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: Medicine
Newborn 7 days old with vomiting and dehydration clinical examination was normal except for hyperpigmentation of nipple. Electrolytes Na: 120 meq. K: 9 meq. most likely diagnosis - 
Select and explain. 
A. Cong, adrenal hyperplasia, B. Primary hypothyroidism, C. Panhypopituitarism, D. Pyloric stenosis
A: Cong, adrenal hyperplasia
Answer: A: Cong, adrenal hyperplasia 
Explanation: Congenital adrenal hyperplasia due to a defect in CYP21A2 is suggested by episodes of acute adrenal insufficiency with hyponatremia, hyperkalemia, dehydration & vomiting