Blood plasma osmolarity is higher than intracellular fluid osmolarity.

A. The neural tube and yolk sac have formed: By 8 weeks, the neural tube and yolk sac have formed, but this is not the criterion for defining a fetus.

B. The amniotic cavity has formed: The formation of the amniotic cavity occurs earlier in development and is not the defining feature of a fetus.

C. All of the organ systems are present: By the end of 8 weeks, the basic structures of all major organ systems are present, which is why the embryo is referred to as a fetus from this point onwards.

D. The mesoderm has formed: The formation of the mesoderm occurs earlier and is not the defining factor for fetal status. By 8 weeks, more significant development, including the presence of organ systems, has occurred.

A. Glomerulus is involved in filtering blood but not in producing hypertonic urine.

B. Collecting duct reabsorbs water and concentrates urine, making it hypertonic.

C. Nephron loop (loop of Henle) creates a concentration gradient in the medulla that helps in water reabsorption but is not the primary site for final urine concentration. This option is partially correct but less specific than the collecting duct.

D. Proximal convoluted tubule reabsorbs a significant portion of water and solutes but does not produce hypertonic urine.