**Core Concept:**
Hyperglycemia in pregnancy refers to high blood sugar levels that occur during pregnancy. In this case, the patient and her mother and sister all have a similar history of hyperglycemia during pregnancy, indicating an inherited genetic disorder.

**Why the Correct Answer is Right:**
The correct answer is **C.** Inherited disorders of glucose homeostasis, such as in this case, are often caused by mutations in genes that encode enzymes involved in glucose regulation. One such enzyme is **alpha-glucosidase (α-GSI) which plays a crucial role in the digestion and absorption of carbohydrates.** The correct answer, C, refers to a deficiency of α-glucosidase, leading to impaired carbohydrate digestion and subsequent hyperglycemia during pregnancy.

**Why Each Wrong Option is Incorrect:**
A. The other options (A, B, and D) do not specifically address the enzyme defect causing hyperglycemia in pregnancy.

A. Insulin resistance is a condition in which the body does not respond properly to insulin, but it is not an enzyme defect and does not relate to pregnancy.

B. Gestational diabetes mellitus (GDM) is a form of diabetes that occurs during pregnancy, but it is a temporary condition and not an inherited enzyme defect.

D. The other genetic disorders mentioned are not related to the enzyme defect causing hyperglycemia in pregnancy.

**Clinical Pearl:**
A thorough understanding of inherited genetic disorders, particularly those related to carbohydrate, fat, or amino acid homeostasis can help identify the underlying cause of hyperglycemia in patients and their family members. In this case, the deficiency of α-glucosidase leads to impaired carbohydrate digestion and absorption, resulting in elevated blood glucose levels during pregnancy. This knowledge aids in providing appropriate medical management and counseling for affected individuals and their families regarding the inheritance pattern and potential future complications.