**Core Concept**
Multifactorial inheritance refers to a pattern of inheritance where multiple genetic and environmental factors contribute to the development of a disorder, rather than a single gene or mutation. This type of inheritance is often seen in complex traits that have a polygenic basis, where multiple genes interact with each other and with environmental factors to produce the phenotype.

**Why the Correct Answer is Right**
Cleft lip and palate are classic examples of multifactorial inheritance. They are complex traits that result from the interaction of multiple genetic and environmental factors, including genetic mutations, maternal diet, and exposure to teratogens during pregnancy. Research has shown that multiple genetic loci contribute to the risk of cleft lip and palate, and that environmental factors such as smoking and alcohol consumption during pregnancy can also increase the risk. The genetic contribution to cleft lip and palate is estimated to be around 30-70%, with the remaining risk attributed to environmental factors.

**Why Each Wrong Option is Incorrect**
**Option B:** Marfan's syndrome is a genetic disorder caused by a mutation in the FBN1 gene, which encodes for fibrillin-1. It is an autosomal dominant disorder, meaning that a single copy of the mutated gene is sufficient to cause the condition. Therefore, it does not exhibit multifactorial inheritance.

**Option C:** Erythroblastosis fetalis is a condition caused by the immune system of the mother attacking the red blood cells of the fetus. It is typically caused by Rh incompatibility between the mother and the fetus, and is not a condition that is influenced by multiple genetic and environmental factors.

**Option D:** Down's syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 21. It is a chromosomal disorder, rather than a multifactorial condition, and is typically caused by a nondisjunction event during meiosis.

**Clinical Pearl / High-Yield Fact**
When considering the inheritance pattern of a condition, it is essential to look for evidence of a complex interplay between genetic and environmental factors. Multifactorial inheritance is often seen in conditions that have a late age of onset, variable expression, and a complex family history.

**✓ Correct Answer: A. Cleft lip**