**Core Concept**
Antibiotics can alter the normal gut flora, leading to an overgrowth of certain bacteria, such as Clostridium difficile (C. difficile). This disruption in the gut microbiome can cause antibiotic-associated diarrhea (AAD).

**Why the Correct Answer is Right**
C. difficile is a Gram-positive, spore-forming bacterium that produces toxins A and B, which cause inflammation and damage to the intestinal lining, resulting in diarrhea. The overgrowth of C. difficile occurs when the normal gut flora is disrupted by antibiotics, such as clindamycin, cephalosporins, or penicillins. The toxins produced by C. difficile inhibit the absorption of water and electrolytes in the colon, leading to watery diarrhea.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a particular cause of diarrhea. While antibiotic-associated diarrhea can be caused by various factors, C. difficile is a common and well-documented cause.

**Option B:** This option is incorrect because it suggests that the patient's diarrhea is a result of a viral infection. While viral infections can cause diarrhea, the patient's history of antibiotic use makes C. difficile a more likely cause.

**Option C:** This option is incorrect because it implies that the patient's diarrhea is a result of a bacterial infection other than C. difficile. While other bacteria can cause diarrhea, C. difficile is a common cause of antibiotic-associated diarrhea.

**Clinical Pearl / High-Yield Fact**
The use of antibiotics can disrupt the normal gut flora, leading to an overgrowth of C. difficile and antibiotic-associated diarrhea. To prevent this, it is essential to use antibiotics judiciously and to consider the use of probiotics or other measures to maintain the normal gut flora.

**Correct Answer: C. Clostridium difficile**