This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pediatrics
A 1–month old baby present with frequent vomiting and failure to thrive. There are features of moderate dehydration. Blood sodium in 122 mEq/1 and potassium is 6.1 mEq/1. The most likely diagnosis is – 
Select and explain. 
A. Gitelman syndrome, B. Bartter syndrome, C. 21–hydroxylase deficiency, D. 11–β hydroxylase deficiency
C: 21–hydroxylase deficiency
Answer: C: 21–hydroxylase deficiency 
Explanation: Blood parameters indicate Hyponatremia and Hyperkalemia in this patient. All conditions other than 21 hydroxylase deficiency present with Hypokalemia.