RDN Physiology Flashcards: 140 Practice Questions & Answers

Absorption of fat-soluble vitamins (A, D, E, K) requires bile salts and pancreatic lipases for micelle formation. In chronic pancreatitis, lipase deficiency prevents the hydrolysis of triglycerides and retinyl esters. Without micelles, these vitamins cannot reach the brush border of the enterocytes for passive diffusion. CDR Tip: Distinguish between digestion (lipase) and absorption (micelle/diffusion). Distractors often mention the portal vein; remember, fat-soluble vitamins (except some K2) primarily enter the lymphatic system via chylomicrons.

Phosphorus acts as a titratable acid or urinary buffer. The dibasic phosphate (HPO4 2-) in the renal tubule filtrate reacts with secreted hydrogen ions (H+) to form monobasic phosphate (H2PO4-), which is then excreted. This allows the kidney to eliminate H+ without excessively lowering urine pH, crucial for preventing systemic acidosis.

In skeletal muscle, pyruvate (from glycolysis) is transaminated to Alanine, utilizing nitrogen derived from BCAA catabolism. Alanine is released into the blood and transported to the liver. In the liver, Alanine is deaminated back to pyruvate for gluconeogenesis (to maintain blood glucose), while the nitrogen group is diverted to the Urea Cycle for excretion. Exam Tip: This cycle allows the muscle to export potentially toxic ammonia safely while providing a carbon skeleton for glucose production.

Renal Compensation: The kidneys increase the reabsorption of bicarbonate (HCO3-) and increase the secretion of hydrogen ions (H+) into the urine. This increases the serum pH back toward the normal range. Timeline: Unlike the respiratory system which reacts in minutes, renal compensation is slow, beginning within hours but taking 3 to 5 days to reach maximal effectiveness. Clinical Note: This is why chronic respiratory disorders show higher HCO3 levels than acute ones. CDR emphasizes the difference in speed between pulmonary and renal buffering.

1) Cow's milk is low in iron and its iron is poorly absorbed. 2) High calcium and casein in cow's milk inhibit non-heme iron absorption by competing for transport proteins. 3) Heat-treated proteins in cow's milk can cause microscopic intestinal bleeding in infants <12 months. This triple threat leads to Iron Deficiency Anemia (IDA). CDR Tip: Cow's milk introduction must be >12 months. Focus on the competition between calcium and iron for absorption sites as a key physiological distractor.