**Core Concept**
A sine wave pattern on a cardiotocograph (CTG) is indicative of a specific type of fetal heart rate (FHR) pattern associated with a particular condition during pregnancy. This pattern is characterized by a smooth, undulating waveform with a consistent frequency and amplitude, resembling a sine wave. The sine wave pattern is often seen in cases of fetal distress or hypoxia.

**Why the Correct Answer is Right**
The sine wave pattern is typically observed in cases of **intrauterine growth restriction (IUGR)**, where the fetus is experiencing chronic hypoxia due to placental insufficiency. This condition leads to a decrease in fetal movement and a corresponding decrease in the variability of the FHR. The sine wave pattern is a result of the fetal heart rate being dominated by the vagal tone, leading to a smooth and regular waveform. The mechanism behind this pattern is the result of the fetal heart rate being controlled by the parasympathetic nervous system, which is mediated by the release of acetylcholine.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the baseline fetal heart rate is not a sine wave pattern, but rather a steady rate that can be affected by various factors such as maternal position and uterine contractions.
**Option B:** This option is incorrect because variable decelerations are typically seen in cases of umbilical cord compression, which can cause a sudden decrease in fetal heart rate.
**Option C:** This option is incorrect because early decelerations are typically seen in cases of head compression during uterine contractions, which can cause a gradual decrease in fetal heart rate.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that a sine wave pattern on a CTG is a critical indicator of fetal distress and requires immediate attention and intervention to prevent potential complications.

**Correct Answer:** C. Intrauterine growth restriction (IUGR)