**Core Concept**
Transport maximum (Tm) is a pharmacological concept referring to the maximum rate at which a substance can be transported across a biological membrane, such as a cell membrane or a renal tubule. It is a measure of the maximum capacity of a transport mechanism to move a substance against its concentration gradient.

**Why the Correct Answer is Right**
The transport maximum is a saturable process, meaning that as the concentration of the substance increases, the rate of transport also increases until it reaches a maximum value. This is because the transport mechanism has a limited number of binding sites, and once these sites are occupied, further increases in concentration do not result in increased transport. The transport maximum is often represented by the Michaelis-Menten equation, which describes the relationship between substrate concentration and transport rate.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not related to the concept of transport maximum. While it may be a term from pharmacology, it does not describe the maximum rate of transport.

**Option B:** This option is incorrect because it is a measure of the maximum concentration of a substance that can be attained in the blood after administration, not the maximum rate of transport.

**Option C:** This option is incorrect because it is a term from molecular biology, referring to the maximum number of molecules that can be bound to a protein, not the maximum rate of transport across a membrane.

**Clinical Pearl / High-Yield Fact**
The transport maximum is an important concept in pharmacology because it determines the maximum therapeutic effect of a drug. Understanding the transport maximum can help clinicians optimize dosing regimens and avoid toxicity.

**Correct Answer: D. The maximum rate of transport of a substance across a biological membrane.**