Person having heterozygous sickle cell trait is protected from infection of
**Core Concept**
Sickle cell trait, also known as sickle cell heterozygote, is a condition where an individual inherits one normal hemoglobin gene and one sickle hemoglobin gene (HbS). This condition is characterized by the presence of both normal and sickle hemoglobin in red blood cells.
**Why the Correct Answer is Right**
Individuals with sickle cell trait have a selective advantage against certain infections, particularly those caused by Plasmodium falciparum, the parasite responsible for malaria. This is due to the fact that sickle hemoglobin has a different affinity for the parasite than normal hemoglobin, making it less conducive to the parasite's growth and survival. As a result, individuals with sickle cell trait have a reduced risk of contracting malaria.
**Why Each Wrong Option is Incorrect**
* **Option A:** There is no evidence to suggest that individuals with sickle cell trait are protected against infections caused by Streptococcus pneumoniae.
* **Option B:** While individuals with sickle cell trait may have a reduced risk of malaria, they are not protected against infections caused by Haemophilus influenzae.
* **Option C:** There is no known association between sickle cell trait and protection against infections caused by Escherichia coli.
**Clinical Pearl / High-Yield Fact**
Individuals with sickle cell trait can still develop sickle cell disease if their offspring inherit two sickle hemoglobin genes (one from each parent), which is known as sickle cell anemia. This highlights the importance of genetic counseling for individuals with sickle cell trait who are planning to start a family.
**Correct Answer: D. Malaria.