**Core Concept**
A contracted pelvis is defined by a smaller-than-average pelvic size, which can affect childbirth and other obstetric procedures. This condition is typically characterized by reduced diameters of the pelvic inlet, outlet, or mid-pelvis.

**Why the Correct Answer is Right**
The correct answer, A, refers to a transverse diameter of the pelvic inlet measuring 10 cm or less. This is a key feature of a contracted pelvis, as the transverse diameter is an important measurement in assessing the pelvic capacity for birth. A smaller transverse diameter can restrict the passage of the fetal head, making childbirth more challenging. In a normal pelvis, the transverse diameter of the inlet is typically around 12-15 cm.

**Why Each Wrong Option is Incorrect**
**Option B:** An anteroposterior (AP) diameter of 12 cm is actually within the normal range for the pelvic inlet. This measurement does not indicate a contracted pelvis.
**Option C:** A platypelloid pelvis is a type of pelvis characterized by a wide, shallow shape. While this type of pelvis may have a smaller transverse diameter, it is not a defining feature of a contracted pelvis.
**Option D:** A gynaecoid pelvis is considered the "classic" or "normal" female pelvis, with a round, spacious shape. This type of pelvis is not a feature of a contracted pelvis.

**Clinical Pearl / High-Yield Fact**
When assessing pelvic capacity for birth, it's essential to remember that the pelvic inlet, outlet, and mid-pelvis must all be adequate to accommodate the fetal head. A contracted pelvis can be a significant risk factor for complications during childbirth, such as obstructed labor or the need for cesarean delivery.

**✓ Correct Answer: A. Transverse diameter of inlet is 10 cm**