**Core Concept:** Insulins are classified based on their onset, peak, and duration of action into rapid-acting, short-acting, intermediate-acting, and long-acting insulins.

**Why the Correct Answer is Right:** Intermediate-acting insulin has an onset of action between 1-4 hours and a peak between 12-18 hours. It maintains a steady level of insulin in the bloodstream for around 18-20 hours, providing a stable postprandial blood glucose control.

**Why Each Wrong Option is Incorrect:**

A. Rapid-acting insulin (insulin aspart, insulin lispro, insulin glulisine) has a very short onset of action, usually within 5-15 minutes, and a peak within 1-2 hours. It is used for immediate postprandial glucose control.

B. Short-acting insulin (neutral protamine hagedorn, NPH) has a slower onset of action (1-4 hours) and a peak between 6-12 hours. It is suitable for postprandial glucose control but has a shorter duration compared to intermediate-acting insulin.

C. Long-acting insulin (insulin glargine, insulin detemir) has a very long duration of action (around 24 hours) and a peak at around 6-12 hours. It is used for basal insulin therapy and provides continuous glucose control.

D. Intermediate-acting insulin (regular insulin) is the correct answer, with an onset of action between 1-4 hours, a peak between 12-18 hours, and a duration of action around 18-20 hours. It is used as a basal-bolus regimen, combining with short-acting insulin for adequate glucose control.

**Clinical Pearls:**

1. Understanding the pharmacokinetic characteristics of different insulin types is essential for optimal glucose control in patients with diabetes mellitus.
2. Proper selection of insulin type and dosage timing is crucial to achieve a balance between postprandial and basal glucose control.
3. Combining short-acting and intermediate-acting insulin can provide a more stable and sustained glucose control, known as the basal-bolus regimen.

**Correct Answer:** D. Regular insulin, also known as insulin human, is an intermediate-acting insulin with an onset of action between 1-4 hours, a peak between 12-18 hours, and a duration of action around 18-20 hours. It is commonly used as a basal insulin in the basal-bolus regimen along with short-acting insulin (insulin aspart, insulin lispro, or insulin glulisine) for postprandial glucose control.