**Core Concept:** Trypanosoma brucei is a protozoan parasite causing African sleeping sickness in humans. It has two forms: the infective form (procyclic form) and the pathogenic form (tsetse fly stage and bloodstream form).

**Why the Correct Answer is Right:** The correct answer is **Option C: Bloodstream form**. The bloodstream form of T. brucei is the pathogenic form responsible for causing disease in humans. It multiplies rapidly in the bloodstream, leading to the symptoms of African sleeping sickness. This form has a higher molecular weight and lacks the protective glycoproteins present in the procyclic form.

**Why Each Wrong Option is Incorrect:**

A. **Option A: Procyclic form** is the infective form of T. brucei, present in the gut of the tsetse fly. It is not the cause of African sleeping sickness in humans.

B. **Option B: Infective form** is incorrect because the correct answer is the bloodstream form (option C), which causes the disease in humans.

D. **Option D: Tsetse fly stage** refers to the life cycle stage where the procyclic form is transmitted to humans, but this is not the form causing African sleeping sickness in humans.

**Clinical Pearl:** Understanding the life cycle and different forms of T. brucei is crucial for diagnosing and treating African sleeping sickness. Knowledge of the infective form and its transition between tsetse fly, vertebrate host, and invertebrate host is essential for recognizing the disease and selecting appropriate therapy.

**Correct Answer:** Option C: Bloodstream form