**Question:** Most severely affected child in Rh-isoimmunisation patients:

A. Newborn
B. Premature
C. Low birth weight
D. Asphyxiated

**Core Concept:**
Rh-isoimmunisation is a condition in which a pregnant woman's immune system produces antibodies against her fetus's Rh-positive blood type, leading to complications during pregnancy and birth. The severity of the condition depends on the stage of pregnancy and the baby's physiological and anatomical development.

**Why the Correct Answer is Right:**
The correct answer is A. Newborn, as the most severely affected child in Rh-isoimmunisation patients is the newborn baby. This is due to the fact that the baby is the direct recipient of the mother's anti-RhD antibodies, which cross the placenta and cause RhD alloimmunisation. This results in RhD alloimmunisation of the baby, leading to RhD haemolytic disease of the newborn (HDN).

**Why Each Wrong Option is Incorrect:**

A. Premature (Option B) is not the most severely affected child in Rh-isoimmunisation patients, as the severity of the condition depends on the mother's antibody titer and the baby's gestational age. Premature babies may have additional complications but are not more severely affected in Rh-isoimmunisation than term babies.

B. Premature (Option B) is not the most severely affected child in Rh-isoimmunisation patients, as mentioned above. Premature babies may have additional complications but are not more severely affected in Rh-isoimmunisation than term babies.

C. Low birth weight (Option C) is a risk factor for RhD haemolytic disease of the newborn, but it is not the most severely affected child in Rh-isoimmunisation patients. The severity of the condition depends on the mother's antibody titer and the baby's gestational age.

D. Asphyxiated (Option D) is not the most severely affected child in Rh-isoimmunisation patients. The severity of the condition depends on the mother's antibody titer and the baby's gestational age. Asphyxia is a separate clinical entity and does not directly relate to the severity of RhD haemolytic disease of the newborn caused by RhD isoimmunisation.

**Clinical Pearl:**
While low birth weight and premature birth are risk factors for HDN, the severity of the condition depends on the mother's antibody titer and the baby's gestational age. If the mother has high titre of anti-D antibodies and the baby is born at a late gestational age, the severity of HDN is likely to be more severe.

Answer: A. Newborn