A fossa is?

Correct Answer: D

Rationale: A fossa is a depression or indentation in a bone, such as the mandibular fossa of the temporal bone where the mandible articulates, serving as a surface for muscle attachment or joint articulation. An air-filled cavity describes sinuses (e.g., frontal sinus), not all fossae. A hollow passage suggests a canal or foramen (e.g., carotid canal), not a fossa's shallow nature. Fused cartilage refers to structures like synchondroses, unrelated to fossae. The term 'fossa' (Latin for ditch) denotes a sunken area, making 'd' the precise anatomical definition.

Correct Answer: A

Rationale: Parathyroid hormone (PTH), secreted by the parathyroid glands when blood calcium drops, regulates calcium and phosphate balance. In the kidneys, PTH enhances calcium reabsorption in the distal tubules, reducing urinary calcium loss and raising blood levels. Concurrently, it inhibits phosphate reabsorption in the proximal tubules, increasing phosphate excretion and lowering blood phosphate. This dual action maintains mineral homeostasis elevated calcium supports bone and muscle function, while reduced phosphate prevents soft tissue calcification. The kidneys filter 180 liters of plasma daily, and PTH fine-tunes reabsorption to counter hypocalcemia, often triggered by dietary or hormonal shifts. Calcium loss with phosphate retention would worsen hypocalcemia, contradicting PTH's purpose. Losing both minerals isn't selective enough for balance, and retaining both would raise phosphate, risking imbalances. PTH also mobilizes calcium from bone and boosts vitamin D activation, but its renal effect is immediate and precise. Endocrine physiology texts confirm this mechanism, highlighting PTH's critical role in mineral metabolism, distinct from other hormones like calcitonin, which lowers calcium.

Correct Answer: B

Rationale: Epithelial tissue is avascular, lacking blood vessels, and relies on diffusion from underlying connective tissue for nutrients, as seen in the epidermis or gut lining. Vascular or highly vascularized applies to connective tissues like dermis, not epithelium. Nerve endings may penetrate epithelium, but innervation isn't a defining trait avascularity is. This lack of vasculature, ensuring a protective or selective barrier, makes 'b' the correct characteristic.

Correct Answer: B

Rationale: Melanin, produced by melanocytes in the stratum basale, determines skin color, ranging from light to dark, and protects against UV by absorbing it. Hemoglobin colors blood, visible through skin but not primary. Carotene adds yellowish tones, minorly. Keratin is a structural protein, not a pigment. Melanin's dominant role in pigmentation and photoprotection, per dermatology, makes 'b' correct.

Correct Answer: C

Rationale: Osteoclasts resorb bone, breaking down matrix via enzymes/acids to release calcium, balancing remodeling. Osteoblasts form bone, osteocytes maintain it, and chondrocytes produce cartilage, not bone resorbers. Osteoclasts' destructive role, per bone physiology, makes 'c' correct.