Final answer:
The major organ missing from the acardiac twin is the heart.
Step-by-step explanation:
The major organ missing from the acardiac twin is the heart.
An acardiac twin, also known as a parasitic twin or a twin reversed arterial perfusion (TRAP) sequence, occurs when one twin lacks a heart or has an underdeveloped one. In this condition, blood is supplied to the acardiac twin through connections with the healthy twin. The acardiac twin relies on the healthy twin's heart for its survival.
This condition is a rare occurrence in monochorionic pregnancies (where both twins share the same placenta) and can have serious implications for the healthy twin.