**Core Concept**
Fetal tachycardia refers to an abnormally rapid heart rate in the fetus, which can be a sign of distress or underlying fetal pathology. This condition is typically defined in terms of a heart rate exceeding a certain threshold, which is a critical parameter in fetal monitoring.

**Why the Correct Answer is Right**
The correct answer, 160 beats per minute (bpm), is based on the American College of Obstetricians and Gynecologists (ACOG) guidelines, which define fetal tachycardia as a heart rate greater than 160 bpm. This threshold is used to identify fetuses that may be at risk for distress or other complications. The mechanism underlying fetal tachycardia is complex and multifactorial, involving the interplay of various physiological and pathological factors. However, a heart rate exceeding 160 bpm is generally considered to be outside the normal range for fetal heart rates, which typically range from 110 to 160 bpm.

**Why Each Wrong Option is Incorrect**
**Option A:** 140 bpm is within the normal range for fetal heart rates, and a heart rate of 140 bpm would not be considered tachycardic.
**Option C:** 180 bpm is significantly higher than the threshold for fetal tachycardia and would be considered a severe and potentially life-threatening abnormality.
**Option D:** 200 bpm is an extremely high heart rate that would be associated with severe fetal distress or other life-threatening conditions.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that fetal heart rate monitoring is a critical component of prenatal care, and any abnormal heart rate patterns, including tachycardia, should be promptly investigated and managed by a healthcare provider.

**✓ Correct Answer: B. 160**