**Question:** Which among the following is not a cause of fasting hypoglycemia?

A. Insulinoma
B. Cushing's syndrome
C. Acromegaly
D. Hyperthyroidism

**Core Concept:** Fasting hypoglycemia is a state of low blood glucose levels that occurs when the body is not consuming energy (e.g., fasting) and has not produced enough glucose. The underlying mechanisms include impaired gluconeogenesis, glycogenolysis, or both.

**Why the Correct Answer is Right:** D. Hyperthyroidism is not a direct cause of fasting hypoglycemia, as it primarily leads to increased basal glucose production (gluconeogenesis) and impaired glucose utilization, resulting in hyperglycemia.

**Why Each Wrong Option is Incorrect:**

A. Insulinoma: This is a benign tumor of the pancreas producing excessive amounts of insulin, leading to hypoglycemia due to increased glucose uptake and decreased glucose production.
B. Cushing's syndrome: This is a condition caused by prolonged exposure to excessive cortisol, leading to impaired glucose tolerance and increased glucose production, typically resulting in hyperglycemia.
C. Acromegaly: This is a condition caused by excessive growth hormone secretion, resulting in impaired glucose tolerance and increased glucose production, often leading to hyperglycemia.

**Clinical Pearl:** Hypoglycemia due to Cushing's syndrome is known as euglycemic hypoglycemia, as blood glucose levels remain within the normal range due to increased glucose production.

**Correct Answer:** B. Cushing's syndrome (hyperglycemic cause)

**Explanation:** Cushing's syndrome is not a direct cause of fasting hypoglycemia, but rather, it leads to hyperglycemia due to prolonged exposure to excessive cortisol. Fasting hypoglycemia is more commonly associated with conditions like insulinoma (hypoglycemic cause) or acromegaly (hypoglycemic cause).