**Core Concept**
The crural index is a ratio used in forensic anthropology to estimate the sex of an individual from skeletal remains. It is based on the proportion of the lower leg (tibia) to the thigh (femur) in length.

**Why the Correct Answer is Right**
The crural index is calculated by dividing the length of the tibia by the length of the femur and multiplying by 100. This ratio is useful in forensic analysis because the proportions of the lower limb bones can vary between males and females. On average, females tend to have a shorter tibia relative to the femur compared to males. By calculating the crural index, investigators can make an educated guess about the sex of an individual based on their skeletal remains.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the crural index specifically refers to the ratio between the tibia and femur, not the radius and humerus. The radial index, which is another ratio used in forensic anthropology, involves the length of the radius and humerus.
**Option B:** This option is incorrect because it refers to the radial index, which is a different ratio used in forensic anthropology.
**Option C:** This option is incorrect because the ratio between the fibula and tibia is not a commonly used index in forensic anthropology.
**Option D:** This option is incorrect because it refers to the ratio between the radius and ulna, which is not a relevant index in this context.

**Clinical Pearl / High-Yield Fact**
A useful mnemonic to remember the crural index is "Tibia to Femur, not Radius or Humerus!" This highlights the importance of the correct ratio in forensic anthropology.

**✓ Correct Answer: A. Length of tibia/femur x 100**