**Core Concept**
Preimplantation genetic diagnosis (PGD) is a technique used during in vitro fertilization (IVF) to screen embryos for specific genetic abnormalities before implantation. It targets inherited or monogenic disorders by analyzing the DNA of early-stage embryos.

**Why the Correct Answer is Right**
PGD is specifically designed to detect single gene mutations responsible for inherited disorders such as cystic fibrosis, sickle cell anemia, or Huntington’s disease. It involves amplifying DNA from a few cells of a blastocyst and using polymerase chain reaction (PCR) or other molecular techniques to identify mutations in specific genes. This allows selection of embryos free from the targeted genetic defect, making it ideal for familial single-gene disorders.

**Why Each Wrong Option is Incorrect**
Option A: Fetal gender determination is not the primary purpose of PGD; it is typically used for sex-linked disorders or family planning, but not the core indication.
Option C: HLA typing is used in bone marrow transplantation, not standard PGD, and is typically performed in a different context (e.g., umbilical cord blood).
Option D: Karyotyping detects chromosomal abnormalities (e.g., trisomy 21), which is the domain of PGS (preimplantation genetic screening), not PGD.

**Clinical Pearl / High-Yield Fact**
PGD is distinct from PGS (chromosomal screening); PGD targets **specific single-gene mutations**, while PGS screens for chromosomal number/structure. Always remember: PGD = monogenic disorders, PGS = aneuploidy.

✓ Correct Answer: B. Single gene mutation analysis