mrcem2-0516

A 3-week-old girl presents with vomiting, poor weight gain, and decreased muscle tone. She is hypotensive and has a fast heart rate. During the examination, you notice that she has enlarged labia and increased pigmentation. Blood tests show high potassium, low sodium, and elevated levels of 17-hydroxyprogesterone. A venous blood gas reveals the presence of metabolic acidosis, and her blood glucose level is slightly low. Intravenous fluids have already been started.

What is the SINGLE most appropriate next step in management?