Which finding is characteristic of all forms of clinical hypoglycemia?

In clinical hypoglycemia, neuroglycopenic symptoms are a key characteristic. These symptoms arise when the brain lacks sufficient glucose, which is its primary energy source. Common neuroglycopenic symptoms include confusion, dizziness, headache, irritability, and in severe cases, seizures or loss of consciousness. Since these symptoms can occur whenever blood glucose levels are low, they are a common and defining feature across all forms of hypoglycemia.

Fasting blood glucose levels below 55 mg/dL may be indicative of hypoglycemia, but not all cases of hypoglycemia will present with this specific threshold; individual patients may exhibit different thresholds for symptoms. High fasting insulin levels can be seen in some types of hypoglycemia, particularly those related to insulinomas or certain medication-induced states, but they are not universal to all forms. Decreased serum C peptide is associated with conditions like insulin-dependent diabetes or other situations of low or no insulin production, but again, it does not apply to all hypoglycemic scenarios.

Thus, neuroglycopenic symptoms are the most consistent finding across different causes and presentations of hypoglycemia, making this the correct answer.