Skeletal System — Set 1

Correct Answer: D. Clavicle

• **Clavicle** = The clavicle is the S-shaped bone that runs horizontally between the sternum and the shoulder, forming the visible 'collar' at the top of the chest — hence the common name 'collar bone.' • **Key fact** — The clavicle is the most frequently fractured bone in the body and acts as a strut that holds the arm away from the trunk, allowing a full range of shoulder motion. • It also transmits compressive forces from the upper limb to the axial skeleton. • 💡 Option A (Scapula) is wrong because it is the flat triangular shoulder blade, not a collar; Option B (Sternum) is wrong because it is the breastbone in the centre of the chest; Option C (Radius) is wrong because it is a forearm bone.

Correct Answer: A. 80

• **80** = The axial skeleton forms the central axis of the body and contains exactly 80 bones: 29 skull bones, 26 vertebrae, 24 ribs, and 1 sternum. • **Key fact** — The axial skeleton contrasts with the appendicular skeleton (126 bones), and together they make up the total adult count of 206 bones. • The axial skeleton's primary role is to protect vital organs — brain, spinal cord, heart, and lungs. • 💡 Option B (126) is wrong because 126 is the count of the appendicular skeleton; Option C (206) is wrong because that is the total bones in the whole body; Option D (22) is wrong because 22 is only the number of skull bones.

Correct Answer: B. Pivot joint

• **Pivot joint** = The atlanto-axial joint is a pivot (trochoid) joint where the ring-like atlas (C1) rotates around the peg-like odontoid process (dens) of the axis (C2), enabling the head to rotate left and right. • **Key fact** — This is the only joint in the body that allows pure axial rotation of the head — the movement you make when shaking your head 'no.' • Another pivot joint example is the proximal radio-ulnar joint that allows forearm pronation and supination. • 💡 Option A (Hinge joint) is wrong because hinge joints allow flexion/extension only, like the knee; Option C (Gliding joint) is wrong because gliding joints allow small sliding motions between flat surfaces; Option D (Saddle joint) is wrong because saddle joints allow biaxial movement, like the thumb's carpometacarpal joint.

Correct Answer: A. Calcium and Phosphorus

• **Calcium and Phosphorus** = Bone mineral is primarily hydroxyapatite — Ca₁₀(PO₄)₆(OH)₂ — a crystalline compound of calcium and phosphorus that fills the collagen matrix and gives bone its compressive strength and rigidity. • **Key fact** — About 99% of the body's calcium and 85% of its phosphorus are stored in the bones, making the skeleton a vital mineral reservoir that regulates blood calcium levels. • Vitamin D is essential to absorb calcium from the gut; deficiency leads to soft bones (rickets in children, osteomalacia in adults). • 💡 Option B (Sodium and Potassium) is wrong because these are electrolytes critical for nerve and muscle function, not bone hardness; Option C (Iron and Magnesium) is wrong because iron is in haemoglobin and magnesium is in enzymes, neither hardens bone; Option D (Silica and Carbon) is wrong because these are not significant components of human bone mineral.

Correct Answer: D. Osteology

• **Osteology** = Osteology is the branch of anatomy that deals with the detailed study of the structure, function, development, and diseases of bones — the word comes from the Greek 'osteon' (bone) + 'logos' (study). • **Key fact** — Osteologists study bone morphology, bone density, growth plates, and forensic identification; the field is critical in palaeontology and forensic medicine. • Orthopaedics is the medical specialty that treats bone diseases and injuries, while osteology is the foundational scientific discipline. • 💡 Option A (Geology) is wrong because geology studies Earth's rocks and minerals; Option B (Histology) is wrong because histology is the microscopic study of tissues in general; Option C (Ornithology) is wrong because ornithology is the study of birds.

Correct Answer: B. Red Bone Marrow

• **Red Bone Marrow** = Red bone marrow is the site of haematopoiesis — the production of red blood cells (erythrocytes), white blood cells (leukocytes), and platelets from haematopoietic stem cells. • **Key fact** — In adults, red bone marrow is found mainly in flat bones such as the sternum, ribs, vertebrae, pelvis, and skull; long bone shafts predominantly contain yellow marrow (fat). • Red marrow can produce up to 200 billion red blood cells per day to replace those that die. • 💡 Option A (Endosteum) is wrong because the endosteum is a thin membrane lining the inner surface of bone, not the site of RBC production; Option C (Yellow Bone Marrow) is wrong because yellow marrow stores fat and does not normally produce blood cells; Option D (Periosteum) is wrong because the periosteum is the outer fibrous covering of bone involved in growth and repair, not blood cell production.

Correct Answer: A. Tibia

• **Tibia** = The tibia is the larger and stronger of the two lower-leg bones; its anterior crest lies just beneath the skin and is the bony ridge you feel when you run a finger down your shin — giving rise to the common name 'shin bone.' • **Key fact** — The tibia is the primary weight-bearing bone of the lower leg and articulates with the femur above (at the knee), the fibula laterally, and the talus below (at the ankle). • 'Shin splints' (medial tibial stress syndrome) is a common injury among runners and involves pain along the tibia's medial border. • 💡 Option B (Femur) is wrong because the femur is the thigh bone, the longest in the body; Option C (Fibula) is wrong because the fibula is the slender lateral lower-leg bone, not the shin; Option D (Humerus) is wrong because the humerus is the upper-arm bone.

Correct Answer: C. 2 pairs

• **2 pairs** = The last two pairs of ribs (pairs 11 and 12) are called 'floating ribs' because they are attached to the thoracic vertebrae posteriorly but have no anterior attachment to the sternum or cartilage — they 'float' freely. • **Key fact** — Ribs 1–7 are 'true ribs' (directly attached to sternum via costal cartilage); ribs 8–10 are 'false ribs' (attached indirectly via shared costal cartilage); ribs 11–12 are 'floating ribs.' • Floating ribs protect the kidneys and upper abdominal organs and are occasionally surgically removed to relieve thoracic outlet syndrome. • 💡 Option A (5 pairs) is wrong because 5 pairs would include the false ribs; Option B (7 pairs) is wrong because 7 pairs refers to the true ribs; Option D (3 pairs) is wrong because only 2 pairs, not 3, lack any anterior attachment.

Correct Answer: A. Ossification

• **Ossification** = Ossification (osteogenesis) is the biological process by which new bone tissue is formed — osteoblasts secrete the collagen matrix and then mineralise it with calcium phosphate to create hard bone. • **Key fact** — There are two types: intramembranous ossification (flat skull bones form directly from mesenchyme) and endochondral ossification (most long bones replace a cartilage model, including the growth plate activity responsible for height increase). • Bone remodelling continues throughout life through the balanced actions of osteoblasts (builders) and osteoclasts (destroyers). • 💡 Option B (Digestion) is wrong because digestion is the breakdown of food in the alimentary canal; Option C (Respiration) is wrong because respiration is gas exchange between the body and the environment; Option D (Circulation) is wrong because circulation refers to blood moving through the vascular system.

Correct Answer: B. Mandible

• **Mandible** = The mandible (lower jaw) is the only movable bone in the skull; it articulates with the temporal bone at the temporomandibular joint (TMJ), enabling the movements of chewing, speaking, and yawning. • **Key fact** — The mandible is also the only bone in the skull that is not fused to its neighbours via sutures; all other skull bones are connected by immovable fibrous joints called sutures. • The TMJ is one of the most-used joints in the body, performing roughly 2,000 movements per day during eating and speaking. • 💡 Option A (Maxilla) is wrong because the maxilla forms the upper jaw and is fused immovably to the skull; Option C (Zygomatic bone) is wrong because the cheekbone is fixed via sutures; Option D (Frontal bone) is wrong because the frontal bone forms the forehead and is sutured in place.