Skeletal System — Set 3

Correct Answer: C. Appendicular skeleton

• **Appendicular skeleton** = The appendicular skeleton consists of the 126 bones that form the appendages (limbs) and the girdles that attach them to the axial skeleton: pectoral girdle (clavicles + scapulae), upper limbs, pelvic girdle, and lower limbs. • **Key fact** — 'Appendicular' comes from the Latin 'appendere' (to hang upon); the limbs literally hang off the axial skeleton via the shoulder and hip girdles. • The appendicular skeleton's primary function is locomotion and manipulation of the environment. • 💡 Option A (Axial skeleton) is wrong because the axial skeleton is the central core — skull, vertebral column, ribs, and sternum (80 bones); Option B (Exoskeleton) is wrong because an exoskeleton is an external skeleton found in arthropods like insects, not inside the human body; Option D (Endoskeleton) is wrong because endoskeleton is a general term for any internal skeleton — it is not a specific division of the human skeleton.

Correct Answer: A. Stapes

• **Stapes** = The stapes (stirrup) is the smallest bone in the human body, measuring only about 3 mm in length and weighing approximately 2–3 mg; it is the innermost of the three ossicles in the middle ear. • **Key fact** — The three auditory ossicles — malleus (hammer), incus (anvil), and stapes (stirrup) — form a mechanical lever chain that amplifies sound vibrations from the eardrum and transmits them to the oval window of the cochlea, enabling hearing. • The stapes inserts into the oval window; when it vibrates, it creates pressure waves in the cochlear fluid that stimulate hair cells. • 💡 Option B (Incus) is wrong because the incus is the middle ossicle and slightly larger than the stapes; Option C (Hyoid) is wrong because the hyoid is the horseshoe-shaped throat bone, much larger than the stapes; Option D (Malleus) is wrong because the malleus is the largest of the three ossicles, attached to the eardrum.

Correct Answer: D. 22

• **22** = The adult human skull is composed of 22 bones in total: 8 cranial bones (that encase the brain) and 14 facial bones (that form the face). • **Key fact** — The 8 cranial bones are: frontal, 2 parietal, occipital, 2 temporal, sphenoid, ethmoid. The 14 facial bones include the mandible, vomer, 2 maxillae, 2 palatine, 2 zygomatic, 2 nasal, 2 lacrimal, and 2 inferior nasal conchae. • All skull bones except the mandible are connected by immovable suture joints. • 💡 Option A (20) is wrong because 20 does not correspond to any recognised skull bone count; Option B (14) is wrong because 14 is only the facial bone count, not the total; Option C (8) is wrong because 8 is only the cranial bone count, not the full skull total.

Correct Answer: C. Hinge joint

• **Hinge joint** = The elbow (humeroulnar joint) is a classic hinge (ginglymus) joint where the trochlea of the humerus articulates with the trochlear notch of the ulna, permitting only flexion and extension in a single plane — like a door hinge. • **Key fact** — The elbow is actually a compound joint: the humeroulnar hinge joint and the humeroradial condyloid joint (for flexion/extension) share the same capsule with the proximal radioulnar pivot joint (for pronation/supination). • Normal elbow extension slightly exceeds 180° (hyperextension), and carrying angle (valgus angle) of about 10–15° is normal in women. • 💡 Option A (Gliding joint) is wrong because gliding (plane) joints allow flat-surface sliding movements, as seen in the tarsal bones; Option B (Ball and socket joint) is wrong because the shoulder and hip are ball-and-socket joints that allow multidirectional movement; Option D (Pivot joint) is wrong because a pivot joint allows rotation, as at the atlas-axis articulation — not the main elbow movement.

Correct Answer: A. Fibula

• **Fibula** = The fibula is the slender lateral bone of the lower leg (running alongside the tibia from the knee to the ankle); it bears little body weight but provides attachment for many lower-leg muscles and forms the lateral malleolus of the ankle joint. • **Key fact** — The fibula is the most commonly fractured bone in the ankle; a 'Pott's fracture' involves the fibula (and sometimes the tibia), caused by twisting forces at the ankle. • The fibula can be harvested as a bone graft for reconstructive surgery of the jaw (mandibular reconstruction) because its periosteal blood supply allows it to survive as a free flap. • 💡 Option B (Ulna) is wrong because the ulna is a forearm bone of the upper limb; Option C (Humerus) is wrong because the humerus is the upper-arm bone; Option D (Radius) is wrong because the radius is the other forearm bone on the thumb side of the upper limb.

Correct Answer: B. Frontal bone

• **Frontal bone** = The frontal bone is a single cranial bone that forms the forehead (squamous part), the superior rims of the eye sockets (orbital part), and the anterior part of the cranial floor; it is one of the 8 bones of the neurocranium. • **Key fact** — The frontal bone contains the frontal sinuses — air-filled cavities that reduce skull weight and warm/humidify inhaled air; sinusitis commonly affects these cavities, causing forehead pain. • In infants, the frontal bone is in two halves separated by the metopic suture, which fuses during the first 2 years of life. • 💡 Option A (Occipital bone) is wrong because the occipital bone forms the back and base of the skull, containing the foramen magnum; Option C (Temporal bone) is wrong because the temporal bones form the sides of the skull and house the ear structures; Option D (Parietal bone) is wrong because the two parietal bones form the top and sides of the skull vault.

Correct Answer: B. Rickets

• **Rickets** = Rickets is a childhood bone disease caused by vitamin D (or calcium/phosphorus) deficiency, leading to defective mineralisation of the growth plate cartilage and osteoid, resulting in soft, weak bones that bow under body weight — producing the classic bow-legs or knock-knees appearance. • **Key fact** — Vitamin D is needed to absorb calcium from the gut; without it, osteoid (bone protein matrix) cannot mineralise, the growth plates widen, and bones deform. Sunlight exposure triggers vitamin D synthesis in the skin (via cholesterol → vitamin D₃). • The adult equivalent of rickets is osteomalacia — same vitamin D deficiency, same soft bones, but the growth plates are already closed so there is no bowing. • 💡 Option A (Beriberi) is wrong because beriberi is caused by vitamin B₁ (thiamine) deficiency, causing nerve and heart problems; Option C (Pellagra) is wrong because pellagra results from vitamin B₃ (niacin) deficiency, causing dermatitis, diarrhoea, and dementia; Option D (Scurvy) is wrong because scurvy is caused by vitamin C deficiency, impairing collagen synthesis and causing bleeding gums.

Correct Answer: B. Ligament

• **Ligament** = Ligaments are dense, regular connective tissue bands composed primarily of collagen fibres (type I) that connect bone to bone at a joint, stabilising it and limiting excessive movement. • **Key fact** — The word 'ligament' comes from the Latin 'ligare' (to bind); major examples include the anterior cruciate ligament (ACL) of the knee and the lateral ligaments of the ankle — both commonly torn in sports injuries. • Unlike tendons, ligaments have a relatively poor blood supply, making healing after sprains slow and sometimes incomplete. • 💡 Option A (Nerve) is wrong because nerves transmit electrical impulses and do not physically connect bones; Option C (Tendon) is wrong because tendons connect muscle to bone (not bone to bone); Option D (Muscle) is wrong because muscles generate movement by contracting — they connect to bones via tendons, not directly.

Correct Answer: C. 24

• **24** = Humans have 12 pairs of ribs, giving a total of 24 ribs; all 12 pairs articulate with the 12 thoracic vertebrae posteriorly. • **Key fact** — Anteriorly, ribs 1–7 attach directly to the sternum ('true ribs'), ribs 8–10 attach indirectly via shared costal cartilage ('false ribs'), and ribs 11–12 have no anterior attachment ('floating ribs'). Both males and females have exactly 12 pairs. • A common misconception (from the biblical story of Adam) holds that men have one fewer rib than women — anatomically this is false; both sexes have 24. • 💡 Option A (22) is wrong because 22 is the number of skull bones, not ribs; Option B (10) is wrong because only 10 pairs reach the anterior chest (ribs 11–12 float), but the total count is 24 not 10; Option D (12) is wrong because 12 is the number of pairs, not the total individual ribs.

Correct Answer: A. Femur

• **Femur** = The femur (thigh bone) is the longest, largest, and strongest bone in the human body; in an average adult it measures about 45–50 cm (roughly 25% of a person's height) and can withstand compressive forces of about 1,700 kg. • **Key fact** — The femur articulates proximally with the hip (ball-and-socket joint with the acetabulum) and distally with the tibia and patella at the knee (hinge joint); it transmits the entire body weight from the pelvis to the lower leg. • A fractured femur is a medical emergency because the thigh muscles can cause significant internal bleeding into the thigh — up to 1–2 litres of blood can be lost. • 💡 Option B (Tibia) is wrong because the tibia is the second-longest bone but is shorter and lighter than the femur; Option C (Hip bone) is wrong because the hip bone (ilium + ischium + pubis) is the largest flat bone but is not a long bone and is not longer than the femur; Option D (Humerus) is wrong because the humerus is the upper-arm bone and is considerably shorter and lighter than the femur.