**Core Concept:** Down syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 21. Ultrasound markers are signs detected during pregnancy that may increase the likelihood of certain birth defects, including Down syndrome.

**Why the Correct Answer is Right:** Option C, "Hydrops fetalis," is not a marker for Down syndrome. Hydrops fetalis refers to the presence of generalized edema, ascites, and pleural and pericardial effusions in a fetus, which can be associated with various genetic and non-genetic conditions, including neural tube defects and single gene disorders. However, it is not specifically linked to Down syndrome.

**Why Each Wrong Option is Incorrect:**

A. **Option A: Transverse diameter ratio (TDR):** Increased transverse diameter ratio (TDR) between the fetal long bones is considered a marker for Down syndrome, reflecting the characteristic short, broad hands and feet seen in individuals with Down syndrome.

B. **Option B: Placenta accreta:** Placenta accreta is a placental abnormality that occurs in pregnancies where the placenta attaches too deeply to the uterine wall, increasing the risk of bleeding during delivery. It is not a marker for Down syndrome.

C. **Option C: Hydrops fetalis:** As explained above, hydrops fetalis is not specific to Down syndrome and can be associated with various genetic and non-genetic conditions.

D. **Option D: Polyhydramnios:** Polyhydramnios refers to excessive amniotic fluid accumulation and is associated with various conditions, such as meconium-stained liquor, lung hypoplasia, and gastrointestinal obstruction. It is not a specific marker for Down syndrome.

**Clinical Pearl:** Though ultrasound markers can provide valuable information for risk assessment, it is essential to remember that a single abnormal marker should not lead to definitive conclusions about a fetus's diagnosis. A comprehensive assessment involving multiple markers and clinical findings is crucial for accurate diagnosis and management planning.