Skeletal System — Set 5

Correct Answer: C. Radius

• **Radius** = The radius is the lateral bone of the forearm, running on the thumb side from the elbow to the wrist; it is the primary bone that transfers force from the wrist to the elbow during weight-bearing. • **Key fact** — The distal end of the radius forms the main part of the wrist joint; a 'Colles' fracture' — a break near the wrist end of the radius — is one of the most common fractures in humans. • The radius rotates around the ulna, allowing the hand to pronate (palm down) and supinate (palm up). • 💡 Option A (Fibula) is wrong because the fibula is the thin lateral bone of the lower leg, not the arm; Option B (Ulna) is wrong because the ulna runs on the little-finger (medial) side of the forearm; Option D (Tibia) is wrong because the tibia is the weight-bearing shin bone of the lower leg.

Correct Answer: B. Fingers and Toes

• **Fingers and Toes** = Phalanges are the small tubular bones that form the fingers (14 per hand) and toes (14 per foot); each finger (except the thumb) has three phalanges — proximal, middle, and distal — while the thumb and big toe each have only two. • **Key fact** — The human body has a total of 56 phalanges (28 in the hands + 28 in the feet), making them among the most numerous bones in the body. • The word 'phalanx' is Greek for a military formation — referring to the rows of finger bones lined up like soldiers. • 💡 Option A (Arms) is wrong because the arm bones are the humerus, radius, and ulna — not phalanges; Option C (Back) is wrong because back bones are the vertebrae of the spinal column; Option D (Legs) is wrong because leg bones are the femur, tibia, and fibula.

Correct Answer: C. Cranium

• **Cranium** = The cranium is the rounded, dome-shaped portion of the skull formed by eight fused bones (frontal, parietal ×2, temporal ×2, occipital, sphenoid, ethmoid) that encase and protect the brain. • **Key fact** — The cranium's interlocking sutures distribute impact forces across the entire structure, significantly reducing the risk of brain injury from blows to the head. • Inside the cranium, the brain is further cushioned by three meningeal layers and cerebrospinal fluid. • 💡 Option A (Pelvis) is wrong because the pelvis protects the bladder, reproductive organs, and rectum — not the brain; Option B (Vertebral column) is wrong because the vertebral column encases and protects the spinal cord; Option D (Rib cage) is wrong because the rib cage encloses the heart and lungs.

Correct Answer: D. Cartilaginous joint

• **Cartilaginous joint** = In a cartilaginous joint, bones are connected by cartilage (either hyaline or fibrocartilage); they permit slight, controlled movement rather than full free motion — classic examples are the intervertebral discs between vertebrae and the pubic symphysis. • **Key fact** — The intervertebral discs act as shock absorbers; while each disc allows only a tiny amount of movement, the combined flexibility of all 23 discs gives the spine its wide range of motion. • Cartilaginous joints are classified as slightly movable (amphiarthroses) — more mobile than fibrous joints but less mobile than synovial joints. • 💡 Option A (Synovial joint) is wrong because synovial joints have a fluid-filled cavity and allow the greatest range of motion (e.g., knee, shoulder); Option B (Fibrous joint) is wrong because fibrous joints are bound by collagen fibres and allow almost no movement (e.g., skull sutures); Option C (Ball and socket joint) is wrong because it is a subtype of synovial joint that allows the widest range of movement in all directions (e.g., hip, shoulder).

Correct Answer: A. 26

• **26** = The adult vertebral column has 26 bones: 24 individual vertebrae (7 cervical + 12 thoracic + 5 lumbar), plus the sacrum (5 fused vertebrae) and the coccyx (3–5 fused vertebrae, counted as 1). • **Key fact** — At birth, the spine has 33–34 separate vertebrae; the sacral and coccygeal bones fuse during adolescence and early adulthood, reducing the count to 26. • The vertebral column encloses the spinal cord, supports the skull, and is the central attachment point for the ribs and many back muscles. • 💡 Option B (22) is wrong because it falls short of the actual count and does not match any accepted anatomical grouping; Option C (33) is closer to the infant/child count before fusion, not the adult count; Option D (24) is wrong because it counts only the individual (pre-fusion) vertebrae without the sacrum and coccyx as single bones.

Correct Answer: C. Calcaneus

• **Calcaneus** = The calcaneus is the largest and strongest tarsal bone, forming the heel and providing the posterior pillar of the foot's arch; the Achilles tendon — the body's strongest tendon — attaches directly to it. • **Key fact** — The calcaneus is the most commonly fractured tarsal bone, typically due to falls from a height where the heel strikes the ground with great force. • It works with the talus above it to form the subtalar joint, which allows side-to-side rocking motion of the foot essential for walking on uneven ground. • 💡 Option A (Talus) is wrong because the talus sits on top of the calcaneus and forms the ankle joint with the tibia and fibula; Option B (Patella) is wrong because the patella is the kneecap — a sesamoid bone embedded in the quadriceps tendon; Option D (Cuboid) is wrong because the cuboid is a lateral tarsal bone that articulates with the calcaneus in front, not the heel itself.

Correct Answer: B. Osteoclasts

• **Osteoclasts** = Osteoclasts are large, multinucleated cells derived from monocytes; they break down (resorb) bone tissue by secreting acids and enzymes that dissolve the mineral matrix, a process called bone resorption. • **Key fact** — Bone remodeling is a continuous cycle: osteoclasts resorb old or damaged bone first, then osteoblasts move in to deposit new bone matrix in its place — the skeleton is completely renewed roughly every 10 years. • Osteoclast overactivity (e.g., in osteoporosis or cancer metastasis) leads to weakened, porous bones prone to fracture. • 💡 Option A (Chondrocytes) is wrong because chondrocytes maintain cartilage tissue, not bone; Option C (Osteocytes) is wrong because osteocytes are mature bone cells embedded in the matrix that sense mechanical stress but do not break down bone; Option D (Osteoblasts) is wrong because osteoblasts are the bone-building cells that deposit new bone matrix — the opposite function of osteoclasts.

Correct Answer: C. Sternum

• **Sternum** = The sternum is a flat, dagger-shaped bone in the center of the chest consisting of three parts — manubrium (top), body (middle), and xiphoid process (bottom); the ribs attach to it via costal cartilages, forming the rib cage. • **Key fact** — During CPR (cardiopulmonary resuscitation), compressions are applied to the lower half of the sternum to pump blood; the sternum is also the site of sternal bone marrow biopsies. • The sternum contains red bone marrow in adults and is one of the sites where blood cells are produced throughout life. • 💡 Option A (Scapula) is wrong because the scapula is the shoulder blade on the back of the thorax; Option B (Clavicle) is wrong because the clavicle is the collarbone running horizontally from the sternum to the shoulder; Option D (Femur) is wrong because the femur is the thigh bone — the longest bone in the body.

Correct Answer: D. Middle ear

• **Middle ear** = The middle ear is an air-filled cavity behind the eardrum (tympanic membrane) that houses three tiny ossicles: malleus (hammer), incus (anvil), and stapes (stirrup); these bones amplify and transmit sound vibrations from the eardrum to the inner ear. • **Key fact** — The stapes is the smallest bone in the body at only about 3 mm long; all three ossicles together weigh less than 0.1 gram yet amplify sound pressure by about 22 times. • The Eustachian tube connects the middle ear to the throat to equalize air pressure — which is why ears 'pop' during altitude changes. • 💡 Option A (Ear canal) is wrong because the ear canal is a tube of the outer ear that carries sound waves to the eardrum — it contains no bones; Option B (Inner ear) is wrong because the inner ear (cochlea, semicircular canals) is fluid-filled and houses sensory cells, not bones; Option C (Outer ear) is wrong because the outer ear consists of the pinna (cartilage) and the ear canal — no ossicles.

Correct Answer: A. Femur

• **Femur** = The femur (thigh bone) is the longest and heaviest bone in the body; it belongs to the appendicular skeleton — the division that includes the bones of the limbs and the girdles connecting them to the trunk. • **Key fact** — The axial skeleton (80 bones) forms the central axis of the body: skull, vertebral column, ribs, and sternum; everything else — including all four limbs — is appendicular. • The head of the femur fits into the acetabulum of the pelvis to form the hip joint, which is the point where appendicular meets axial skeleton. • 💡 Option B (Ribs) is wrong because all 12 pairs of ribs are part of the axial skeleton forming the thoracic cage; Option C (Sternum) is wrong because the sternum is the central chest bone and a core part of the axial skeleton; Option D (Skull) is wrong because the skull (cranium + facial bones) is the topmost component of the axial skeleton.