**Core Concept**
Pituitary apoplexy is a medical emergency characterized by the sudden hemorrhage or infarction of the pituitary gland, leading to a range of clinical presentations. The condition often results from the rupture of a pituitary adenoma, which can be either functioning or non-functioning. The clinical manifestations of pituitary apoplexy can be diverse and may include sudden visual disturbances, headache, and hormonal imbalances.

**Why the Correct Answer is Right**
Pituitary apoplexy typically presents with sudden onset of symptoms, which can include severe headache, visual disturbances such as diplopia, and endocrine dysfunction due to the sudden loss of pituitary function. The condition may also be associated with acute hypopituitarism, leading to deficiencies in various hormones such as cortisol, thyroid-stimulating hormone (TSH), and gonadotropins. The clinical presentation of pituitary apoplexy can be confused with other conditions such as subarachnoid hemorrhage or meningitis.

**Why Each Wrong Option is Incorrect**
**Option A:** Sudden severe headache is a classic presentation of pituitary apoplexy. Therefore, option A is incorrect.
**Option B:** Visual disturbances such as diplopia and blindness are common in pituitary apoplexy due to the compression of the optic chiasm. Thus, option B is incorrect.
**Option C:** Acute hypopituitarism is a hallmark of pituitary apoplexy, leading to deficiencies in various hormones. Therefore, option C is incorrect.

**Clinical Pearl / High-Yield Fact**
Pituitary apoplexy is a medical emergency that requires prompt diagnosis and treatment to prevent long-term damage to the pituitary gland and other endocrine organs.

**Correct Answer:** D.