**Core Concept**
The infant of a diabetic mother (IDM) is a newborn baby whose mother had diabetes mellitus during pregnancy. This condition can lead to various complications and congenital anomalies due to the altered intrauterine environment.

**Why the Correct Answer is Right**
Infants of diabetic mothers are at risk of developing macrosomia (excessive birth weight), hypoglycemia (low blood sugar), and polycythemia (elevated red blood cell count). One of the key features of IDM is the presence of hypertrophic cardiomyopathy, which is a thickening of the heart muscle leading to left ventricular hypertrophy. This condition is thought to be due to the increased workload on the heart caused by the high blood sugar levels in the mother, leading to an increase in cardiac output and subsequent hypertrophy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while IDM babies may have various complications, they are not typically characterized by a decrease in birth weight.

**Option B:** This option is incorrect because while IDM babies may have an increased risk of congenital anomalies, microcephaly (small head size) is not a typical feature.

**Option C:** This option is incorrect because while IDM babies may have an increased risk of hypoglycemia, they are not typically characterized by an increase in birth weight.

**Clinical Pearl / High-Yield Fact**
Remember that IDM babies are at risk of developing various complications, including hypertrophic cardiomyopathy, macrosomia, and hypoglycemia. It is essential to monitor these babies closely after birth to prevent and manage these complications.

**Correct Answer:** C. Hypertrophic cardiomyopathy