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1. General Principles & Basic Science MCQs

Practice Set 505 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 10081

Topic: Physiology & Rehabilitation
A 40-year-old male is stabbed in the back, resulting in a spinal cord hemisection (Brown-Séquard syndrome) at the T8 level on the right side. Assuming a pure hemisection, which of the following physical examination findings will be distinctly present in the right lower extremity?
. Loss of pain and temperature sensation
. Loss of proprioception and vibration sensation
. Flaccid paralysis with absent reflexes
. Normal motor function with profound hyperalgesia
. Loss of all sensory modalities

Correct Answer & Explanation

. Loss of proprioception and vibration sensation

Explanation

Brown-Séquard syndrome is characterized by a spinal cord hemisection. The dorsal columns (proprioception, vibration, light touch) cross in the medulla, so a lesion causes ipsilateral loss. The corticospinal tract (motor) crosses in the lower medulla, causing ipsilateral spastic paralysis below the lesion (not flaccid). The spinothalamic tract (pain, temperature) crosses in the spinal cord within 1-2 levels of entry, leading to contralateral loss of pain and temperature.

Question 10082

Topic: Biology, Genetics & Bone Healing

A 7-year-old child with Osteogenesis Imperfecta Type IV has been on intravenous pamidronate therapy for 3 years. Which of the following radiographic findings is a direct consequence of this pharmacological treatment?

. Multiple radiolucent bone cysts
. Transverse sclerotic metaphyseal bands
. Cortical thickening and sclerosis
. Periosteal elevation
. Loss of the provisional zone of calcification

Correct Answer & Explanation

. Transverse sclerotic metaphyseal bands

Explanation

Intravenous bisphosphonates (like pamidronate) are commonly used in OI to increase bone mineral density and reduce fracture rates. Because bisphosphonates transiently inhibit osteoclast-mediated bone resorption, they lead to the formation of transverse dense sclerotic bands (often called 'zebra lines') in the metaphyses of growing bones. Each line represents a cycle of IV administration.

Question 10083

Topic: Biology, Genetics & Bone Healing

A 15-year-old girl with Osteogenesis Imperfecta Type I presents with progressive hearing loss. What is the most common etiology of hearing impairment in patients with OI?

. Sensorineural hearing loss due to primary cochlear nerve degeneration
. Conductive hearing loss due to ossicular dislocation or fixation
. Tympanic membrane rupture from minor trauma
. Chronic otitis media secondary to immunodeficiency
. Ototoxicity from long-term bisphosphonate use

Correct Answer & Explanation

. Conductive hearing loss due to ossicular dislocation or fixation

Explanation

Hearing loss in OI typically begins in the second or third decade of life and is most commonly conductive (though sensorineural and mixed forms also occur). Conductive hearing loss is usually due to otosclerosis-like fixation of the stapes footplate, stapedial crural fracture, or ossicular discontinuity. There is no known direct ototoxicity from bisphosphonates, nor is OI associated with immunodeficiency.

Question 10084

Topic: Biology, Genetics & Bone Healing

In Osteogenesis Imperfecta, treatment with bisphosphonates reduces fracture frequency and increases bone mineral density. However, long-term use has been associated with which of the following orthopedic complications?

. Avascular necrosis of the femoral head
. Delayed healing of osteotomies
. Increased rate of telescoping rod failure
. Premature physeal closure
. Development of secondary osteosarcoma

Correct Answer & Explanation

. Delayed healing of osteotomies

Explanation

Bisphosphonates inhibit osteoclast function, which impairs bone remodeling. While this increases bone density and decreases fracture rates, it can lead to delayed union or nonunion at osteotomy sites (e.g., following Fassier-Duval rodding). Surgeons often temporarily withhold bisphosphonate therapy around the time of surgery to minimize this risk. AVN of the jaw (ONJ) is a known but rare complication, mostly in adults.

Question 10085

Topic: 1. General Principles & Basic Science

The medication Zolgensma (Onasemnogene abeparvovec) is an FDA-approved gene therapy for SMA. What is its mechanism of action?

. It uses a lentiviral vector to integrate the SMN2 gene into the host genome.
. It utilizes an adeno-associated virus 9 (AAV9) vector to deliver a functional copy of the SMN1 gene.
. It is an antisense oligonucleotide injected intrathecally to modify RNA splicing.
. It is an oral small molecule that enhances SMN2 transcription.
. It replaces defective muscle tissue using autologous stem cells.

Correct Answer & Explanation

. It utilizes an adeno-associated virus 9 (AAV9) vector to deliver a functional copy of the SMN1 gene.

Explanation

Zolgensma is an intravenous gene replacement therapy for SMA. It uses an adeno-associated virus 9 (AAV9) vector to deliver a functional, episomal copy of the human SMN1 gene to motor neurons. It does not integrate into the host DNA. Nusinersen is an intrathecal ASO (antisense oligonucleotide), and Risdiplam is an oral small molecule that also targets SMN2 splicing.

Question 10086

Topic: Biology, Genetics & Bone Healing

A 10-year-old boy presents with forearm deformity and restricted pronosupination. Radiographs reveal calcification of the interosseous membrane and a history of hyperplastic callus formation following a previous femur fracture. Which genetic mutation is most likely responsible for this specific phenotype?

. COL1A1
. IFITM5
. CRTAP
. SERPINF1
. LEPRE1

Correct Answer & Explanation

. IFITM5

Explanation

The patient exhibits classic signs of Osteogenesis Imperfecta (OI) Type V, which is uniquely characterized by hyperplastic callus formation, calcification of the interosseous membrane of the forearm (leading to radioulnar synostosis and restricted rotation), and radial head dislocation. Unlike the majority of OI types caused by type I collagen defects, Type V is caused by a dominant mutation in the IFITM5 gene, which encodes the BRIL protein.

Question 10087

Topic: Biology, Genetics & Bone Healing

A 3-year-old child with severe Osteogenesis Imperfecta is started on cyclical intravenous Pamidronate. Which of the following best describes the cellular mechanism of action of this medication?

. Stimulates osteoblast proliferation and matrix synthesis
. Inhibits osteoclast activity by disrupting the mevalonate pathway
. Directly cross-links abnormal type I collagen fibers to increase tensile strength
. Increases intestinal calcium absorption via vitamin D receptor agonism
. Enhances the secretion of osteoprotegerin (OPG) from osteocytes

Correct Answer & Explanation

. Inhibits osteoclast activity by disrupting the mevalonate pathway

Explanation

Pamidronate is a nitrogen-containing bisphosphonate. These drugs localize to sites of bone resorption and are ingested by osteoclasts. Inside the osteoclast, they inhibit the enzyme farnesyl pyrophosphate synthase in the mevalonate pathway. This prevents the prenylation of small GTPase proteins (like Ras, Rho, Rac) essential for osteoclast function and survival, ultimately leading to osteoclast apoptosis and decreased bone resorption.

Question 10088

Topic: Biology, Genetics & Bone Healing
A histomorphometric analysis of an iliac crest bone biopsy from a patient with severe osteogenesis imperfecta (Type III) would most likely demonstrate which of the following findings compared to a healthy, age-matched control?
. Increased cortical thickness and lamellar bone formation
. Decreased number of osteoblasts and osteoclasts (hypocellularity)
. Increased trabecular bone volume with thickened trabeculae
. Decreased cortical thickness, decreased trabecular volume, and hypercellularity
. Absent osteoid seams and severe mineralization defects

Correct Answer & Explanation

. Decreased cortical thickness, decreased trabecular volume, and hypercellularity

Explanation

Bone biopsies in osteogenesis imperfecta reveal an uncoupling of bone turnover favoring resorption. Histomorphometry typically shows severe osteopenia with decreased cortical thickness and reduced trabecular bone volume. Because the bone attempts to repair itself rapidly but with defective collagen, the tissue is hypercellular (increased numbers of both osteoblasts and osteoclasts) and features a persistence of disorganized woven bone rather than mature lamellar bone.

Question 10089

Topic: 1. General Principles & Basic Science
Onasemnogene abeparvovec (Zolgensma) is a transformative therapeutic option for patients with SMA. What is the specific mechanism of action of this therapy?
. It modifies the pre-mRNA splicing of the SMN2 gene to produce functional protein
. It delivers a fully functional copy of the human SMN1 gene via an AAV9 viral vector
. It is a small molecule that inhibits the proteasomal degradation of the SMN protein
. It acts as a monoclonal antibody neutralizing myostatin to increase muscle mass
. It directly edits and repairs the mutated SMN1 gene in vivo using CRISPR-Cas9 technology

Correct Answer & Explanation

. It delivers a fully functional copy of the human SMN1 gene via an AAV9 viral vector

Explanation

Onasemnogene abeparvovec (Zolgensma) is a systemic gene replacement therapy. It utilizes a self-complementary adeno-associated virus serotype 9 (AAV9) vector, which can cross the blood-brain barrier, to deliver a functional, episomal copy of the human SMN1 gene directly to motor neurons. This is administered as a one-time intravenous infusion. In contrast, Nusinersen modifies SMN2 splicing.

Question 10090

Topic: Biology, Genetics & Bone Healing

A patient diagnosed with Osteogenesis Imperfecta Type VI is noted to have a paradoxically poor response to bisphosphonate therapy and exhibits an isolated mineralization defect on bone histology that mimics osteomalacia. Which gene is most likely mutated in this patient?

. COL1A1
. CRTAP
. LEPRE1
. SERPINF1
. IFITM5

Correct Answer & Explanation

. SERPINF1

Explanation

Osteogenesis Imperfecta Type VI is a rare, autosomal recessive form of the disease caused by loss-of-function mutations in the SERPINF1 gene. This gene encodes pigment epithelium-derived factor (PEDF). The hallmark of OI Type VI is an osteomalacia-like mineralization defect on bone biopsy (increased unmineralized osteoid) despite normal calcium and phosphate levels, and these patients notoriously respond poorly to standard bisphosphonate therapy.

Question 10091

Topic: Biology, Genetics & Bone Healing

The Sofield-Millar operation remains a foundational concept in the surgical management of severe long bone deformities in Osteogenesis Imperfecta. What does this classic procedure fundamentally entail?

. Single-level closing wedge osteotomy stabilized by dynamic compression plating
. Multiple sequential osteotomies of the diaphysis, realignment, and intramedullary rod fixation
. Guided growth via temporary hemiepiphysiodesis using tension band plates
. Gradual distraction osteogenesis using a circular external fixator
. Percutaneous drilling and injection of bone morphogenetic protein (BMP) to stimulate hypertrophy

Correct Answer & Explanation

. Multiple sequential osteotomies of the diaphysis, realignment, and intramedullary rod fixation

Explanation

First described in 1959, the Sofield-Millar procedure is the classic 'shish kebab' operation used to treat severe bowing in OI. It involves subperiosteal exposure of the long bone, performing multiple diaphyseal osteotomies to correct the bowing, threading the bone fragments onto a straight intramedullary rod to realign them, and allowing them to heal in a straight configuration.

Question 10092

Topic: Biology, Genetics & Bone Healing
Dentinogenesis imperfecta (DI) is a common manifestation of certain types of osteogenesis imperfecta. Which of the following accurately describes the dental findings and its most common associated OI types?
. The teeth appear chalky white and are highly resistant to caries; commonly seen in OI Type I.
. The primary dentition has delayed eruption and macrognathia; associated exclusively with OI Type V.
. The primary and secondary dentition appear opalescent, yellow-brown, and wear rapidly; commonly associated with OI Types III and IV.
. It only affects the primary dentition and is reversed with cyclical bisphosphonate therapy.
. The teeth have a distinct blue discoloration but normal enamel integrity; commonly seen in OI Type VI.

Correct Answer & Explanation

. The primary and secondary dentition appear opalescent, yellow-brown, and wear rapidly; commonly associated with OI Types III and IV.

Explanation

Dentinogenesis imperfecta (DI) is characterized by teeth that are opalescent (bluish-gray or yellow-brown) due to defective dentin. The enamel is normal but tends to flake off because the underlying dentin is soft, leading to rapid wear and breakage. It affects both primary and secondary dentition and is most commonly associated with COL1A1/COL1A2 mutations, particularly in OI Types III and IV.

Question 10093

Topic: Biology, Genetics & Bone Healing

While bisphosphonates (antiresorptive agents) are the standard of care for severe Osteogenesis Imperfecta, recent research has explored anabolic agents to increase bone formation. Which of the following drugs represents an anabolic approach by acting as a monoclonal antibody against sclerostin?

. Denosumab
. Teriparatide
. Romosozumab
. Burosumab
. Asfotase alfa

Correct Answer & Explanation

. Romosozumab

Explanation

Romosozumab is a monoclonal antibody that targets and binds to sclerostin. Sclerostin is a glycoprotein secreted by osteocytes that normally inhibits the Wnt signaling pathway, thereby inhibiting osteoblast bone formation. By blocking sclerostin, Romosozumab has a dual effect: it strongly stimulates bone formation (anabolic) and moderately decreases bone resorption. It is currently being investigated as a bone-building therapy for OI.

Question 10094

Topic: Biology, Genetics & Bone Healing

A 14-year-old female with Osteogenesis Imperfecta has been treated with intravenous pamidronate for 10 years. Radiographs of her distal femur reveal multiple transverse radiodense lines.

What is the mechanism of action of this medication in producing these lines?

. Inhibition of osteoblast apoptosis
. Stimulation of calcium deposition in the physis
. Inhibition of farnesyl pyrophosphate synthase in osteoclasts
. Direct binding to the COL1A1 promoter
. Neutralization of RANK ligand

Correct Answer & Explanation

. Inhibition of farnesyl pyrophosphate synthase in osteoclasts

Explanation

Nitrogen-containing bisphosphonates like pamidronate inhibit farnesyl pyrophosphate synthase, leading to osteoclast apoptosis. The transverse dense lines (zebra lines) correspond to the cycles of IV administration causing temporary failure of primary spongiosa resorption.

Question 10095

Topic: Biology, Genetics & Bone Healing
A 3-year-old child presents with frequent fractures, blue sclerae, and hyperplastic callus formation following a recent tibia fracture. Genetic analysis reveals a mutation in the IFITM5 gene. Which type of osteogenesis imperfecta does this patient have?
. Type I
. Type III
. Type IV
. Type V
. Type VI

Correct Answer & Explanation

. Type V

Explanation

OI Type V is uniquely characterized by hyperplastic callus formation, calcification of the interosseous membrane, and an autosomal dominant mutation in the IFITM5 gene. Sclerae can be variable but hyperplastic callus is the hallmark.

Question 10096

Topic: Biology, Genetics & Bone Healing

A 16-year-old male with Osteogenesis Imperfecta Type VI presents with a femur fracture. He has a known SERPINF1 mutation. How does the pathophysiology and medical management of Type VI differ from classical OI (Types I-IV)?

. It involves an isolated defect in osteoclast activity, treatable with Denosumab
. It features a mineralization defect resembling osteomalacia and responds poorly to bisphosphonates
. It is characterized by hyperplastic callus formation treated with NSAIDs
. It causes widespread soft tissue ossification requiring surgical excision
. It is an X-linked recessive condition requiring bone marrow transplant

Correct Answer & Explanation

. It features a mineralization defect resembling osteomalacia and responds poorly to bisphosphonates

Explanation

OI Type VI is caused by an autosomal recessive mutation in SERPINF1, resulting in a severe mineralization defect with increased unmineralized osteoid (similar to osteomalacia). Unlike classical OI, it typically has a poor response to bisphosphonate therapy.

Question 10097

Topic: Biology, Genetics & Bone Healing

A 15-year-old with severe OI has experienced multiple long bone fractures despite years of maximum-dose IV bisphosphonate therapy. The multidisciplinary team considers switching to Denosumab. What is the mechanism of action of Denosumab?

. Directly stimulates osteoblast differentiation via the Wnt pathway
. Binds and neutralizes RANK ligand (RANKL), preventing osteoclast activation
. Binds directly to the hydroxyapatite matrix, physically blocking osteoclasts
. Modulates the calcium-sensing receptor on the parathyroid gland
. Suppresses sclerostin, increasing bone formation

Correct Answer & Explanation

. Binds and neutralizes RANK ligand (RANKL), preventing osteoclast activation

Explanation

Denosumab is a fully human monoclonal antibody that binds to RANKL, preventing it from interacting with RANK on osteoclasts. This effectively inhibits osteoclast formation, function, and survival, serving as an alternative treatment for severe OI.

Question 10098

Topic: Biology, Genetics & Bone Healing
A 5-year-old boy with osteogenesis imperfecta type III has been receiving cyclic intravenous pamidronate therapy. Radiographs demonstrate horizontal sclerotic bands in the metaphyses. What is the primary cellular mechanism of action of this medication?
. Stimulates osteoblast differentiation via the Wnt pathway
. Inhibits osteoclast farnesyl pyrophosphate synthase
. Binds directly to RANKL, preventing osteoclast activation
. Increases the renal reabsorption of calcium
. Inhibits matrix metalloproteinase cleavage of type I collagen

Correct Answer & Explanation

. Inhibits osteoclast farnesyl pyrophosphate synthase

Explanation

Pamidronate is a nitrogen-containing bisphosphonate that inhibits farnesyl pyrophosphate synthase in the mevalonate pathway. This leads to osteoclast apoptosis and produces the characteristic metaphyseal sclerotic 'zebra lines' on radiographs.

Question 10099

Topic: 1. General Principles & Basic Science
An 8-year-old patient with SMA type III is prescribed risdiplam. By what mechanism does this medication improve the patient's muscle function?
. It acts as a small molecule modifying pre-mRNA splicing of the SMN2 gene
. It delivers a viral vector containing the SMN1 gene
. It is an intrathecal antisense oligonucleotide preventing exon 7 skipping
. It enhances acetylcholine release at the neuromuscular junction
. It directly stimulates myoblast proliferation

Correct Answer & Explanation

. It acts as a small molecule modifying pre-mRNA splicing of the SMN2 gene

Explanation

Risdiplam is an orally administered small molecule that modifies the pre-mRNA splicing of the SMN2 gene. It promotes the inclusion of exon 7, resulting in an increased production of functional, full-length SMN protein.

Question 10100

Topic: Biology, Genetics & Bone Healing
If a bone biopsy is taken from the iliac crest of a patient with severe osteogenesis imperfecta (type III), which of the following histologic findings is most expected?
. Thickened cortical bone with narrowed Haversian canals
. Presence of woven bone retaining primary trabeculae rather than mature lamellar bone
. Significant decrease in osteocyte density per unit of bone
. Complete absence of osteoclasts
. Accumulation of unmineralized osteoid resembling rickets

Correct Answer & Explanation

. Presence of woven bone retaining primary trabeculae rather than mature lamellar bone

Explanation

Histologically, bone in severe OI exhibits thin cortices and sparse trabeculae with hypercellularity (increased osteocyte density). The bone frequently fails to remodel into mature lamellar bone, remaining predominantly as disorganized woven bone.

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