**Core Concept:**
Hypophysectomy is the surgical removal of the pituitary gland, which leads to the deficiency of various hormones including vasopressin (also known as antidiuretic hormone or ADH). Vasopressin is essential for maintaining proper fluid and electrolyte balance in the body.

**Why the Correct Answer is Right:**
Correct Answer: D. Vasopressin deficiency post-hypophysectomy leads to increased urine output (diuresis) and reduced urine concentration, which can result in dehydration and electrolyte imbalances. Intravenous (IV) administration of vasopressin would directly deliver the hormone to the target tissues, bypassing the pituitary gland and ensuring prompt restoration of the hormone levels in the bloodstream.

**Why Each Wrong Option is Incorrect:**
A. Injection of vasopressin via the intramuscular (IM) route would not effectively reach the target tissues due to its rapid degradation in the bloodstream.
B. Administering vasopressin orally would have poor bioavailability and cannot overcome the pituitary gland deficiency.
C. Oral administration of vasopressin is not suitable for rapid restoration of hormone levels in the bloodstream, as oral medications typically have slower absorption and systemic circulation.

**Why the Correct Answer is Right:**
Intravenous vasopressin administration can directly deliver the hormone to the target tissues, ensuring prompt restoration of the hormone levels in the bloodstream. This is crucial in cases where rapid correction of deficiency is necessary, such as post-hypophysectomy.

**Clinical Pearl:**
It is essential to consider the mode of administration based on the clinical scenario and the patient's condition. In situations where rapid restoration of hormone levels is crucial, such as post-hypophysectomy, intravenous administration might be the preferred option. However, in stable patients with minor hormone deficiencies, oral administration may suffice.