Here are the questions with their correct answers:

16) Total body water is measured by ✅ A. Deuterium oxide (Heavy water / D₂O) Deuterium oxide is a dilution indicator that distributes throughout all body water compartments, making it the standard tracer for measuring total body water (TBW).

17) The cell responsible for bone resorption are ✅ B. Osteoclast Osteoclasts are large multinucleated cells derived from monocyte/macrophage lineage. They secrete acids and proteases (cathepsin K) to break down bone matrix - this is bone resorption.

18) Receptor for stretch reflex is ✅ A. Muscle spindle Muscle spindles (intrafusal fibers) are the sensory receptors that detect muscle stretch. When a muscle is stretched, Ia afferent fibers from spindles fire and cause a reflex contraction - this is the classic stretch reflex (e.g., knee-jerk reflex).

19) The primary receptor responsible for colour vision is ✅ B. Cones Cones are concentrated in the fovea centralis and contain three types of photopigments (S, M, L) sensitive to different wavelengths of light, enabling colour discrimination. Rods are for low-light/scotopic vision only.

20) Sertoli cell secrete ✅ A. Androgen binding protein (ABP) Sertoli cells (nurse cells of the testis) secrete Androgen Binding Protein under FSH stimulation. ABP concentrates testosterone in the seminiferous tubules to support spermatogenesis.