**Core Concept**
Weight gain is a complex phenomenon influenced by various factors including hormonal changes, genetic predisposition, and environmental factors. It is often associated with conditions such as hypothyroidism, Cushing's syndrome, and polycystic ovary syndrome (PCOS).

**Why the Correct Answer is Right**
The correct answer is related to a condition where weight loss is commonly observed. In the case of untreated hyperthyroidism, the body's metabolic rate increases, leading to excessive burning of calories and subsequent weight loss. This is due to the excessive production of thyroid hormones, which stimulates the body's metabolic processes.

**Why Each Wrong Option is Incorrect**
**Option A:** Weight gain is indeed seen in hypothyroidism, as the reduced production of thyroid hormones slows down the body's metabolic rate, leading to increased caloric storage and weight gain.
**Option B:** Cushing's syndrome, a condition caused by excess cortisol production, is characterized by weight gain, particularly in the abdominal area, due to increased fat storage and muscle breakdown.
**Option C:** Polycystic ovary syndrome (PCOS) is often associated with weight gain, insulin resistance, and hormonal imbalances, making it a condition where weight gain is commonly observed.

**Clinical Pearl / High-Yield Fact**
Mnemonic: "CUSHING" to remember the symptoms of Cushing's syndrome: **C**ortisol excess, **U**pper body obesity, **S**kin thinning, **H**ypertension, **I**ntracranial hypertension, **N**egative feedback loop disruption, **G**lucose intolerance.

**Correct Answer: D. Hyperthyroidism. Weight loss is commonly observed in untreated hyperthyroidism.**