**Core Concept**
The post-coital test (PCT) is a diagnostic tool used to assess the interaction between sperm and cervical mucus. It evaluates the cervical factor in infertility, which includes the quality and receptivity of cervical mucus to sperm penetration. This test is particularly useful in identifying cervical factor abnormalities.

**Why the Correct Answer is Right**
The post-coital test cannot detect fallopian tube blockages. Fallopian tube patency is typically assessed through imaging modalities such as hysterosalpingography (HSG) or laparoscopy. These methods directly visualize the fallopian tubes and help diagnose blockages or other abnormalities. In contrast, the PCT focuses on the interaction between sperm and cervical mucus, making it unsuitable for detecting fallopian tube issues.

**Why Each Wrong Option is Incorrect**
**Option B:** Cervical factor abnormality is actually what the post-coital test is designed to detect. It evaluates the receptivity and quality of cervical mucus to sperm penetration, making option B incorrect.
**Option C:** While the PCT indirectly assesses sperm count by observing the number of motile sperm in the cervical mucus, it is not a primary or accurate method for determining sperm count. Sperm count is typically assessed through semen analysis.
**Option D:** The PCT also evaluates sperm abnormality by assessing the motility and morphology of sperm in the cervical mucus. Therefore, option D is incorrect as the PCT does indeed detect sperm abnormalities.

**Clinical Pearl / High-Yield Fact**
When using the post-coital test, it's essential to consider the timing of the test. The test is typically performed 24-48 hours after intercourse, when the cervical mucus is most receptive to sperm penetration. This timing is crucial for accurate interpretation of the results.

**✓ Correct Answer: A. Fallopian tube block.**