How does the nephron regulate the acid-base balance of blood?

A. It assists in the absorption of vitamin A in the liver: Vitamin A is a fat-soluble vitamin absorbed with dietary fats; the liver is a storage site, but intrinsic factor is not involved.

B. It aids in the absorption of vitamin D in the kidneys: Vitamin D is synthesized in the skin and activated in the kidneys; intrinsic factor plays no role in its metabolism.

C. It helps in the absorption of vitamin C in the stomach: Vitamin C is a water-soluble vitamin absorbed primarily in the small intestine via active transport, independent of intrinsic factor.

D. It binds to vitamin B12 to facilitate its absorption in the ileum: Intrinsic factor is a glycoprotein produced by the parietal cells of the stomach. It must bind to B12 to protect it from digestion and allow it to be absorbed in the terminal ileum.

A. Obstruction in the lower airway: While possible, accessory muscle use is a general sign of increased work of breathing, not specific only to lower airway obstructions.

B. Adequate alveolar ventilation: Labored breathing suggests that ventilation is inadequate or under severe strain.

C. Normal respiratory effort: Normal breathing (eupnea) is quiet and uses only the diaphragm and external intercostals, not accessory muscles like the sternocleidomastoid.

D. The patient is compensating for impaired gas exchange: The body uses accessory muscles to increase the volume of the thoracic cavity to move more air when gas exchange is insufficient to meet metabolic demands.