**Core Concept**
Turner's syndrome is a genetic disorder that affects females, characterized by the complete or partial absence of one X chromosome. This leads to a range of physical and developmental abnormalities.

**Why the Correct Answer is Right**
Turner's syndrome is associated with a variety of clinical features, including short stature, infertility, and cardiovascular anomalies. The syndrome is caused by the loss or alteration of genes on the X chromosome, which affects the development of female sexual characteristics and reproductive function. The most common karyotype associated with Turner's syndrome is 45,X, where the individual has only one X chromosome.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is typically incorrect because Turner's syndrome is not associated with an increased risk of cancer. While individuals with Turner's syndrome may have an increased risk of certain medical conditions, such as autoimmune disorders and cardiovascular disease, cancer is not a characteristic feature of the syndrome.

**Option B:** This option is incorrect because individuals with Turner's syndrome typically have a normal intelligence quotient (IQ). While some individuals with Turner's syndrome may have learning disabilities or difficulties with certain cognitive tasks, the majority have average or above-average intelligence.

**Option C:** This option is incorrect because Turner's syndrome is not associated with an increased risk of polycystic ovary syndrome (PCOS). PCOS is a hormonal disorder that affects women of reproductive age, characterized by irregular menstrual cycles, cysts on the ovaries, and excess androgen levels.

**Option D:** This option is incorrect because Turner's syndrome is not associated with an increased risk of sickle cell disease. Sickle cell disease is a genetic disorder that affects the production of hemoglobin, leading to abnormal red blood cells and an increased risk of anemia and other complications.

**Clinical Pearl / High-Yield Fact**
Turner's syndrome is often associated with a characteristic "shield" chest shape, which is due to the underdevelopment of the breast tissue. This is a key clinical feature that can aid in the diagnosis of the syndrome.

**Correct Answer: C. Turner's syndrome is not associated with an increased risk of polycystic ovary syndrome (PCOS).**