formation and remodeling.

Explanation

Bisphosphonates inhibit osteoclast activity, which is a critical component of the bone remodeling phase during fracture or osteotomy healing. While bisphosphonates do not typically prevent the formation of the primary soft callus, they can significantly delay the remodeling of this callus into mature woven and lamellar bone, potentially leading to delayed union or non-union. Therefore, the standard orthopedic consensus is to withhold intravenous bisphosphonate infusions for a period (typically 3 to 6 months) post-operatively to allow the osteotomy sites to heal and remodel adequately before resuming therapy.

Question 9325

Topic: Biology, Genetics & Bone Healing

A 4-year-old boy with Osteogenesis Imperfecta Type IV is started on a regimen of intravenous pamidronate. At the cellular level, this medication primarily improves bone mineral density by:

. Stimulating osteoblast differentiation and proliferation.
. Enhancing intestinal calcium and phosphate absorption.
. Cross-linking abnormal type I collagen fibrils to increase tensile strength.
. Binding to hydroxyapatite and inducing apoptosis in active osteoclasts.
. Upregulating the expression of the COL1A1 gene.

Correct Answer & Explanation

. Binding to hydroxyapatite and inducing apoptosis in active osteoclasts.

Explanation

Correct Answer: DBisphosphonates, such as pamidronate and zoledronic acid, are potent antiresorptive agents. They have a very high affinity for bone mineral (hydroxyapatite) and preferentially accumulate at sites of active bone remodeling. When osteoclasts attempt to resorb this bone, they internalize the bisphosphonate. Nitrogen-containing bisphosphonates (like pamidronate) then inhibit the mevalonate pathway within the osteoclast, disrupting essential intracellular signaling proteins, which ultimately leads to osteoclast apoptosis. This profound inhibition of bone resorption allows osteoblast-mediated bone formation to continue unopposed, thereby increasing overall bone mineral density.

Question 9326

Topic: Biology, Genetics & Bone Healing

A 15-year-old male with Osteogenesis Imperfecta Type I and a history of recurrent vertebral compression fractures is being evaluated for medical management. While intravenous bisphosphonates are the standard of care, the family asks about teriparatide (recombinant human parathyroid hormone), which a relative takes for osteoporosis. Why is teriparatide strictly contraindicated in this pediatric patient?

. It accelerates the closure of the physes, leading to severe, irreversible short stature.
. It carries a black box warning for an increased risk of osteosarcoma in patients with open epiphyses.
. It paradoxically decreases bone mineral density in patients with known COL1A1 mutations.
. It induces severe, symptomatic hypocalcemia requiring prolonged hospitalization.
. It irreversibly neutralizes the effects of any previously administered bisphosphonates.

Correct Answer & Explanation

. It carries a black box warning for an increased risk of osteosarcoma in patients with open epiphyses.

Explanation

Correct Answer: BTeriparatide is an anabolic agent (recombinant PTH 1-34) used to treat severe osteoporosis in adults. However, it is strictly contraindicated in pediatric patients, young adults with open epiphyses, and patients with prior radiation therapy or Paget's disease. This is due to a black box warning based on preclinical studies in rats that demonstrated a dose-dependent increase in the incidence of osteosarcoma. Because children and adolescents have actively growing skeletons and open physes, the risk of osteosarcoma is theoretically elevated, making teriparatide unsafe for use in pediatric Osteogenesis Imperfecta.

Question 9327

Topic: Biology, Genetics & Bone Healing
A neonate presents with severe, lethal bone fragility, multiple intrauterine fractures, and white sclerae. Genetic testing reveals no mutations in the COL1A1 or COL1A2 genes. Further whole-exome sequencing identifies a homozygous mutation in the CRTAP gene. This presentation is most consistent with an underlying defect in which of the following processes?
. Defective cleavage of the procollagen C-propeptide by bone morphogenetic protein 1 (BMP-1).
. Abnormal post-translational 3-hydroxylation of proline residues in the collagen triple helix.
. Overexpression of osteoclast-activating factors leading to massive bone resorption.
. Impaired mineralization of the osteoid matrix due to tissue-nonspecific alkaline phosphatase deficiency.
. Defective cross-linking of collagen fibrils by the copper-dependent enzyme lysyl oxidase.

Correct Answer & Explanation

. Abnormal post-translational 3-hydroxylation of proline residues in the collagen triple helix.

Explanation

While the vast majority (85-90%) of Osteogenesis Imperfecta cases are autosomal dominant and caused by mutations in COL1A1 or COL1A2, there are rare autosomal recessive forms (Types VII, VIII, IX, etc.). Mutations in the CRTAP (cartilage-associated protein) or LEPRE1 (prolyl 3-hydroxylase 1) genes cause severe, often lethal, recessive OI. These genes encode proteins that form the prolyl 3-hydroxylase complex in the endoplasmic reticulum. This complex is responsible for the post-translational 3-hydroxylation of specific proline residues (e.g., Pro986) on the collagen alpha-1(I) chain. Deficiencies in this process lead to abnormal collagen folding, delayed secretion, and severe bone fragility, often presenting with white sclerae, distinguishing it clinically from severe dominant forms which often have blue/grey sclerae.

Question 9328

Topic: Physiology & Rehabilitation
A 10-year-old boy with Spinal Muscular Atrophy Type III (Kugelberg-Welander disease) is ambulatory but exhibits a pronounced waddling gait with excessive lateral trunk sway over the stance limb during each step. This specific gait deviation is primarily a compensatory mechanism for weakness in which of the following muscle groups?
. Ankle dorsiflexors (Tibialis anterior)
. Knee extensors (Quadriceps femoris)
. Hip abductors (Gluteus medius and minimus)
. Hip extensors (Gluteus maximus)
. Ankle plantarflexors (Gastrocnemius-soleus complex)

Correct Answer & Explanation

. Hip abductors (Gluteus medius and minimus)

Explanation

SMA Type III patients are typically ambulatory but develop progressive proximal muscle weakness. The described gait is a compensated Trendelenburg gait (or "abductor lurch"). During the stance phase of gait, the hip abductors (gluteus medius and minimus) of the stance leg must contract to keep the pelvis level. When these muscles are weak, the pelvis drops on the contralateral (swing) side. To compensate for this drop and maintain balance, the patient laterally flexes their trunk over the weak stance limb, shifting their center of gravity over the hip joint to reduce the demand on the weak abductors. This results in the classic waddling appearance.

Question 9329

Topic: Biology, Genetics & Bone Healing

A 5-year-old boy with severe bone fragility is diagnosed with a rare form of Osteogenesis Imperfecta. Genetic testing reveals a homozygous mutation in the SERPINF1 gene. Bone biopsy shows a distinctive 'fish-scale' pattern of bone lamellae. Which of the following clinical characteristics is most unique to this specific OI subtype?

. Massive hyperplastic callus formation following minor fractures
. Blue sclerae that persist into late adulthood
. Calcification of the interosseous membrane of the forearm
. Poor or no clinical response to standard bisphosphonate therapy
. Associated profound conductive hearing loss in early childhood

Correct Answer & Explanation

. Poor or no clinical response to standard bisphosphonate therapy

Explanation

SERPINF1 mutations cause OI Type VI, which is characterized by a severe mineralization defect and fish-scale bone lamellae. Unlike classic OI types, Type VI typically does not respond to bisphosphonate therapy.

Question 9330

Topic: Biology, Genetics & Bone Healing

Intravenous bisphosphonates are a mainstay of medical treatment for moderate-to-severe Osteogenesis Imperfecta. By which of the following mechanisms do nitrogen-containing bisphosphonates (e.g., pamidronate) exert their primary effect on bone metabolism?

. Inhibition of RANKL expression by osteoblasts
. Direct stimulation of osteoblast bone matrix synthesis
. Inhibition of farnesyl pyrophosphate synthase in osteoclasts
. Activation of the Wnt/beta-catenin signaling pathway
. Enzymatic cross-linking of type I collagen fibers

Correct Answer & Explanation

. Inhibition of farnesyl pyrophosphate synthase in osteoclasts

Explanation

Nitrogen-containing bisphosphonates inhibit farnesyl pyrophosphate synthase within the mevalonate pathway. This disrupts osteoclast function and leads to osteoclast apoptosis, decreasing bone resorption.

Question 9331

Topic: Biology, Genetics & Bone Healing
A 3-year-old boy with Osteogenesis Imperfecta Type III is started on intravenous pamidronate. This pharmacological therapy primarily increases bone mineral density through which of the following cellular mechanisms?
. Stimulation of osteoblast differentiation via the Wnt/beta-catenin pathway.
. Inhibition of farnesyl pyrophosphate synthase, leading to osteoclast apoptosis.
. Direct binding to the RANK ligand, preventing osteoclast activation.
. Upregulation of osteoprotegerin (OPG) secretion by osteoblasts.
. Enhanced absorption of dietary calcium and phosphorus in the gastrointestinal tract.

Correct Answer & Explanation

. Inhibition of farnesyl pyrophosphate synthase, leading to osteoclast apoptosis.

Explanation

Nitrogen-containing bisphosphonates like pamidronate inhibit farnesyl pyrophosphate synthase within the mevalonate pathway. This disrupts protein prenylation in osteoclasts, ultimately leading to their apoptosis and a profound decrease in bone resorption.

Question 9332

Topic: Biology, Genetics & Bone Healing

A 16-year-old female with Osteogenesis Imperfecta has been treated with intravenous bisphosphonates for the past 12 years. She presents with a new, atraumatic transverse fracture of the lateral cortex of her proximal femoral diaphysis. What is the most likely pathophysiological mechanism contributing to this specific fracture pattern?

. Secondary hyperparathyroidism accelerating cortical bone resorption.
. Severe suppression of bone turnover leading to microdamage accumulation.
. Development of a secondary osteosarcoma within a healing callus.
. Malnutrition-induced rickets superimposed on osteogenesis imperfecta.
. Progressive basilar invagination causing an altered gait pattern.

Correct Answer & Explanation

. Severe suppression of bone turnover leading to microdamage accumulation.

Explanation

Prolonged bisphosphonate therapy can cause severe, long-term suppression of bone turnover. This prevents the normal remodeling of microdamage, leading to increased bone brittleness and the development of atypical subtrochanteric or diaphyseal femur fractures.

Question 9333

Topic: 1. General Principles & Basic Science

A 55-year-old female presents with vague generalized bone pain and proximal muscle weakness. Pelvic radiograph shows a transverse radiolucency in the superior pubic ramus.

Which of the following laboratory profiles is most consistent with her diagnosis?

. Normal calcium, normal phosphate, normal alkaline phosphatase, normal parathyroid hormone
. Low calcium, low phosphate, elevated alkaline phosphatase, elevated parathyroid hormone
. High calcium, low phosphate, normal alkaline phosphatase, elevated parathyroid hormone
. Normal calcium, high phosphate, low alkaline phosphatase, normal parathyroid hormone
. High calcium, normal phosphate, elevated alkaline phosphatase, normal parathyroid hormone

Correct Answer & Explanation

. Low calcium, low phosphate, elevated alkaline phosphatase, elevated parathyroid hormone

Explanation

The patient has osteomalacia, characterized by defective bone mineralization resulting in Looser zones (pseudofractures). Laboratory findings typically include low to normal calcium, low phosphate, elevated alkaline phosphatase, and secondary hyperparathyroidism.

Question 9334

Topic: Infection, Pharmacology & VTE
A 2-year-old child presents with a limp and refuses to bear weight. Laboratory tests show an ESR of 45 mm/h and a CRP of 3.2 mg/dL. Joint aspiration reveals 60,000 WBCs/mcL. Standard agar cultures show no growth at 48 hours. What is the most likely causative organism?
. Staphylococcus aureus
. Streptococcus pyogenes
. Kingella kingae
. Haemophilus influenzae type b
. Neisseria gonorrhoeae

Correct Answer & Explanation

. Kingella kingae

Explanation

Kingella kingae is a fastidious Gram-negative organism and is now a leading cause of septic arthritis in children aged 6 to 36 months. It requires inoculation into BACTEC blood culture bottles or prolonged culture for optimal isolation.

Question 9335

Topic: 1. General Principles & Basic Science

A newborn is noted to have short-limbed dwarfism, bilateral clubfeet, hitchhiker thumbs, and swelling of the external ear pinnae. A defect in which of the following is most likely responsible for this condition?

. Type II collagen synthesis
. Fibroblast growth factor receptor 3
. Cartilage oligomeric matrix protein
. Sulfate transport
. CBFA1 transcription factor

Correct Answer & Explanation

. Sulfate transport

Explanation

Diastrophic dysplasia is an autosomal recessive disorder caused by a mutation in the SLC26A2 gene, which encodes a sulfate transporter. This leads to under-sulfated proteoglycans in cartilage, manifesting clinically with cauliflower ears and hitchhiker thumbs.

Question 9336

Topic: 1. General Principles & Basic Science

A 50-year-old female with systemic sclerosis presents with a painfully ischemic, cyanotic long finger digit despite conservative warming measures. Which of the following medications acts as a first-line therapy to promote vasodilation in this acute setting?

. Beta-blockers
. Epinephrine
. Nifedipine
. Corticosteroids
. Methotrexate

Correct Answer & Explanation

. Nifedipine

Explanation

Raynaud's phenomenon in scleroderma can lead to severe digital ischemia and tissue loss. Calcium channel blockers like nifedipine are the first-line medical therapy to induce vasodilation and improve peripheral perfusion.

Question 9337

Topic: Biology, Genetics & Bone Healing

A 5-year-old boy presents with progressive bowing of his legs and short stature. His mother has a similar history and required bilateral corrective osteotomies as a teenager. Genetic testing confirms a PHEX mutation. Which of the following is the hallmark of the indicated medical treatment?

. Intravenous bisphosphonates
. Oral calcium and cholecalciferol
. Oral phosphate and calcitriol
. Growth hormone therapy
. Parathyroidectomy

Correct Answer & Explanation

. Oral phosphate and calcitriol

Explanation

X-linked hypophosphatemic rickets is caused by a PHEX mutation resulting in elevated FGF-23 and renal phosphate wasting. Treatment requires supplementation with both oral phosphate and active vitamin D (calcitriol) to prevent secondary hyperparathyroidism.

Question 9338

Topic: Infection, Pharmacology & VTE

A 65-year-old male undergoes a two-stage revision for a chronic prosthetic joint infection. The organism isolated is Methicillin-sensitive Staphylococcus aureus. The success of targeted antibiotic therapy in this setting depends heavily on penetrating a biofilm. Which of the following antibiotics is most critical to include in the regimen for its unique anti-biofilm activity?

. Cefazolin
. Vancomycin
. Gentamicin
. Rifampin
. Linezolid

Correct Answer & Explanation

. Rifampin

Explanation

Staphylococcal species produce a thick exopolysaccharide glycocalyx biofilm that protects sessile bacteria from standard antibiotics. Rifampin has unique efficacy in penetrating biofilms and is typically used adjunctively in orthopaedic implant infections.

Question 9339

Topic: Biology, Genetics & Bone Healing

A 14-year-old female presents for evaluation of shoulder hypermobility. She can easily approximate her shoulders anteriorly in the midline. Examination reveals delayed eruption of permanent teeth. What is the genetic inheritance pattern and associated gene mutation for this condition?

. Autosomal recessive; SLC26A2
. Autosomal dominant; COMP
. Autosomal dominant; RUNX2 (CBFA1)
. X-linked dominant; PHEX
. Autosomal dominant; FGFR3

Correct Answer & Explanation

. Autosomal dominant; RUNX2 (CBFA1)

Explanation

Cleidocranial dysplasia presents with hypoplastic or absent clavicles, delayed cranial suture closure, and dental abnormalities. It is inherited in an autosomal dominant pattern due to a mutation in the RUNX2 (CBFA1) transcription factor.

Question 9340

Topic: Infection, Pharmacology & VTE

A 7-year-old child with sickle cell disease presents with fever, focal tibial pain, and swelling. MRI suggests diaphyseal osteomyelitis. Aspiration and blood cultures yield a Gram-negative, motile, non-lactose fermenting bacillus. What is the most appropriate directed antibiotic therapy?

. Vancomycin
. Ceftriaxone
. Clindamycin
. Penicillin G
. Metronidazole

Correct Answer & Explanation

. Ceftriaxone

Explanation

Children with sickle cell disease are uniquely predisposed to Salmonella osteomyelitis, which is a Gram-negative, non-lactose fermenting bacillus. Third-generation cephalosporins (like ceftriaxone) or fluoroquinolones are the treatments of choice.

Test Yourself

Switch to an interactive, timed exam simulation to truly master this topic.

On this Page