MCAT® ExamBiological and Biochemical Foundations of Living SystemsHard
A patient is diagnosed with a rare genetic disorder that results in chronic metabolic acidosis. The primary defect is found in the cells responsible for secreting bicarbonate into the renal tubules, located in the distal convoluted tubule and collecting duct. Which of the following is the most direct consequence of this defect on the patient's blood pH regulation?
- AIncreased reabsorption of hydrogen ions in the proximal tubule
- BEnhanced production of carbonic acid in the blood
- CDecreased excretion of ammonia by the kidneys
- DImpaired ability to excrete acid and conserve bicarbonate
Show answer & explanationAnswer & explanation
Correct answer: D. Impaired ability to excrete acid and conserve bicarbonate
The cells in the distal convoluted tubule and collecting duct are crucial for fine-tuning acid-base balance, specifically by secreting hydrogen ions and reabsorbing bicarbonate. A defect in bicarbonate secretion into the tubules implies an inability to reabsorb it from the filtrate (or an inability to generate new bicarbonate for buffering), leading to a net loss of bicarbonate and thus an impaired ability to excrete acid and conserve bicarbonate, resulting in acidosis.
Why the other options are wrong
- A. Increased reabsorption of hydrogen ions would worsen acidosis, but the primary defect described is related to bicarbonate handling, not H+ reabsorption in the proximal tubule.
- B. Enhanced production of carbonic acid would occur if CO2 levels were high (respiratory acidosis) or if metabolism generated more acid, which is not the primary defect described.
- C. Ammonia excretion is another mechanism for acid excretion, but the direct defect is in bicarbonate handling, which is a more fundamental buffer system.
Renal Acid-Base Regulation
The kidneys regulate blood pH by reabsorbing bicarbonate, excreting hydrogen ions, and producing ammonia to buffer excreted acids.
- Proximal tubule reabsorbs most bicarbonate.
- Distal tubule and collecting duct fine-tune H+ excretion and bicarbonate reabsorption/generation.
- Ammonia buffering aids H+ excretion.
- Crucial for maintaining acid-base homeostasis.
Memory trick: Bicarbonate's Balance is Kidney's Key