This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Gastrointestinal System, Subject name: Physiology
The pump responsible for gastric acid secretion is 
Select and explain. 
A. H+ ATPase, B. Na+ - H+ ATPase, C. H+ - K+ ATPase, D. Ca2+ - H+ ATPase
C: H+ - K+ ATPase
Answer: C: H+ - K+ ATPase 
Explanation: Gastric parietal cells are highly specialized for their unusual task of secreting concentrated acid. The cells are packed with mitochondria that supply energy to drive the apical H+,K+-ATPase, or proton pump, that moves H+ ions out of the parietal cell against a concentration gradient. The apical membrane also contains potassium channels, which supply the K+ ions to be exchanged for H+, and Cl- channels that supply the counterion for HCl secretion.