Basic Science in Orthopaedics — MCQs

Basic Science in Orthopaedics Indian Medical PG Practice Questions and MCQs

Question 31: The posterior iliac horn is characteristic of which condition?

A. Nail-patella syndrome (Correct Answer)
B. Marfan's syndrome
C. Hurler syndrome
D. Acromegaly

Explanation: **Explanation:** **Nail-Patella Syndrome (Fong’s Disease)** is an autosomal dominant disorder caused by a mutation in the **LMX1B gene**. It is characterized by a classic clinical tetrad: nail dysplasia, patellar hypoplasia/aplasia, elbow abnormalities, and the presence of **iliac horns**. 1. **Why A is correct:** Posterior iliac horns (bilateral, symmetrical bony outgrowths from the posterior surface of the ilium) are considered **pathognomonic** for Nail-Patella Syndrome. They are present in approximately 80% of cases and are often the most specific radiographic finding for the diagnosis. 2. **Why the others are wrong:** * **Marfan’s Syndrome:** Characterized by arachnodactyly, ectopia lentis, and aortic root dilation, but does not involve iliac horns. * **Hurler Syndrome (MPS I):** Features include "J-shaped" sella turcica, ovoid vertebrae with anterior beaking, and spatulate ribs (Dysostosis Multiplex), but not iliac horns. * **Acromegaly:** Shows "tufting" of the distal phalanges, increased heel pad thickness, and frontal bossing due to GH excess. **High-Yield Clinical Pearls for NEET-PG:** * **The Tetrad:** 1. Hypoplastic/Absent nails (index finger most common); 2. Small/Absent patella; 3. Elbow dysplasia (limited supination/extension); 4. Iliac horns. * **Renal Involvement:** About 40% of patients develop nephropathy (similar to Glomerulonephritis), which can progress to renal failure. * **Genetics:** LMX1B gene mutation on Chromosome 9q. * **Radiology:** Iliac horns are best seen on an AP view of the pelvis.

Question 32: The basic pathology in Myositis Ossificans Progressiva is located in which of the following?

A. Muscle fibres (Correct Answer)
B. Serum chemistry
C. Body collagen
D. None of the above

Explanation: **Explanation:** **Myositis Ossificans Progressiva (MOP)**, also known as Fibrodysplasia Ossificans Progressiva (FOP), is a rare genetic disorder characterized by the progressive replacement of soft tissues—specifically skeletal muscles, tendons, and ligaments—with mature heterotopic bone. **Why Option A is Correct:** The primary pathology lies within the **muscle fibers** and their surrounding connective tissue. The condition is caused by a mutation in the **ACVR1 gene**, which leads to over-activation of the Bone Morphogenetic Protein (BMP) signaling pathway. This triggers an inflammatory response in the muscle (myositis), followed by fibroblastic proliferation and eventual endochondral ossification, turning muscle into bone. **Why Other Options are Incorrect:** * **Option B (Serum Chemistry):** In MOP, serum levels of calcium, phosphorus, and alkaline phosphatase are typically **normal**. The pathology is a localized cellular transformation rather than a systemic metabolic or biochemical imbalance. * **Option C (Body Collagen):** While collagenous tissues (tendons/ligaments) are eventually involved, the hallmark and initiating site of the "ossificans" process is the skeletal muscle tissue. It is not a generalized collagen vascular disease. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Dominant (though most cases are sporadic). * **Classic Presentation:** Characterized by the "Great Toe Sign"—congenital **shortening and hallux valgus** of the big toe. * **Progression:** Ossification usually starts in the neck and shoulders, moving cranio-caudally and proximo-distally. * **Management Warning:** Avoid biopsies or intramuscular injections, as **trauma triggers** new flare-ups and rapid ossification.

Question 33: What is the major mineral component of bone?

A. Calcite
B. Hydroxyapatite (Correct Answer)
C. Calcium oxide
D. Calcium carbonate

Explanation: **Explanation:** Bone is a specialized connective tissue composed of an organic matrix (osteoid) and an inorganic mineral phase. The **inorganic component** accounts for approximately 65-70% of the dry weight of bone and provides its characteristic compressive strength. **Why Hydroxyapatite is correct:** The primary mineral constituent of bone is **Hydroxyapatite**, a crystalline form of calcium phosphate with the chemical formula **$Ca_{10}(PO_4)_6(OH)_2$**. These crystals are deposited within the gaps of the collagen fibrils (Type I collagen) during mineralization. This structural arrangement allows bone to act as a reservoir for 99% of the body’s calcium and 85% of its phosphorus. **Why other options are incorrect:** * **Calcite & Calcium Carbonate:** While calcium carbonate is a major component of the shells of marine organisms and is found in trace amounts in bone, it is not the primary structural mineral. * **Calcium Oxide:** This is a caustic chemical compound (quicklime) not found naturally in human biological tissues. **High-Yield Clinical Pearls for NEET-PG:** * **Organic Matrix:** Composed 90% of **Type I Collagen** (mnemonic: "Bone is One"). * **Non-collagenous proteins:** **Osteocalcin** is the most abundant non-collagenous protein and is a marker of bone formation. * **Mineralization:** Regulated by alkaline phosphatase, which increases local concentrations of inorganic phosphate. * **Stiffness:** The stiffness of bone is directly proportional to its mineral content, whereas its toughness (resistance to fracture) is derived from the collagen fibers.

Question 34: Which of the following is a true statement about the physis?

A. Gives tensile strength to bone
B. Metabolically active part of the bone (Correct Answer)
C. Helps in bone resorption
D. Has lowest turnover activity in the bone

Explanation: **Explanation:** The **physis**, or epiphyseal plate, is the primary site of longitudinal bone growth in children. It is a highly organized cartilaginous structure located between the epiphysis and metaphysis. **1. Why Option B is Correct:** The physis is one of the most **metabolically active** regions of the skeletal system. It undergoes a continuous, rapid cycle of chondrocyte proliferation, hypertrophy, and extracellular matrix calcification. This process requires a robust blood supply (primarily from the perichondrial ring of LaCroix and epiphyseal vessels) and high enzymatic activity (e.g., alkaline phosphatase) to facilitate the transformation of cartilage into bone (endochondral ossification). **2. Why Other Options are Incorrect:** * **Option A:** Tensile strength is primarily provided by the **Type I collagen** found in the cortical bone and osteoid. The physis, being cartilaginous (Type II collagen), is actually the "weak link" in the pediatric skeleton, making it susceptible to fractures (Salter-Harris injuries) rather than providing strength. * **Option C:** Bone resorption is the primary function of **osteoclasts**, typically located in the endosteum or at sites of remodeling. While the physis involves "remodeling" at the zone of vascular invasion, its primary role is bone *formation*, not resorption. * **Option D:** The physis has the **highest turnover activity** in the growing skeleton. Low turnover is characteristic of mature cortical bone in adults. **Clinical Pearls for NEET-PG:** * **Weakest Zone:** The **Zone of Hypertrophy** is the weakest layer of the physis and the most common site for fractures. * **Collagen Type:** The physis contains **Type II collagen** (cartilage), whereas the bone it creates contains **Type I**. * **Scurvy:** Affects the physis by inhibiting osteoblast activity and collagen synthesis, leading to the "Trummerfeld zone" seen on X-ray.

Question 35: Which of the following statements is FALSE regarding myositis ossificans progressiva?

A. Pneumonia is a common complication.
B. Life longevity is typically normal. (Correct Answer)
C. The most common site involved is the spine.
D. Onset usually occurs before 6 years of age.

Explanation: **Explanation:** **Myositis Ossificans Progressiva (MOP)**, also known as **Fibrodysplasia Ossificans Progressiva (FOP)**, is a rare, autosomal dominant genetic disorder characterized by the progressive transformation of soft tissues (muscles, tendons, and ligaments) into heterotopic bone. **Why Option B is FALSE (The Correct Answer):** Life longevity is **not** normal. Most patients are severely disabled by their 20s and typically succumb to the disease by their 40s. The primary cause of death is **Thoracic Insufficiency Syndrome**, resulting from the ossification of intercostal muscles and the chest wall, leading to restricted lung expansion and respiratory failure. **Analysis of Other Options:** * **Option A (Pneumonia):** This is a common and often fatal complication. Due to the rigid chest wall and restricted ventilation, patients cannot clear secretions effectively, making them highly susceptible to recurrent pulmonary infections. * **Option C (Spine):** The disease characteristically follows a cranio-caudal and axial-to-appendicular pattern. The **spine, neck, and shoulders** are indeed the earliest and most common sites of involvement, leading to a "stone man" appearance. * **Option D (Onset < 6 years):** True. The condition typically manifests in early childhood, usually before the age of 6, often starting with painful soft tissue swellings (flare-ups). **Clinical Pearls for NEET-PG:** * **Hallmark Sign:** Congenital **short/malformed great toe** (hallux valgus/microdactyly) is the earliest diagnostic clue present at birth. * **Genetic Mutation:** Caused by a mutation in the **ACVR1 gene** (encoding the ALK2 receptor). * **Contraindication:** Avoid **intramuscular injections** and biopsies, as any local trauma can trigger a massive flare-up of heterotopic ossification.

Question 36: When is peak bone mass achieved?

A. Infancy
B. Early adolescence
C. Early adulthood (Correct Answer)
D. Early fetal life

Explanation: ### Explanation **Correct Answer: C. Early adulthood** **Medical Concept:** Peak Bone Mass (PBM) refers to the maximum amount of bone tissue an individual accumulates during their lifetime. Bone mass increases steadily throughout childhood and accelerates during puberty due to the influence of growth hormones and sex steroids. The consolidation phase continues after longitudinal growth stops, with PBM typically achieved in **early adulthood**, specifically between the ages of **25 and 30 years**. Approximately 90% of PBM is acquired by age 18 in girls and age 20 in boys, but the final density is reached in the third decade. **Analysis of Incorrect Options:** * **A. Infancy:** This is a period of rapid bone modeling and turnover, but the total mineral content is very low compared to adult levels. * **B. Early adolescence:** While this is the period of the "pubertal growth spurt" where bone mineral accrual is at its highest rate, the bones have not yet reached their maximum density or mineralization. * **D. Early fetal life:** At this stage, the skeleton is primarily cartilaginous (primordial) or undergoing initial intramembranous/endochondral ossification. **NEET-PG High-Yield Pearls:** * **Determinants:** Genetics is the most significant determinant of PBM (up to 80%), followed by nutrition (Calcium/Vitamin D) and weight-bearing exercise. * **Gender Difference:** Men generally achieve a higher PBM than women. * **Clinical Significance:** A higher PBM is the best defense against **osteoporosis** later in life. After age 30–35, bone resorption begins to exceed bone formation, leading to a gradual decline in bone mass (approximately 0.3–0.5% per year). * **The "Window of Opportunity":** The pre-pubertal and adolescent years are the most critical times to optimize PBM through lifestyle interventions.

Question 37: What type of primary healing is described?

A. Contact healing
B. Gap healing
C. Both contact and gap healing (Correct Answer)
D. Neither contact nor gap healing

Explanation: **Explanation:** Bone healing is broadly classified into **Primary (Direct)** and **Secondary (Indirect)** healing. Primary healing occurs when there is absolute stability (no motion at the fracture site), typically achieved through rigid internal fixation like compression plating. **Primary healing consists of two distinct mechanisms:** 1. **Contact Healing:** Occurs when the gap between bone ends is **less than 0.01 mm** and interfragmentary strain is less than 2%. Under these conditions, "cutting cones" (osteoclasts followed by osteoblasts) can cross the fracture line directly to create new haversian systems without any precursor callus. 2. **Gap Healing:** Occurs when the gap is **between 0.01 mm and 1 mm**. Here, the space is first filled by lamellar bone oriented perpendicular to the long axis, which is later remodeled into longitudinal haversian systems. Since both mechanisms occur under conditions of absolute stability and result in bone union without the formation of an external callus, the correct answer is **Both contact and gap healing.** **Why other options are incorrect:** * **Options A & B:** These are components of primary healing, but neither occurs in isolation during a clinical fracture repair; they often coexist depending on the precision of the reduction. * **Option D:** This describes secondary healing, which involves callus formation and occurs under conditions of relative stability (e.g., casting, intramedullary nailing). **High-Yield NEET-PG Pearls:** * **Absolute Stability:** Leads to Primary Healing (No Callus). * **Relative Stability:** Leads to Secondary Healing (Callus formation). * **Cutting Cones:** The functional unit of primary bone remodeling. * **Strain Theory (Perren):** Bone formation requires <2% strain; Primary healing requires absolute stability.

Question 38: Bone apposition is best in which of the following scenarios?

A. Osteoblastic activity at the area of stress
B. Endochondral ossification
C. Subperiosteal cambium layer
D. Osteoblastic activity in Howship's lacunae (Correct Answer)

Explanation: **Explanation:** Bone remodeling is a continuous physiological process involving a delicate balance between bone resorption and bone formation. This process occurs at **Basic Multicellular Units (BMUs)**. **Why Option D is Correct:** Bone resorption is carried out by osteoclasts, which create shallow pits or depressions on the bone surface known as **Howship’s lacunae**. Once resorption is complete, osteoblasts are recruited to these exact sites to deposit new bone matrix (osteoid). This coupled process—where **osteoblastic activity occurs within Howship’s lacunae**—is the fundamental mechanism of bone apposition and remodeling in mature bone. It ensures that the bone replaced is equal to the bone removed, maintaining skeletal integrity. **Analysis of Incorrect Options:** * **Option A:** According to **Wolff’s Law**, bone remodels in response to mechanical stress. While stress stimulates osteoblasts, "bone apposition" specifically refers to the cellular coupling that occurs during the remodeling cycle in resorption pits. * **Option B:** Endochondral ossification is the process of bone formation from a hyaline cartilage model (seen in long bone growth and fracture callus). It is a method of bone development, not the standard mechanism for appositional remodeling. * **Option C:** The cambium (inner) layer of the periosteum contains osteoprogenitor cells responsible for increasing the *girth* of bones. While this is appositional growth, the most precise description of the bone remodeling cycle (apposition following resorption) is linked to Howship’s lacunae. **High-Yield Clinical Pearls for NEET-PG:** * **Howship’s Lacunae:** Pits formed by **Osteoclasts** (derived from monocyte-macrophage lineage). * **Volkmann’s Canals:** Channels that transmit blood vessels from the periosteum to the Haversian canals. * **Cutting Cone:** The mechanism of remodeling in cortical bone; the "head" contains osteoclasts, and the "tail" contains osteoblasts. * **Marker of Bone Formation:** Serum Alkaline Phosphatase and Osteocalcin. * **Marker of Bone Resorption:** Urinary Pyridinoline and N-telopeptide.

Question 39: Marie and Sainton's disease is also known as:

A. Dentinogenesis imperfecta.
B. Cleidocranial dysostosis. (Correct Answer)
C. Turners hypoplasia.
D. Amelogenesia imperfecta.

Explanation: **Explanation:** **Marie and Sainton’s disease** is the eponym for **Cleidocranial Dysostosis (CCD)**. It is an autosomal dominant skeletal dysplasia caused by a mutation in the **RUNX2 gene** (located on chromosome 6), which is essential for osteoblast differentiation and intramembranous ossification. **Why Cleidocranial Dysostosis is correct:** The disease primarily affects bones formed via intramembranous ossification (skull and clavicles). Key clinical features include: * **Aplasia or hypoplasia of clavicles:** Patients can often approximate their shoulders in the midline. * **Skull abnormalities:** Delayed closure of fontanelles, presence of multiple **Wormian bones**, and frontal bossing. * **Dental anomalies:** Failure of eruption of permanent teeth and the presence of multiple **supernumerary teeth**. **Why other options are incorrect:** * **Dentinogenesis imperfecta:** A genetic disorder of tooth development resulting in discolored (opalescent) and weak teeth; it is often associated with Osteogenesis Imperfecta but is not Marie and Sainton’s disease. * **Turner’s hypoplasia:** An enamel defect in permanent teeth caused by periapical infection or trauma to the preceding deciduous tooth. * **Amelogenesis imperfecta:** A group of hereditary disorders affecting enamel formation in the absence of systemic features. **High-Yield Clinical Pearls for NEET-PG:** * **Gene:** RUNX2 (CBFA1). * **Radiology:** "Hot cross bun" appearance of the skull (due to prominent sutures) and widened symphysis pubis. * **Classic Sign:** Ability to touch shoulders together anteriorly. * **Inheritance:** Autosomal Dominant (though 1/3rd are spontaneous mutations).

Question 40: The Trendelenburg test is primarily performed to assess the integrity of which muscle group?

A. Gluteus maximus
B. Gluteus medius (Correct Answer)
C. Gluteus minimus
D. All of the above

Explanation: ### Explanation The **Trendelenburg test** is a clinical examination used to assess the integrity of the **hip abductor mechanism**. **Why Gluteus Medius is the Correct Answer:** The primary muscle responsible for stabilizing the pelvis during the stance phase of the gait cycle is the **Gluteus medius** (assisted by the gluteus minimus and tensor fasciae latae). When a person stands on one leg, the gluteus medius on the weight-bearing side contracts to prevent the opposite (unsupported) side of the pelvis from sagging. A positive Trendelenburg sign occurs when the pelvis drops on the unsupported side, indicating weakness or paralysis of the gluteus medius on the **standing (weight-bearing) side**. **Analysis of Incorrect Options:** * **Gluteus maximus (A):** This is the primary extensor of the hip. While crucial for climbing stairs and rising from a seated position, it does not play a primary role in lateral pelvic stabilization. * **Gluteus minimus (C):** Although it is an abductor, it is significantly weaker than the gluteus medius. While it contributes to the mechanism, the test is clinically defined by the failure of the gluteus medius. * **All of the above (D):** Incorrect because the test specifically targets the abductor group, excluding the maximus. **NEET-PG High-Yield Pearls:** 1. **Nerve Supply:** The gluteus medius and minimus are supplied by the **Superior Gluteal Nerve (L4, L5, S1)**. Injury to this nerve results in a positive Trendelenburg sign. 2. **Trendelenburg Gait:** Also known as a "lurching gait." To compensate for the pelvic drop, the patient tilts their trunk *toward* the affected side to shift the center of gravity. 3. **Causes of Positive Sign:** Polio, Superior Gluteal Nerve injury, L5 radiculopathy, Congenital Dislocation of the Hip (CDH/DDH), and ununited fractures of the femoral neck.

Practice by Chapter

Bone Structure and Function

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Cartilage Biology and Physiology

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Muscle and Tendon Physiology

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Joint Biomechanics

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Fracture Healing Process

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Bone Metabolism and Turnover

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Orthopaedic Biomaterials

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Tribology in Orthopaedics

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Gait Analysis

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Biomechanics of Spine

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Applied Surgical Anatomy

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Bone Banking and Grafting

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