Skeletal System — Set 4

Correct Answer: C. Cervical

• **Cervical** = The cervical region forms the neck portion of the spine and contains 7 vertebrae (C1–C7); the atlas (C1) supports the skull and allows nodding, while the axis (C2) has the odontoid peg that enables head rotation. • **Key fact** — The atlas-axis joint is unique in the entire spine because it allows about 50% of all head rotation. • Together, these two vertebrae form the most mobile segment of the vertebral column. • 💡 Option A (Thoracic) is wrong because thoracic vertebrae (T1–T12) attach to the ribs and are in the upper back; Option B (Sacral) is wrong because sacral vertebrae are fused into the sacrum at the base of the spine; Option D (Lumbar) is wrong because lumbar vertebrae are the five large lower-back bones, far below the neck.

Correct Answer: B. Fibula

• **Fibula** = The fibula is a slender bone of the lower leg running parallel to the tibia; it bears little body weight and primarily provides attachment points for muscles of the leg. • **Key fact** — The human arm has three bones: humerus (upper arm), and radius + ulna (forearm), none of which is the fibula. • The fibula is often used as a donor bone in reconstructive surgeries because it can be removed without significantly affecting walking. • 💡 Option A (Radius) is wrong because it is the lateral forearm bone on the thumb side; Option C (Humerus) is wrong because it is the single bone of the upper arm; Option D (Ulna) is wrong because it is the medial forearm bone forming the elbow point.

Correct Answer: C. Scapula

• **Scapula** = The scapula is a flat, triangular bone sitting on the posterior (back) side of the thorax; it forms the shoulder joint with the humerus and serves as the attachment point for 17 muscles. • **Key fact** — The scapula has a prominent spine and acromion process that you can feel on your upper back; it connects the arm bones to the clavicle. • Together with the clavicle, the scapula forms the pectoral (shoulder) girdle that links the upper limb to the axial skeleton. • 💡 Option A (Ilium) is wrong because it is the large flaring bone of the pelvis; Option B (Clavicle) is wrong because it is the collarbone connecting the sternum to the scapula; Option D (Sternum) is wrong because it is the flat breastbone in the center of the chest.

Correct Answer: C. Fontanelles

• **Fontanelles** = Fontanelles are membrane-covered gaps between the incompletely fused skull bones of a newborn; they allow the skull to compress during birth and the brain to grow rapidly in early life. • **Key fact** — The anterior (front) fontanelle is the largest and closes by 18–24 months; the posterior fontanelle closes by 2–3 months after birth. • Doctors use fontanelle tension as a clinical indicator — a bulging fontanelle may signal raised intracranial pressure. • 💡 Option A (Sutures) is wrong because sutures are the rigid, interlocking joints between already-fused skull bones in adults; Option B (Sinuses) is wrong because sinuses are air-filled cavities in skull bones; Option D (Foramens) is wrong because foramens are small openings in bones through which nerves and blood vessels pass.

Correct Answer: D. Calcium

• **Calcium** = Calcium is stored in bones as hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂], making up about 70% of bone's dry weight; bones and teeth together hold 99% of the body's total calcium. • **Key fact** — The skeleton acts as a calcium reservoir — when blood calcium falls, parathyroid hormone triggers osteoclasts to dissolve bone and release calcium back into the blood. • Adequate calcium intake (with Vitamin D) throughout life is critical to prevent osteoporosis in old age. • 💡 Option A (Sodium) is wrong because sodium is mainly an extracellular electrolyte in body fluids, not a structural bone mineral; Option B (Zinc) is wrong because zinc is a trace mineral involved in enzyme function, not a major bone component; Option C (Iron) is wrong because iron is primarily found in hemoglobin in red blood cells, not bones.

Correct Answer: D. Cartilage

• **Cartilage** = Articular (hyaline) cartilage is a smooth, glassy connective tissue that covers the ends of bones at synovial joints; its slippery surface reduces friction to nearly zero, protecting bones during movement. • **Key fact** — Unlike bone, cartilage has no blood vessels or nerves and receives nutrients by diffusion; this is why it heals very slowly when damaged. • The knee's menisci are another form of cartilage (fibrocartilage) that act as shock absorbers between the femur and tibia. • 💡 Option A (Ligament) is wrong because ligaments are tough fibrous bands that connect bone to bone for joint stability; Option B (Muscle) is wrong because muscles generate movement and do not cover bone ends; Option C (Tendon) is wrong because tendons connect muscles to bones, transmitting force.

Correct Answer: B. 206

• **206** = The adult human body has 206 bones; a newborn starts with about 270–300 bones, and many fuse together during childhood and adolescence to reach the adult count. • **Key fact** — The last bones to fuse are typically the clavicles and the epiphyseal plates of long bones, completing around age 25. • The 206 bones are divided into the axial skeleton (80 bones: skull, spine, ribs, sternum) and the appendicular skeleton (126 bones: limbs and girdles). • 💡 Option A (180) is wrong because it is too low and does not match any known developmental stage; Option C (300) is closer to a newborn's bone count before fusion occurs; Option D (210) is wrong because it overstates the adult number by four bones.

Correct Answer: B. Hyoid

• **Hyoid** = The hyoid is a U-shaped bone suspended in the upper neck by muscles and ligaments; it is the only bone in the body that does not articulate (form a joint) with any other bone. • **Key fact** — The hyoid anchors the tongue muscles above and the larynx (voice box) below, making it essential for swallowing, speaking, and breathing. • In forensic medicine, a fractured hyoid is a strong indicator of death by strangulation. • 💡 Option A (Mandible) is wrong because the mandible is the lower jawbone that holds the lower teeth; Option C (Atlas) is wrong because the atlas is the first cervical vertebra that supports the skull; Option D (Maxilla) is wrong because the maxilla forms the upper jaw and part of the hard palate.

Correct Answer: C. Ilium

• **Ilium** = The ilium is the largest and uppermost of the three bones (ilium, ischium, pubis) that fuse to form each hip bone (os coxae); together the two hip bones and sacrum form the pelvic girdle. • **Key fact** — The iliac crest (top rim of the ilium) is the landmark doctors use for bone marrow biopsies and is the reference point for measuring limb-length discrepancy. • The pelvic girdle transfers the weight of the upper body to the lower limbs and protects the pelvic organs. • 💡 Option A (Clavicle) is wrong because the clavicle (collarbone) is part of the pectoral girdle connecting the arm to the axial skeleton; Option B (Sternum) is wrong because the sternum (breastbone) is part of the axial skeleton; Option D (Scapula) is wrong because the scapula (shoulder blade) is part of the pectoral girdle, not the pelvis.

Correct Answer: B. Lumbar

• **Lumbar** = The five lumbar vertebrae (L1–L5) are the largest and strongest in the spine because they bear the greatest compressive load — supporting the entire weight of the upper body. • **Key fact** — The lumbar vertebrae have large, kidney-shaped bodies and thick, sturdy processes; L4–L5 is the most common site of herniated discs. • The lumbar region has a natural forward curve (lordosis) that helps distribute mechanical stress during walking and standing. • 💡 Option A (Sacral) is wrong because the sacral vertebrae are fused into a single triangular bone (sacrum) and are not individual large vertebrae; Option C (Thoracic) is wrong because the 12 thoracic vertebrae are medium-sized and are mainly associated with rib attachment; Option D (Cervical) is wrong because the 7 cervical vertebrae are the smallest in the spine.