A 10-day-old female neonate presents to the emergency department with severe vomiting, dehydration, and lethargy. Laboratory investigations reveal serum sodium 118 mEq/L, potassium 6.8 mEq/L, and blood glucose 42 mg/dL. Imaging shows bilateral adrenal enlargement. The structure marked **C** in the diagram (Zona Reticularis) is involved in a metabolic block that shunts precursor hormones into the androgen pathway. Which of the following is the PRIMARY DIAGNOSTIC MARKER of the enzyme deficiency affecting this zone?