**Core Concept**
Immunosuppression refers to the reduction or blockade of the immune system's function, which is crucial for fighting infections and diseases. In a clinical setting, patients undergoing immunosuppression are at an increased risk of various complications due to their compromised immune status.

**Why the Correct Answer is Right**
The most common complication of immunosuppression is indeed infection. This is because the immune system plays a vital role in protecting against microbial pathogens, such as bacteria, viruses, and fungi. When the immune system is suppressed, patients are more susceptible to infections, which can range from mild to life-threatening. The compromised immune status can lead to an increased risk of opportunistic infections, which are caused by pathogens that typically do not cause disease in healthy individuals.

**Why Each Wrong Option is Incorrect**
**Option A:** Malignancy is a potential complication of long-term immunosuppression, but it is not the most common complication. While immunosuppression can increase the risk of certain types of cancer, such as skin cancer and lymphoma, it is not the primary concern in the immediate postoperative period.

**Option B:** Graft rejection is a concern in transplant patients, but it is not the most common complication of immunosuppression. Graft rejection occurs when the immune system recognizes the transplanted organ as foreign and mounts an immune response against it.

**Option D:** Thrombocytopenia, or low platelet count, can be a complication of immunosuppressive therapy, particularly in patients receiving chemotherapy or certain medications. However, it is not the most common complication of immunosuppression.

**Clinical Pearl / High-Yield Fact**
It's essential for surgeons and healthcare providers to be aware of the increased risk of infection in immunosuppressed patients and take steps to prevent and manage infections promptly. This includes proper wound care, use of antibiotics, and close monitoring for signs of infection.

**Correct Answer:**
✓ Correct Answer: C. Infection